GAME DEVELOPMENT TUTORIAL: DAY 2-4: Enter the Robot

10/13/2012

Lesson #2-9: Creating the Main Character

Welcome to the fourth day! Now that we got the basic frameworks set up, we will be adding our main character.

To do so, we will be creating a new class. Within this class, we will do the following:

1. Manage x, y coordinates and speed.
2. Make updates to speed and position.
3. Allow other classes to retrieve its x, y related variables.

Therefore, the methods of this class will be divided into these categories:

1. Constantly updated methods that are called on each iteration of the game loop.
2. Methods that are only called upon player input.
3. Helper methods that retrieve and change the value of variables in the class.

Let's get started.

We begin by creating a new class called Robot.java in the kiloboltgame package (or whatever you named your package).

Copy and paste the following:

Figure 2-13: Robot.java

package kiloboltgame;

import java.awt.Graphics;

public class Robot {
//In Java, Class Variables should be private so that only its methods can change them.
private int centerX = 100;
private int centerY = 382;
private boolean jumped = false;

private int speedX = 0;
private int speedY = 1;

public void update() {

// Moves Character or Scrolls Background accordingly.
if (speedX < 0) {
centerX += speedX;
} else if (speedX == 0) {
System.out.println("Do not scroll the background.");

} else {
if (centerX <= 150) {
centerX += speedX;
} else {
System.out.println("Scroll Background Here");
}
}

// Updates Y Position

if (centerY + speedY >= 382) {
centerY = 382;
}else{
centerY += speedY;
}

// Handles Jumping
if (jumped == true) {
speedY += 1;

if (centerY + speedY >= 382) {
centerY = 382;
speedY = 0;
jumped = false;
}

}

// Prevents going beyond X coordinate of 0
if (centerX + speedX <= 60) {
centerX = 61;
}
}

public void moveRight() {
speedX = 6;
}

public void moveLeft() {
speedX = -6;
}

public void stop() {
speedX = 0;
}

public void jump() {
if (jumped == false) {
speedY = -15;
jumped = true;
}

}
}

What do those variables mean?

A brief description of the variables (there will be more a in depth discussion further down):
1. centerX, centerY are the x, y coordinates of our robot character's center.
2. speedX, speed Y are the rate at which these x and y positions change.
3. jumped changes to true if the character is in the air, and reverts to false when grounded.

Now let's talk about each of the methods above.

1. Always called methods:

update() : this method will be called on each iteration of the for loop. This is a very important method, so let's spend time taking about it. But before we do that, PLEASE look through it one more time and try to make sense of the if statements.

*When examining the update() method again, Keep the following in mind.

1. Speed can be negative, which means that a character with negative speedX would move to the left.

2. The Origin (0,0) pixel is at the TOP LEFT. I will talk about this below. This means that if a character has a positive speedY, he is FALLING, not RISING.

The origin is at the top left of the screen.

3. Recall the meaning of += :
x += 1;
is equivalent to:
x = x + 1;

4. We are arbitrarily defining the ground at about 440 pixels. That means if the character's centerY is at about 382, his feet would reach the ground at ~440 pixels.

5. If the character's centerX is lesser than 60, his left hand will be outside the left edge of the screen.

Now have a look at a diagram:

Basic Diagram

Examine this update() method once more before moving on!

public void update() {

// Moves Character or Scrolls Background accordingly.
if (speedX < 0) {
centerX += speedX; //This changes centerX by adding speedX.
} else if (speedX == 0) {
System.out.println("Do not scroll the background.");

} else {
if (centerX <= 150) { //If the character's centerX is in the left 150 pixels
centerX += speedX; //Change centerX by adding speedX.
} else {
System.out.println("Scroll Background Here"); /
}
}

// Updates Y Position
if (centerY + speedY >= 382) {
  //382 is where the character's centerY would be if he were standing on the ground.
centerY = 382;
}else{
centerY += speedY; //Add speedY to centerY to determine its new position
}

// Handles Jumping
if (jumped == true) {
speedY += 1; //While the character is in the air, add 1 to his speedY.
//NOTE: This will bring the character downwards!

if (centerY + speedY >= 382) {
centerY = 382;
speedY = 0;
jumped = false;
}

}

// Prevents going beyond X coordinate of 0
if (centerX + speedX <= 60) { //If speedX plus centerX would bring the character //outside the screen,
centerX = 61;
//Fix the character's centerX at 60 pixels.
}
}

Dissecting the update() method:

First, let's discuss the section labeled:
// Moves Character or Scrolls Background accordingly.

1. Our first if statement:
For our game to draw the character in the proper location, centerX must constantly be updated; however, if the character is not moving (speedX == 0), then there is no need to update centerX by adding speed to it.

2. Watch a few seconds of this video:

In Metal Slug, the character freely moves in the left half of the screen, but once he starts moving to the right, the character stays at the same location while the background scrolls. This is what we are doing here. If speed is zero, then we will not scroll the background. If the character's centerX coordinate is less than 150, he can move freely. Else, we will scroll the background and stop moving the character.

Now, let's discuss the sections labeled:
//Updates Y Position and //Handles Jumping.
Since gravity is always present, the character is constantly being pushed to the ground. We assume that the ground is located at about 440 pixels down from the top, and if the character's Y position plus his Y speed will bring him below the ground, we use the statement: centerY = 382; to manually set the character at a height that will stop him from moving.

The Handles jumping section will check the current speed and position to test whether the character is in mid-jump or on the ground.

Finally, let's discuss the section labeled:// Prevents going beyond X coordinate of 0

This section just checks if the character is moving beyond the left edge of the screen and fixes his centerX coordinate at 61 if he tries to move off the screen.

Now that we have discussed the update() method, we will move on to the other methods:

2. Methods called upon input:

moveRight(), which sets the character's horizontal speed (speedX) as 6.
moveLeft(), which sets the character's speedX as -6.
stop(), which sets the speedX as zero.
jump(), which sets the vertical speed as -15.

3. Helper Methods
We will be adding these later in today's lesson.

Lesson #2-10: Graphics 101

To make sense of those speed values, we need to talk about some basic graphical stuff.

Pixels are the tiny squares on your display. They are the smallest unit of change in your display, and are either on or off and have a single color.

Screen resolution is a representation of your horizontal and vertical pixel counts. For example, a resolution of 1920 by 1080 tells you that a display has 1920 horizontal pixels and 1080 vertical pixels.

Individual pixel coordinates are given in (x,y); however, by convention and technological limitations, we choose the top left corner as (0,0). On a screen with resolution 1920 x 1080, then, the bottom right pixel has coordinates (1919, 1079) NOT (1920, 1080). This is because we begin counting from zero, not one.

I will now apply this to describe how we create an illusion of motion in games.
Within the StartingClass's paint method, we will write a statement that will look something like this:
drawRobot(centerX, centerY). This will draw the Robot at the coordinate (centerX, centerY).

With each iteration of the paint method, which is called roughly 60 times per second, the Robot will appear to move as its centerX and centerY values change. To change the centerX and centerY, we use the speedX and speedY variables, which represent the rate at which centerX and centerY will change with each update of the game (60 times per second).

So when I say that moveRight() sets the character's horizontal speed as 6, I am saying that each time that moveRight() is called, the character's speed will be given a value of 6 pixels. In the case of the robot, the update() method, will add this speed of 6 to the centerX value. Changing the centerX value will mean that the drawRobot(centerX, centerY) statement above will now "move" the robot to a new location with its new centerX. If this happens on a speed that is fast enough, we get the illusion of smooth motion.

Creating a Robot named robot.

With the Robot class created, we can now create a Robot object inside StartingClass.
Locate:

public class StartingClass extends Applet implements Runnable, KeyListener {
and below it, declare:
private Robot robot;
to create a private Robot object called robot.

Now, let us initialize this value by writing:

robot = new Robot(); in the start() method.

Now that we have created an instance of the Robot class, we can take Robot's methods and call them by writing:

robot.methodName(); (methodName() is just a generic name for any method in the Robot class).

Lesson #2-11: Drawing the Main Character from StartingClass.java

There's lots of code to cover here, and I will try to keep the explanations as simple as possible; however, some of the concepts here are a bit elusive and you might wonder how you would come up with such a solution when programming by yourself in the future. This is one of those cases where you are not required to memorize or fully understand how something works, because it is so easy to implement and difficult to fully understand.

We will be working in the StartingClass.java
1. First make sure that you have imported : java.awt.Graphics;
2. Then we will begin by going below the run method (it does not matter where, but I like grouping related methods together) and create the following methods: update(Graphics g) and paint(Graphics g). It is easy to do create these methods by typing update and then pressing Ctrl+Space, and choosing the first option. Do the same thing with paint.

The update() method is implicitly called automatically, and will loop over and over again. The paint() will similarly be always called, with the repaint() statement within the run() method.

I. Defining the update() Method:

Let's first deal with the update() method. We will use this method for double buffering - a technique that is used to prevent tearing and flickering. Feel free to read up more on this subject, as I am not an expert in this topic. The basic concept is that it works by retaining the previous position of the screen's current image for a short amount of time, so that the movement of the image looks smooth and natural.

1. Begin by declaring (below private Robot robot):

private Image image;
private Graphics second;

As class variables (accessible by all methods within the StartingClass).

2. If you receive an error on either Graphics or Images, press Ctrl+Shift+O to auto-import them.
3. Now go down the the update() method and add the following:

Figure 2-14: update() method within StartingClass.java

@Override
public void update(Graphics g) {
if (image == null) {
image = createImage(this.getWidth(), this.getHeight());
second = image.getGraphics();
}

second.setColor(getBackground());
second.fillRect(0, 0, getWidth(), getHeight());
second.setColor(getForeground());
paint(second);

g.drawImage(image, 0, 0, this);

}

The individual statements of this segment of code is easy to understand. What is difficult is discerning how each of these pieces fit together as a whole to create a double buffering system. Feel free to look at this in detail, but my advice to you: ignore it, accept it, and move on. :)

II. Defining the paint() Method:
The paint() method will be used to draw our graphics to the screen. For now, we only need to draw the robot, so it will have one statement inside:

Figure 2-15: paint() method within StartingClass.java

@Override
public void paint(Graphics g) {
g.drawImage(character, robot.getCenterX() - 61, robot.getCenterY() - 63, this);

}

The g.drawImage() method takes in the following parameters:
g.drawImage(img, x, y, observer)

An Image variable, the x and y coordinates where you want to draw the Image, and an ImageObserver (which is slightly beyond the scope of this lesson).

In our example, we will use the character variable to represent our robot image, and then draw the top left corner of the robot 61 pixels to the left, and 63 pixels above the (centerX, centerY), and then use the "this" keyword as our ImageObserver.

At this point, you will have a lot of errors. We will address them each one at a time.

The paint() method should have errors.

I. Addressing the character variable error

1. The character variable is undefined, so we must first define it.
Find the statement: private Image image;
and add , character like so:

private Image image, character;

2. Now we must assign a value to it. We will assign an image to the variable character. To do so, we must create a URL object that will allow us to use addresses (such as C:\Images\image.jpg) to refer to images.

So, below private Image image, character; add:

private URL base;

(As mentioned before, private variables are only accessible from within the class - it is common practice to create class-wide variables as private).

Remember to import URL.

3. Within the init() method, we will define the URL base and the assign value to character.
Make these changes:

Figure 2-16: init() method within StartingClass.java

@Override
public void init() {

setSize(800, 480);
setBackground(Color.BLACK);
setFocusable(true);
addKeyListener(this);
Frame frame = (Frame) this.getParent().getParent();
frame.setTitle("Q-Bot Alpha");
try {
base = getDocumentBase();
} catch (Exception e) {
// TODO: handle exception
}

// Image Setups
character = getImage(base, "data/character.png");

}

4. Finally, we must create a data folder by right clicking on src and creating a folder named data.
Inside it, download, drag, and drop this image:

character.png
File Size:	3 kb
File Type:	png

Download File

That should solve the character errors. The StartingClass.java will look like this now:
(If you have errors, try Rebuilding by doing the following: On the toolbar above, click on Project > Clean > OK).

Figure 2-17: StartingClass.java, After Fixing character Errors

package kiloboltgame;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.net.URL;

public class StartingClass extends Applet implements Runnable, KeyListener {

private Image image, character;
private Graphics second;
private URL base;

@Override
public void init() {

setSize(800, 480);
setBackground(Color.BLACK);
setFocusable(true);
addKeyListener(this);
Frame frame = (Frame) this.getParent().getParent();
frame.setTitle("Q-Bot Alpha");
try {
base = getDocumentBase();
} catch (Exception e) {
// TODO: handle exception
}

// Image Setups
character = getImage(base, "data/character.png");

}

@Override
public void start() {
robot = new Robot();

Thread thread = new Thread(this);
thread.start();
}

@Override
public void stop() {
// TODO Auto-generated method stub
}

@Override
public void destroy() {
// TODO Auto-generated method stub
}

@Override
public void run() {
while (true) {

repaint();
try {
Thread.sleep(17);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

@Override
public void update(Graphics g) {
if (image == null) {
image = createImage(this.getWidth(), this.getHeight());
second = image.getGraphics();
}

second.setColor(getBackground());
second.fillRect(0, 0, getWidth(), getHeight());
second.setColor(getForeground());
paint(second);

g.drawImage(image, 0, 0, this);

}

@Override
public void paint(Graphics g) {
g.drawImage(character, robot.getCenterX() - 61, robot.getCenterY() - 63, this);

}

@Override
public void keyPressed(KeyEvent e) {

switch (e.getKeyCode()) {
case KeyEvent.VK_UP:
System.out.println("Move up");
break;

case KeyEvent.VK_DOWN:
System.out.println("Move down");
break;

case KeyEvent.VK_LEFT:
System.out.println("Move left");
break;

case KeyEvent.VK_RIGHT:
System.out.println("Move right");
break;

case KeyEvent.VK_SPACE:
System.out.println("Jump");
break;

}

}

@Override
public void keyReleased(KeyEvent e) {
switch (e.getKeyCode()) {
case KeyEvent.VK_UP:
System.out.println("Stop moving up");
break;

case KeyEvent.VK_DOWN:
System.out.println("Stop moving down");
break;

case KeyEvent.VK_LEFT:
System.out.println("Stop moving left");
break;

case KeyEvent.VK_RIGHT:
System.out.println("Stop moving right");
break;

case KeyEvent.VK_SPACE:
System.out.println("Stop jumping");
break;

}

}

@Override
public void keyTyped(KeyEvent e) {
// TODO Auto-generated method stub

}

}

II. Addressing the get... errors

We still have these errors in our StartingClass:

In my previous description of the Robot class, I mentioned that we would be creating:

1. Constantly updated methods that are called on each iteration of the game loop.
2. Methods that are only called upon player input.
3. Helper methods that retrieve and change the value of variables in the class.

We never did create the helper methods. So here we do that.

1. Open Robot.java again.
2. Right click anywhere on the white space, click Source >> and select Generate Getters and Setters.

Generate Getters and Setters

3. Select all, sort by getters then setters, and then press OK.

What are getters and setters?
Again, in Java, it is common practice to set class-wide variables as private. For other classes to access these private variables, they must use helper functions known as getters and setters.

Let's have a look at a pair:

public int getSpeedX() {
return speedX;
}

and
public void setSpeedX(int speedX) {
this.height = speedX;
}

Whenever a getter method is called, it returns the value that you "get." When I say:
myNewVariable = getSpeedX(), my new variable will get the value of speedX.

If I say: setSpeed(10); then my speedX will now have a value of 10.

With these three things finished, the result is:

Figure 2-18: StartingClass.java After Fixing the get... Errors

package kiloboltgame;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.net.URL;

public class StartingClass extends Applet implements Runnable, KeyListener {

private Robot robot;
private Image image, character;
private Graphics second;
private URL base;

@Override
public void init() {

setSize(800, 480);
setBackground(Color.BLACK);
setFocusable(true);
addKeyListener(this);
Frame frame = (Frame) this.getParent().getParent();
frame.setTitle("Q-Bot Alpha");
try {
base = getDocumentBase();
} catch (Exception e) {
// TODO: handle exception
}

// Image Setups
character = getImage(base, "data/character.png");

}

@Override
public void start() {
robot = new Robot();

Thread thread = new Thread(this);
thread.start();
}

@Override
public void stop() {
// TODO Auto-generated method stub
}

@Override
public void destroy() {
// TODO Auto-generated method stub
}

@Override
public void run() {
while (true) {

repaint();
try {
Thread.sleep(17);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

@Override
public void update(Graphics g) {
if (image == null) {
image = createImage(this.getWidth(), this.getHeight());
second = image.getGraphics();
}

second.setColor(getBackground());
second.fillRect(0, 0, getWidth(), getHeight());
second.setColor(getForeground());
paint(second);

g.drawImage(image, 0, 0, this);

}

@Override
public void paint(Graphics g) {
g.drawImage(character, robot.getCenterX() - 61, robot.getCenterY() - 63, this);

}

@Override
public void keyPressed(KeyEvent e) {

switch (e.getKeyCode()) {
case KeyEvent.VK_UP:
System.out.println("Move up");
break;

case KeyEvent.VK_DOWN:
System.out.println("Move down");
break;

case KeyEvent.VK_LEFT:
System.out.println("Move left");
break;

case KeyEvent.VK_RIGHT:
System.out.println("Move right");
break;

case KeyEvent.VK_SPACE:
System.out.println("Jump");
break;

}

}

@Override
public void keyReleased(KeyEvent e) {
switch (e.getKeyCode()) {
case KeyEvent.VK_UP:
System.out.println("Stop moving up");
break;

case KeyEvent.VK_DOWN:
System.out.println("Stop moving down");
break;

case KeyEvent.VK_LEFT:
System.out.println("Stop moving left");
break;

case KeyEvent.VK_RIGHT:
System.out.println("Stop moving right");
break;

case KeyEvent.VK_SPACE:
System.out.println("Stop jumping");
break;

}

}

@Override
public void keyTyped(KeyEvent e) {
// TODO Auto-generated method stub

}

}

Lesson #2-12: Character Movement

At this point, you should be able to draw your character to the screen without any flickering:

Our progress so far.

Now we will make our final set of changes to make this character move.

First: In StartingClass go down to the keyPressed() method, and replace:

1. System.out.println("Move left"); with robot.moveLeft();
2. System.out.println("Move right"); with robot.moveRight();
3. System.out.println("Jump"); with robot.jump();

And in the keyReleased() method, replace:

1. System.out.println("Stop moving left");
and
2. System.out.println("Stop moving right");
with
robot.stop();

Second: Within the run() method, we need to call robot.update();

Figure 2-19: run() method within StartingClass.java

@Override
public void run() {
while (true) {
robot.update();
repaint();
try {
Thread.sleep(17);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

Now you should finally have the end result:

Figure 2-20: StartingClass.java, End of Day 4

package kiloboltgame;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.net.URL;

public class StartingClass extends Applet implements Runnable, KeyListener {

private Robot robot;
private Image image, character;
private Graphics second;
private URL base;

@Override
public void init() {

setSize(800, 480);
setBackground(Color.BLACK);
setFocusable(true);
addKeyListener(this);
Frame frame = (Frame) this.getParent().getParent();
frame.setTitle("Q-Bot Alpha");
try {
base = getDocumentBase();
} catch (Exception e) {
// TODO: handle exception
}

// Image Setups
character = getImage(base, "data/character.png");

}

@Override
public void start() {
robot = new Robot();

Thread thread = new Thread(this);
thread.start();
}

@Override
public void stop() {
// TODO Auto-generated method stub
}

@Override
public void destroy() {
// TODO Auto-generated method stub
}

@Override
public void run() {
while (true) {
robot.update();
repaint();
try {
Thread.sleep(17);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

@Override
public void update(Graphics g) {
if (image == null) {
image = createImage(this.getWidth(), this.getHeight());
second = image.getGraphics();
}

second.setColor(getBackground());
second.fillRect(0, 0, getWidth(), getHeight());
second.setColor(getForeground());
paint(second);

g.drawImage(image, 0, 0, this);

}

@Override
public void paint(Graphics g) {
g.drawImage(character, robot.getCenterX() - 61, robot.getCenterY() - 63, this);

}

@Override
public void keyPressed(KeyEvent e) {

switch (e.getKeyCode()) {
case KeyEvent.VK_UP:
System.out.println("Move up");
break;

case KeyEvent.VK_DOWN:
System.out.println("Move down");
break;

case KeyEvent.VK_LEFT:
robot.moveLeft();
break;

case KeyEvent.VK_RIGHT:
robot.moveRight();
break;

case KeyEvent.VK_SPACE:
robot.jump();
break;

}

}

@Override
public void keyReleased(KeyEvent e) {
switch (e.getKeyCode()) {
case KeyEvent.VK_UP:
System.out.println("Stop moving up");
break;

case KeyEvent.VK_DOWN:
System.out.println("Stop moving down");
break;

case KeyEvent.VK_LEFT:
robot.stop();
break;

case KeyEvent.VK_RIGHT:
robot.stop();
break;

case KeyEvent.VK_SPACE:
System.out.println("Stop jumping");
break;

}

}

@Override
public void keyTyped(KeyEvent e) {
// TODO Auto-generated method stub

}

}

Figure 2-21: Robot.java, End of Day 4

package kiloboltgame;

import java.awt.Graphics;

public class Robot {

private int centerX = 100;
private int centerY = 382;
private boolean jumped = false;

private int speedX = 0;
private int speedY = 1;

public void update() {

// Moves Character or Scrolls Background accordingly.
if (speedX < 0) {
centerX += speedX;
} else if (speedX == 0) {
System.out.println("Do not scroll the background.");

} else {
if (centerX <= 150) {
centerX += speedX;
} else {
System.out.println("Scroll Background Here");
}
}

// Updates Y Position

if (centerY + speedY >= 382) {
centerY = 382;
}else{
centerY += speedY;
}

// Handles Jumping
if (jumped == true) {
speedY += 1;

if (centerY + speedY >= 382) {
centerY = 382;
speedY = 0;
jumped = false;
}

}

// Prevents going beyond X coordinate of 0
if (centerX + speedX <= 60) {
centerX = 61;
}
}

public void moveRight() {
speedX = 6;
}

public void moveLeft() {
speedX = -6;
}

public void stop() {
speedX = 0;
}

public void jump() {
if (jumped == false) {
speedY = -15;
jumped = true;
}

}

public int getCenterX() {
return centerX;
}

public int getCenterY() {
return centerY;
}

public boolean isJumped() {
return jumped;
}

public int getSpeedX() {
return speedX;
}

public int getSpeedY() {
return speedY;
}

public void setCenterX(int centerX) {
this.centerX = centerX;
}

public void setCenterY(int centerY) {
this.centerY = centerY;
}

public void setJumped(boolean jumped) {
this.jumped = jumped;
}

public void setSpeedX(int speedX) {
this.speedX = speedX;
}

public void setSpeedY(int speedY) {
this.speedY = speedY;
}

}

That's it for Day 4. This was a tough lesson. Please let me know in the comments section if you need me to clarify anything at all.

If your code is not working for any reason, try Rebuilding by doing the following:
On the toolbar above, click on Project > Clean > OK.

As a general rule, if your code does not work and you cannot find any errors, your first step should be to rebuild the project!

Thanks for reading and please support us if you are learning! Every dollar helps us out tremendously!

kiloboltgame4.zip
File Size:	13 kb
File Type:	zip

Download File

Instructions on importing projects can be found here.

Go to Day 3: Taking User Input

Go to Day 5: Background and Sprites

Comments

David link

10/14/2012 8:41am

I was following the text and noticed:

robot = new Robot();

wasn't mentioned.

Ben

10/14/2012 11:39am

I too noticed this. I also am having other issues but I'm checking to see if its a mistake I have made first.

Ben Brace

10/14/2012 2:10pm

Working fine now, wasn't loading the png file for some reason. Great tutorials!

James C.

10/15/2012 7:34am

This will be fixed soon.

simon

01/08/2014 7:33pm

I wanted to create 3d games.....am I in the right place

Aoi Okami

03/14/2014 11:31am

With my personal experience and say yes. I also want to create 3D games but before jumping to the sea, you need to know how to swim. This GREAT tutorial will help you understand the basic knowledge in game developing like looping, collision and other logic that you need to use in 3D games, especially if your using a game engine.

12/07/2012 2:53pm

Yeah I realized this too.... I ran into a null pointer exception till I realized that the robot object had not be initialized O:

12/08/2012 9:37am

Sorry about that! I thought I fixed that already.

richard

11/08/2013 6:27am

The robot declaration in still missing in one of the "full" code snippets. Great tutorial.

03/08/2014 1:43am

This was the reason why robot doesn't show up in my program. I have no idea why though.

Can someone explain why this causes the image to not to show up?

James link

10/14/2012 1:46pm

I have been following these tutorials and I must say they are great. Keep them coming!

nightmare6m

10/14/2012 7:17pm

Had the same issue with the png but also got it working in the end. Can't wait for day 5.

rolson

01/19/2013 10:48am

How did you get the PNG to load. Mine isn't loading either

01/20/2013 7:50pm

Actually to get the image to load I had to change the path to from "data/character.png" to " ../data/character.png" It worked after that.

Bill Siegel

04/03/2013 3:00am

Thanks rolson.. good find, I also had to add the "../" to "data/character.png" to end up with " ../data/character.png" then it would paint

Son

08/12/2013 4:39am

Mine just works after I changed:
- inside init(): base = getDocumentBase() -> base = getCodeBase()
- Turn Java Compiler from 1.7 -> 1.6

Hope I help somebody ! :)

Jeison

02/12/2014 5:59pm

YO man can any one PLLLLLLZZZZZZZZZ help me like face chat or some thing to help me download the picture icons cause that is my biggest problem.

10/15/2012 1:43am

Just curious about the "height and width" variables mentioned when you set up the getters and setters, I can see them in your code screenshot but not in any of the code itself, what are these used for?

10/15/2012 7:30am

I included them by accident. I will fix that asap. :)

10/15/2012 5:53am

One other thing I've noticed (having copied and adapted the code to create my own game alongside this one) is that movement is jarred, if I suddenly change direction from left to right my character will move a fraction pause for a short while and then move in the direction as normal. Any idea what might be causing this?

10/15/2012 7:32am

Ben, this is normal behavior when you press two buttons and release one. We will be implementing a more complex movement system soon. This game is nowhere near finished so don't worry about little bugs for now.

10/15/2012 9:07am

I actually figured this out shortly after I posted my comment and realised that the stop method was still being invoked on the release of one key even if another one had already been pressed.

Again James, thanks for these tutorials they are awesome. I am using them as a base to build my first android app and game and I will 100% be putting your name and website in my credits. I look forward to the rest of the series.

Matt link

10/16/2012 6:31am

Thanks for your time on the tutorials James, very much appreciated.

Manish

10/17/2012 4:41am

very nice tutorial.

Brendan

10/17/2012 9:06pm

Hey I've just started reading these today. Thankfully I know basic Java so I was able to start unit 2 right away. I like the way these tutorials actually start the game making process right off the bat instead of trying to sit the reader through 150+ pages of theory from a textbook.

For this section I had one problem trying to create a data folder within the package. I got it eventually but maybe this part should be clarified a bit more. Other than that the only problem I have with the tutorials is sometimes there is a lack of depth. If something isn't fully explained or I am told to copy and paste something it doesn't feel like learning. This hasn't happened too much though and it's understandable when you have to try and write out one or two tutorials a week.

Other than that I will try to keep up with these great tutorials until the end. I'd also like to know what games you guys are planning for the future? You have a 3D tutorial planned but no 3D games on the Market yet!

Albert L.

04/17/2013 10:33am

"For this section I had one problem trying to create a data folder within the package. I got it eventually but maybe this part should be clarified a bit more."
Agreed.

"Other than that the only problem I have with the tutorials is sometimes there is a lack of depth. If something isn't fully explained or I am told to copy and paste something it doesn't feel like learning."

I have to disagree with you on this one. I think that some of these concepts are mote easily understood when the product is observed as a whole. So instead of trying to explain all of the defenitions and later trying to peice them all together, he first puts them into context and then explains the meaning of each part and what it does.

ale

10/18/2012 12:09am

I can't solve the image problem, the program runs, the console says: "Do not scroll the background.", but the red "robot" wont show up, if I move the message in te console changes, but it still wont show me the image :( how did you guys solved it?

The image is in KiloboltGame\src\data\character.png

10/18/2012 3:31am

I had this problem as well. I eventually solved it by deleting the image from the data folder. Then opening eclipse, opening my desktop (where the png was orginally stored) and dragging the png to the data folder icon in eclipse. I am not sure why that worked while actually placing the image in the folder did not but maybe worth a try.

James C

10/18/2012 6:27am

Try rebuilding the project by going to Project > Clean. That should solve the issue.

mlorenz

11/23/2012 2:04pm

+1 for this. I tried everything else listed here and nothing, but nothing, worked. Dragging the image from another folder and dropping it into the data folder in Eclipse finally did the trick.

I don't know why this should matter, either.

driff

12/06/2012 7:23pm

u can also just right click the project and select refresh this happens because u are adding outside eclipse and it doesnt check what has changed from outside eclipse unless u close it and open it back again or u hit refresh

Hanashimaru

06/12/2013 4:08pm

Thank you! I tried everything else, but this was the one way to actually get it work! Thanks again!

XOLAROViCH link

04/12/2014 12:03am

Thnx nightmare6m. It worked for me.

UAB-Student

01/23/2014 12:15pm

Actualy the right way to do this is change last line in the "init()" methode to:

// Image Setups
character = getImage(path, "../Data/robot.png");

adding the two points "../" indecates the previous folder, some versions of the operating system won't work without the "../"

10/18/2012 11:15am

still not loading the image :(

. drag and drop won't work, same problem
. project > clean, same problem

any other idea?

10/18/2012 11:54am

Have you tried importing the finished source code I posted at the end?

Mike

01/17/2013 9:03pm

This is a bit late but I was having the same issue with the image not showing. I fixed it by changing the line that reads:

base = getDocumentBase();
- to -
base = getCodeBase();

See: http://docs.oracle.com/javase/1.4.2/docs/api/java/applet/Applet.html

Hopefully this doesn't cause an issue later. :D

R4m80

01/24/2013 8:33am

I even would recommend "getCodeBase()" because it returns the URL to the folder,
while "getDocumentBase()" returns the URL of a generated html-file in this folder.
however, it works both ways for me
(wanted to reply to Mike but didn't find a "Reply"-button :( )

10/18/2012 1:07pm

well that solved it, apparently I was creating the wrong type of "data" folder.

is it ok that it doesn't move up?

ps. cool work with the style of the site :D
can't wait until tomorrow for next lesson :)

Sean

10/18/2012 4:02pm

I have the same problem as Ale although I can't fix it. What type of folder is it that you use to store the robot graphic?

10/18/2012 4:08pm

It is just a regular folder you create by right clicking the Src folder and creating a New folder. Adding new files often causes problems so make sure you rebuild. If you cant figure it out, copy and paste your Two classes and email them to me :)

Jamescho7@Kilobolt.com

10/19/2012 5:05am

It's okay I've fixed it now! :D

10/19/2012 9:59pm

hello, the link to day 5 is not working :p

thanks for another aesome lesson! :D

Richie

10/25/2012 11:07am

Hi there
got a question:
while handling y coordinates,
centerY += speedY;
if (centerY + speedY >= 382)
centerY = 382;
and
f (jumped == true) {
speedY += 1;

if (centerY + speedY >= 382) {
centerY = 382;
speedY = 0;
jumped = false;
}

Why are we using if (centerY + speedY >= 382) {centerY = 382; } twice ? (the one before the boolean condition and the one after it)

10/25/2012 11:56am

The first centerY = 382 is called when jumped is false.
The second is called when jumped is true.

Confused

04/03/2014 4:10am

How does the java know that SpeedX is dedicated for horizontal movement and SpeedY is dedicated for vertical movement?
Since both SpeedX and SpeedY are just int variables, how does java differentiate which variable is used for horizontal movement or vertical movement?
thanks in advance

10/25/2012 10:06pm

Thank You!!

Hymosi

10/26/2012 8:13am

Why the

public void update(Graphics g) method update automatically?

Bob

10/26/2012 8:45pm

It's just a function of an applet

Alpha120_xda link

10/26/2012 1:45pm

Hello, I dont know if my app is working fine.
I nearly cant move the robot to the right, in console i see "Scroll the background", but I can move the robot to the left, out of the screen. Someting wrong with coords maybe? Also, jumping does not work.
Is there anything wrong or is it ok as of now?

Great tutorials, thank you for your hard work, James.

10/26/2012 7:57pm

It sounds like some of your code is wrong. Try downloading the source code to see if there are any differences in those movement related methods.

And the "Scroll the background" is normal behavior. I included a video of Metal Slug, which incorporates such forward movement.

10/27/2012 6:54pm

Hi James, I solved the problem.
There was a pair of misplaced braces.
FIrstly, I replaced all my code by yours, in Robot.java.
I run it and jumping and moving worked.
I compared the two files, side by side in Notepad++ and noticed the misplaced braces.
Now Its working fine, jumping works and I cant go off the screen.
Thank you for your hard work.

abhyudaya srinet

10/31/2012 4:18am

i am not getting the character on my applet. i downloaded your file and cross checked everything but i cannot find anything.
your robot class had imported graphics which i had not so i did that but it shows a warning that it is an unused import plus i have a few warnings in my startingclass too. i have mailed you the two classes i made. please tell me the error.

love you work.. thank you for teaching everyone all of this so clearly and briefly.

10/31/2012 10:55am

there two draw images.. one in update and the other in paint..
i get that paint will make the character at the coordinates by the arguments.. but how do the link together??

10/31/2012 10:59am

The statements in the update method simply create the double buffering effect that I briefly discussed, and the statements inside the paint method are what you are drawing literally to the screen (i.e. the robot).

They aren't technically "linked." They are called sequentially and automatically by the fact that these are runnable java applets.

DeathRage

11/04/2012 12:18pm

thanks alot for these great tutorials
please just add a summery at the end for long lessons like this one
and thanks again!

André

11/18/2012 9:32am

I have a problem ... When I press the right arrow key the robot moves to the right, but when I press the left he doesn't move. If I change the value of speedX to a positive value he already moves with the left key. What is the problem?

Reece

11/19/2012 1:12am

I'm really impressed and happy with the progress made today. thank you so much. I agree with previous comments its so nice to get straight to it not after hundreds of textbook pages.
Thanks heaps!

zahire

05/11/2013 7:01am

Woke up till 1 am? no wonder u are impressed and happy.

11/21/2012 5:40pm

Hi, setfocusable(true) doesnt seem to set the focus onto the applet. This is on a Mac. Any ideas?

Bogdan

11/22/2012 1:14am

Hi,
Your tutorial is really awesome!

I'd just like to point out something that I believe to be an inconsistence in the code:
// Updates Y Position
centerY += speedY;
if (centerY + speedY >= 382) {
centerY = 382;
}
Your intent is to limit centerY to 382, but you're doing it the wrong way. Let's say that centerY=370 and speedY=6. Is this case you add 6 and obtaint centerY=376, but then you check if 376+6>=382 which is true, and thus you instantly teleport the robot to 382.
The right way to do what you want it to simply add speedY and then check only if (centerY>=382), no need to add speedY again.
So correct code is:
// Updates Y Position
centerY += speedY;
if (centerY >= 382) {
centerY = 382;
}

or even:
// Updates Y Position
centerY += speedY;
centerY -= centerY-382; //deduct any extra

Hope this helps somehow. Thanks!

11/22/2012 1:17am

I've just noticed that my very last piece of code only works if you got over 382, so ignore it. Only my first solution is correct.

Thanks

11/22/2012 1:27am

Thanks for your comment!
Actually my intent is to prevent the centerY from going over 382 in the first place. Your solution corrects the position after it passes 382, which is fine until we incorporate deltaTime. Then the character will be jumping more significantly than it does with my current approach.

I do agree that my current approach is oversimplification that would cause the update to happen a tiny fraction of a second early, but for our purposes (we aren't developing a AAA title), it will work :)

11/22/2012 1:35am

if (centerY + speedY >= 382) {
centerY = 382;
}else{
centerY += speedY;
}

This probably works best. Changed the guide :) Thanks!

11/22/2012 3:09am

That makes sense. Thanks!

Eric

12/04/2012 8:43pm

TO ALL THOSE WHO ARENT GETTING THE IMAGE STILL!

the getDocumentBase method, was only getting me to the bin folder. So when i got the image i had to include the path from the bin folder.
character = getImage(base, "kiloboltgame/data/character.png");
I am on a OSX 10.8

Boris

12/20/2012 7:17am

Thanks Eric! That did it.

01/24/2013 8:41am

You also can put the data folder to the bin folder. (hope this wont get any side effects later on, when we put such an applet on a phone)
getDocumentBase() and getCodeBase() got me to the bin folder as well on:
ubuntu 12.04 - java 1.6
ubuntu 12.04 - java 1.7
windows 7 - java 1.7
(didn't try any other OS-Java combination)

12/22/2012 11:13pm

Copy/pasting in: Private Robot robot;
I get an error.

Tooltip suggests I create a Class.

But if I change to: private Robot robot; (lower p private)
It works fine.

Nice site!

12/23/2012 12:04am

Fixed. Thank you!

Eugene

12/30/2012 7:45am

Hi James!
At first, I would like to thank you for this great tutorial! :)
I've noticed a little issue, when I tested the game.
If I press left and then without a pause press right, I've noticed some lag in character's movement. It's a quite annoying. How we can fix it?
Thanks!

John

01/10/2013 2:29pm

I had the same problem and the way I fixed it was by first locating the keyReleased method and under case VK_RIGHT and VK_LEFT I removed the call to robot.stop();. Hope that helps.

01/11/2013 7:03am

Nevermind incorrect. Sorry

Humoud

01/01/2013 2:02am

Thnx man! i Really Really appreciate it. You have no idea how much this is helping me.

Anish

01/03/2013 8:51pm

I have the same code as you in the Starting and Robot class but the image doesn't appear on the window when I run the program. What can be the problem

01/03/2013 9:20pm

never mind, I had the picture saved in the wrong folder.

Chris B

01/08/2013 2:13pm

Hi James - firstly, great tutorial, thanks!
Secondly, I notice that the update() and paint() methods that are overridden in the StartingClass do not call .super(). The javadoc suggests that you have to do this to make sure the container gets painted/updated properly. Why do we not call them? I did try calling them, but it just made the robot image flicker (so I am sure you have a good reason for not calling them!!)

Cheers,
Chris

01/08/2013 2:15pm

Oh there is no reason. It just didn't seem necessary!

No need to worry about fixing what ain't broke ;)

Bruno

01/12/2013 6:14pm

For those of the people that aren't getting the robot image, make sure you are creating the data folder in src and not putting data inside the kiloboltgame package this is what happened to me haha

User

10/01/2013 6:26pm

After reading through a lot of comments, finally a solution

Sandeep Dhaga

01/17/2013 4:22am

Really amazing tutorials and the explanations are also crisp.
Thanks! My robot is moving left & right and jumping too!

Josh

01/18/2013 8:01pm

If your robot image still isn't showing up after the suggestions above, try right clicking the data folder in eclipse and hit "Refresh", this worked for me

Povilas

01/19/2013 5:52am

I was following this tutorial nad when i run the application i get the following error: Exception in thread "Thread-3" java.lang.NullPointerException
at kiloboltgame.StartingClass.run(StartingClass.java:58)
at java.lang.Thread.run(Unknown Source)

Any ideas about that?

01/19/2013 5:58am

Also the robot sprite is not appearing until i change the size of the window

01/20/2013 4:37pm

Check your StartingClass with the example above.

01/21/2013 6:50am

The fun thing is that my code is identical to the code in your project, yet when i run my code nothing shows up until i change the size of the window (good thing the error is now gone), but when i run your code it works perfectly

01/21/2013 6:57am

Okay i found the problem - i missplaced one brace. Anyway thanks for the awesome tutorial

Daniel

02/10/2013 3:59am

I had the same problem, in my case it was related to the start method in starting class.
what i had :
@Override
public void start() {

Thread thread = new Thread(this);
thread.start();
robot = new Robot();

}

what it needs to be :
@Override
public void start() {
robot = new Robot();
Thread thread = new Thread(this);
thread.start();

}

Pipkin link

02/12/2013 7:13am

That was exactly my error. Thanks for helping!

Troy

03/28/2013 2:46pm

Thank you so much! His tutorial says put it below that >_o. That should be updated.

Paula

10/10/2013 5:53am

Thanks a lot! I couldn't figure it out!

Jesus

03/09/2014 11:08pm

Thank you, I had the same error. I didnt know what was wrong with my code

Gman

01/25/2013 4:05pm

Ok, was the most frustrating thing in the entire world!!!!! My robot image would not show up either, and i was raging super hard when none of the fixes worked. I finally said screw it, and downloaded the source from the tutorial. Upon inspection I noticed his data folder was INSIDE his "src" folder. The whole time i had been creating new source folders, that would not go into the src folder! You do not! i repeat DO NOT want a SOURCE FOLDER! You want just a plain old normal folder. The reason this is so deceiving is because when you right click, it does not give you the option to create a normal folder. Simply just click other, then search for folder. Select the normal folder not source, and create it. Then drag and drop the image in! Note:the folder the character.png goes into, is a white box, not a brown or yellow one. Other than that extreme frustration i'm loving these tuts!

Gnome

07/05/2013 7:32am

This totally worked to fix it for me. Thanks a ton

01/29/2013 8:34am

The image problem is basically making sure your source file is were it belongs.
It needs to be placed within the bin, into the the file (Whatever name you picked). there you will find the code headers you created to date.
So if you used (Kilobolt), that would be the file that resides in the bin.
Go into the Kilobolt file and place the data file, that simple.
Great tutorials, defiantly will contribute to the site.

Martin link

02/10/2013 11:59am

Hey thank you so much for the lessons... a great help to a n00b such as myself :-)

I only have one issue from here... working fine as of the end of this lesson, however when moving left or jumping (or moving in general) although the Q-Bot is moving fine... there is an extra pair of legs in the original position that remain... (the moving Q-Bot is not missing his legs...)

Have run the clean thingy and double checked all code... can't quite see what i've done wrong... any ideas?

Scott

02/18/2014 4:03pm

I am having a similar issue. First, when the window opens, it does not fully open to show the whole robot. As I make the window big enough to see the whole robot, when he jumps one pair of legs stay at the original position, but when he jumps he is whole. Finally, when I move him left or right, the top part of him moves. It is as if the lower section of him is behind the background. I have gone through my code line by line with the sample code and cannot find a difference. I hesitate to import the correct code as then I don't find my own error. Any help would be great. Thank you.

IvanL

02/11/2013 4:36pm

This day 4 is a lot harder compared with the previous, at least for me since I'm totally new to programming :(. I can't understand a lot of things, hope they are somewhat explained later. How the program can work without a main method? What are the "g" of Graphics and the "e" of Keyevents? What's the mean of "this"? How to know when is better to use "public" or "private"? And what's the point of use "private" if you are going to need getters later?

02/14/2013 11:32am

Ivan,

"g" is just a Graphics object and "e" is just the variable that we can use to refer to as the current key press.

Graphics g appears when the update/paint methods are called. These are called by the applet (automatically), which passes in a Graphics object by default. We just call this object g and use it to draw to the screen.

"this" usually refers to the current class or the object that calls a method.

If you were to say:
g.drawImage();

"this" would refer to "g" inside the drawImage() method.

The reason we use private is to ensure that we do not have any methods mistakenly changing the values of the private variables. It's just a safety line that prevents us from doing damage to our game.

I hope that clears things up!

Jafar

08/08/2013 8:21am

Could you please elaborate about the image and second variables.
I just can't really understand how the images are drawn on the screen and when.

Thank you for the great tutorial!

newbie

02/12/2013 10:12pm

Hi I just want to know why you call setColor() twice when you do the double buffering. I tried commenting out the first setColor() and the output is the same.

02/14/2013 11:24am

Depending on the situation, commenting that first one out may or may not cause issues. This is a universal solution.

Tumble

02/14/2013 2:54pm

Thanks for a great tutorial, im halfway through day 4.
Ive added getters and setters in the robot class but im still getting errors on the gets in the startingClass. Ive got to the bit where you output it to the applet, all works ok. will the errors go as i progress or have i missed something?

Thanks again

Tom

02/14/2013 2:57pm

all sorted after a second clean. what a tool! cant wait to crack on with this tomorrow!

Weathers

02/22/2013 3:04am

Hey James thanks so much for the tutorial! Hey i'm having a little of a problem when i run the program the applet thats displayed is resizable and when it loads i only see half of the robot i have to resize the window to see the whole robot. Any ideas why this is?

02/22/2013 6:15am

Not sure what is happening. Email me your code at jamescho7@kilobolt.com.

ken

02/25/2013 10:32am

hello
thank u for your effort
i have question
in this code
if (jumped == true) {
speedY += 1; //While the character is in the air, add 1 to his speedY.
//NOTE: This will bring the character downwards!

if (centerY + speedY >= 382) {
centerY = 382;
speedY = 0;
jumped = false;
}

}

this code if (jumped == true)
speedY += 1;

i think we have to use while loop instead of if
so we can make this statement (centerY + speedY >= 382) true

so could u plz explain why we use if not while loop
thanks

rtt

02/25/2013 10:37am

hi
speedY = 1;
why it is not speedY = 0;
thank you

02/27/2013 10:16am

A positive value for speedY is the speed at which your robot is falling. If you set it at a negative, it will float up at that speed. With that said, change the value from 1 to 0 and jump a bunch. Now change it back. You will notice the speed difference.

04/03/2014 4:04am

Matt

02/26/2013 5:53am

Hi, Java newbie here,
I have tried a bunch of different fixes and looked through all of the comments, but i cannot for the life of me get my .png image to show up in the applet. The applet itself loads, and also post message in the console(like "move up" when you press up etc.), but the image does not show up. If anyone could help it would be greatly appreciated!
-Matt

02/27/2013 8:12am

Matt,

Export and send me your project (jamescho7@kilobolt.com). I will look into it for you.

sean

02/26/2013 4:26pm

So I was having a nightmare of a time figuring out why my robot would not move; the key presses was being reported in console but the robot would simply not move. I commented the entire file out and pasted yours in and it worked so I knew it was doing on my part.

I found the culprit:

public void start() {

Thread thread = new Thread(this);
thread.start();

robot = new Robot();

}

Can you explain to me why robot = new Robot(); has to be above the thread creation and not below it? I followed your directions and it somehow had me placing it where it was below and if below it seems the robot will not move; no matter how many times I sigh at it and ask it to kindly move :)

On a side note, you show, during the getters and setters portion a total of 14 but when I ran it, only 10 displayed. Did I miss something or has the code changed since you took the screenshot? Also, can you elaborate more on why and how getters and setters work? Why not simply set something as public if you are going to be passing information from class to class? And does using Eclipse to auto-generate these detract from us understanding how and why they work?

Lots of beginner questions, I know...but it's better to understand how things work rather than simply assuming they will always function as intended. :)

Djiango

03/11/2013 4:35am

Thank you so much. My robot wouldn't move like yours, but your advice seemed to work!

Bruce Lindman

02/11/2014 8:13am

Ah! Thank you for finding that error, I had the same one, and I can explain it simply. Once the thread starts, the program focuses only on what is happening inside the thread, not leaving it until the thread ends. If the robot is defined after the thread starts, the program never reaches it's definition (until after the game thread has completely ended), leaving it as a null variable.

jose

02/26/2013 10:36pm

help me!! I am not getting
@Override
public void update(Graphics g)

or public void paint(Graphics g)

tom link

03/06/2013 4:15pm

I had all kinds of trouble with getting the robot image to appear, but fixed it after a lot of frustration. Since this is such a common topic I would do something to clarify the tutorial in case people don't read through the comments - it is such a great tutorial you don't want people giving it up because of this.

In my case I had created the folder and moved the image into it entirely outside of eclipse -- ultimately I changed the code to:

try {
// base = getDocumentBase();
base = getCodeBase();
} catch (Exception e) {
// TODO: handle exception
}

// Image Setups
System.out.println("Base: " + base );

character = getImage(base, "../src/data/character.png");

Will

03/07/2013 9:48am

I did this but the Robot only appears (flickers) as I resize the window.

I can't see the robot when not actively resizing the window.

I have tried both JavaSE-1.6 and 1.7, I've double and triple checked my code, every thing looks as it should.

03/07/2013 11:25am

Turns out I had missed "paint(second);" in the update(graphics g) method.

Kieran

03/12/2013 6:23pm

I did everything you said but still get errors on getCenterX and Y. I started over and did it again, but still the same errors.

Erv

03/27/2013 11:24pm

Same thing happened to me, I just wanna solve it already so I can move on lol

Bob Dole

06/05/2013 10:10pm

I had the same problem. I just renamed centerX and centerY to CenterX/CenterY and it resolved it. I'm guessing that was the problem. Is it required that we capitalize the second word in names like that? As in getCenterX as opposed to getcenterX

06/06/2013 4:22pm

The convention is to use camelCase but it is not a requirement.

06/06/2013 7:13pm

Thanks, was never sure. I think my changing centerX to CenterX wasn't legal. I don't know why it was giving me those errors but whatever autocorrect eclipse suggested when you mouse over it fixed it, though it didn't look like it changed any text.

Sorin

03/17/2013 4:41am

Hey, I made all you said in the tutorial, but sometimes it works, and sometimes not. It gives me this error :
Exception in thread "Thread-3" java.lang.NullPointerException
at busdriver.StartingClass.run(StartingClass.java:59)
at java.lang.Thread.run(Unknown Source)

Can someone please help me?

aliaksei

04/03/2013 11:26am

Hi amazing tutorials, thanks for spending your time on these;

I get an error on the line:
robot = new Robot();
It says "Unhandled exception type AWTException"
It suggests surrounding it with a try/catch, I have declared "private Robot robot", and tried cleaning the project multiple times

Gary

04/06/2013 10:38am

Thank you so much for this, it is excellent work. I intend to go through this with my 10 year old son so he can learn programming (i dont hink 10 is too young) and then use his imagination in the future to create a game which will make us rich! :-)

Michael

04/13/2013 10:04am

So i read and write down the code as you go and everything. i actually am learning and thank you for that. But whenever i click the run it will just spam my console with "Do not scroll the background." after that i just decided to copy in your code and the same thing happened. I dont know why it does this. I am on a mac, if your wondering what computer I use.

James

04/21/2013 7:14pm

Mike, if I remember correctly, that is the right behavior.

Shane

04/19/2013 10:27am

I could not for the life of me figure out how to get the png's to appear using your method. I put the images in every possible folder. I ended up loading the images using a different approach.

ex: background = new ImageIcon("src/data/background.png").getImage();

Is that method any worse to use that the one you had?

It should be fine. I am sorry it did not work for you (and many others). I will revisit this section in a few weeks and make sure I have multiple ways outlined.

04/22/2013 5:36pm

No ya i figured it out eventually, dont know what was wrong but i just reinput the source code and it just happened to work, will reread this section though.

nelzkie

04/28/2013 1:24am

I am using Intellij and i cant make the image to show? i tried to make it ../data/character.png still not working.. black screen

Lamar

08/15/2013 10:01pm

I'm using Intellij also. you haven't found a solution I found that, for me, Intellij reads from the /tmp directory. So I just moved my data folder there and it all worked. I don't know how to tell it to read and write to the projects directory.

Richard Burns

04/28/2013 8:08am

Would the code learned in this tutorial series help me if I was to make a game with AndEngine?

Jerzku

04/28/2013 10:06am

Hey. Just wondering will you be telling us how to use small standby animation or does it work just by importing .gif file? Probaply would found tutorials for that but though of asking you first :) Great tutorials btw.

Priya

05/08/2013 10:17am

Hi James the site is really wonderful & very helpful.
I am facing problem in loading images I have read previous discussions but still am facing same issue. can u help me in sorting out this problem plz?

Supriya

05/10/2013 11:19pm

In my case image worked when i placed image in folder "bin/data" as "bin/data/character.png"

Esteban

05/09/2013 1:30pm

Hi,
im having trouble with my Getter and setter.. I do as the tutorial says but i get a message saying "the opreation is not applicable to the current selection. Select a fiels which is not
declared as type variable or a type that declaes such fields." Any ideas??

sameer

05/15/2013 11:44pm

HI,
// Moves Character or Scrolls Background accordingly.
if (speedX < 0) {
centerX += speedX;
} else if (speedX == 0) {
System.out.println("Do not scroll the background.");

} else {
if (centerX <= 150) {
centerX += speedX;
} else {
System.out.println("Scroll Background Here");
}
}

// Updates Y Position

if (centerY + speedY >= 382) {
centerY = 382;
}else{
centerY += speedY;
}

// Handles Jumping
if (jumped == true) {
speedY += 1;

if (centerY + speedY >= 382) {
centerY = 382;
speedY = 0;
jumped = false;
}

}
Could u explain these codes in a simple way pls, why we are checking SpeedX<0 and centerX<=150

05/17/2013 2:35am

No probs i understood

javad

05/19/2013 1:07am

Thanks a lot!! Very useful!
I will read all of your tutorials...

shree

05/21/2013 11:01pm

there is a mistake in the code
// Updates Y Position

if (centerY + speedY >= 382) {
centerY = 382;
}else{
centerY += speedY; // it does'nt work here..robot does'nt move
}

however it is correct in the code you have given to download
// Updates Y Position
centerY += speedY;
if (centerY + speedY >= 382) {
centerY = 382;
}
However the tutorial is amazing. Very good job.. learnt a lot from it.Thanks.

05/21/2013 11:31pm

Kevin

05/23/2013 12:21pm

I'm not getting any errors, however the image is not being display. The character.png file is in the workspace\KiloboltGame\src\data folder. Neither of these statements work.

character = getImage(base, "data/character.png");
character = getImage(base, "../data/character.png");

Thank you.

05/23/2013 12:50pm

Fixed.

Bartlee Anderson

06/02/2013 4:47pm

For NetBeans, you need to prefix the path with classes, i.e.

in init
character = getImage(base, "classes/data/character.png");

Justin T.

06/16/2013 11:00pm

Oh man thanks so much for this

Louis N

06/08/2013 1:55am

As per your suggestion in Lesson 8, I started reading the code through in the order it would execute. Something from this lesson has me confused though, so I figured I'd make my comment here.

In the Robot class, we declare and initialize speedY to 1. Later, in the // Handles Jumping section, after checking if the robot is done jumping, we set speedY to 0. Concerned about the inconsistency between the speedY setting, I tried changing the initialization of speedY to a much higher number, as well as trying to set the setting of speedY in the // Handles Jumping section to a much higher number. I found that no matter what I set them to, the robot's jumps aren't affected in any way (as long as I don't set them to a negative value, in which case the robot starts flying).

So that begs the question, why do we set speedY to 0 at one point, and to 1 at another. I can see that it doesn't matter what we set it to as long as it's not a negative value, but shouldn't we keep the number consistent?

Akshay

06/19/2013 10:45am

Only a black window appears with Q bot alpha written on the title bar.. png is not loading.. pls help..

Ilias

06/24/2013 4:38am

I still cant load the image? I tried everything what people said in the comments but it still won't work? Help please!

Ian

06/24/2013 5:09pm

James, just wanted to say this is a great tutorial and I am really enjoying getting to grips with game programming. Had a little bit of trouble with the png as some have also had but I solved that. Can't wait to carry on with Unit 2!

07/03/2013 11:29am

There are 2 data folders in my file system. The images seem to need to be in bin\data. Whenever I clean up the project it seems to remove the PNGs from this data folder not the other. I'm not sure why this is but I am now avoiding the clean up. If you are having display problems check your bin\data folder and make sure all the images are there.

Tyler

06/28/2013 3:11am

My character.png does not appear when running the program. I have tried every suggestion in this thread, but to no avail. At this point, I'm assuming it's an IDE issue. I am using IntelliJ. You wouldn't happen to have an idea as to what would cause this would you?

08/15/2013 9:38pm

I'm having the same problems in Intellij. Has anyone been able to solve this yet? my getCodeBase and getDocumentBase always points my /tmp directory.

08/15/2013 10:02pm

I solved it after writing the last reply. I found that, for me, Intellij reads from the /tmp directory. So I just moved my data folder there and it all worked. I don't know how to tell it to read and write to the projects directory.

08/26/2013 8:58pm

I tried everything I could but I couldn't get anything to work, even after downloading the source code. I noticed that the path in the source code was /data/character.png". Make sure that the path does NOT have the leading slash. Instead, it should be: "data/character.png". d

Andy

02/11/2014 10:37pm

I got the image to appear by changing the path to "../src/data/character.png"

Juan

07/03/2013 2:00am

I've been following this tutorial with NetBeans. I'm currently stuck here since I haven't been able to make the Robot appear. I've tried using the downloadable code but it still doesn't appear. I've tried adding classes in the string url but it still doesn't appear. Please help!

Colby

07/03/2013 10:34am

I had a similar issue. All I did was go to the Package Explorer and refreshed the KiloBoltGame package. The Robot started to appear. Hope this helps!

07/03/2013 9:59pm

I think its called something else. Package Explorer is for Eclipse. But no, it also doesn't work...

08/26/2013 8:57pm

Malcolm

07/07/2013 5:16pm

Absolutely love the tutorials! At first I couldnt get the little man to be displayed but then I saw that the png needs to be in the bin directory not the src. Keep up the awesome work!

Ryan

07/09/2013 8:18am

I've had loads of problems with the character loading and i've figured out why. It seems like the update method in the robot class that was originally posted in the beginning is different then what's in the source so go back and check that out and fix it then your robot should load up.

07/09/2013 8:20am

never mind guess it was me that just miss typed sorry about the inconvenience. Great tutorials :)

Mitz

07/09/2013 6:15pm

Hi!
I'm currently working with an Android game. Luckily I saw this tutorials and they're great! I've been following this for some weeks now.

I have one error, the java.awt.graphics . It says that "the java.awt.graphics cannot be resolved". Do you have any idea on how to fix this?

Thank you. Keep up your work!

ozgur

07/25/2013 4:40am

if you get error th show character.png change this code lines like this.
try {
base=this.getClass() .getResource("/data/character.png");
}
catch (Exception e)
{
//TODO
}

character =getImage(base);

Nanashi

07/26/2013 12:53am

When pressed jump, the character's centerX becomes 382 + (-15) which is 367.
Shouldn't the character appear in the peak of its jump when jumped?
How are we getting this smooth motion when jumping up?

Richard

04/02/2014 12:13pm

I've been scratching my head all day wondering the same thing!

Ricahrd

04/02/2014 12:25pm

I figured it out as soon as I wrote this. It's because -15 is the vertical speed of his jump NOT his max jump height (what I initially though). So every update he speed is getting adjusted by +1 (i.e. -15, -14, -13, ... -2, -1, 0, 1, 2, ... 13, 14, 15) until he hits ground then jump is turned set to false.

Jonathan

07/27/2013 1:32pm

For people that cant get the robot to appear, check under your data folder and see what the character image is actually named. according to the source code it should be character.png but mine was named character[1].png so I had to change the code to "data/character[1].png
Also I need help, my robot will not move very far to the right, its like he only has access to a 1/4 of the screen. copied the source code and same problem?

07/27/2013 5:24pm

Disregard the robot not moving to right problem, everything is working as it should. Thanks a ton for the Tutorial!

Mike link

07/31/2013 10:06pm

Why dont we put "don't scroll the background" when centerX <= 150 (that is, when the robot goes left) and we only don't scroll the background only when speed = 0 ?

Considering the metal slug video above, whenever he stops the background doesnt scroll, but when the character goes left the background doesnt scroll too right?

I find it's a bit inconsistent

Eshrath

08/01/2013 11:37pm

Hi...!!
i tired your tutorial it showing following error in console..
Exception in thread "Thread-4" java.lang.NullPointerException
at sifu.eshrath.StartingClass.run(StartingClass.java:59)
at java.lang.Thread.run(Thread.java:619)

so please help me..
As per my knowledge i think update method is not working...

Tom link

08/11/2013 12:11am

Let's clean up that update() method. It's entirely too confusing.

//moves the character.
public void update() {
// Moves Character.
centerX += speedX;
centerY += speedY;

//prevents going beyond x coordinate of 150.
if (centerX > 150) {
centerX = 150;
/*
* Code to scroll background here. The Robot should NOT be scrolling the background!
* The Robot and the Background should not need to know anything about one another to
* perform their functions. I suggest a GameManager Object to handle the movement of
* the Robot and the Background in relation to one another.
*/
System.out.println("Scrolling the backgound.");
} else {
//do not scroll the background.
System.out.println("Not scrolling the background.");
}

// Prevents going beyond X coordinate of 0
if (centerX <= 10) {
centerX = 10;
}

// Updates Y Position
if (centerY > 352) {
centerY = 352;
speedY = 0;
jumped = false;
}

// Handles Jumping
if (jumped == true) {
speedY += 1;
}
}

pranaykarani

04/03/2014 3:55am

daniel

08/11/2013 1:44pm

How do I get the robot move slower? even slower than if I put movespeed=1

04/03/2014 3:52am

thats easy, declare SpeedX as float and put...
float SpeedX = 0.00001;

Peter

08/16/2013 2:26pm

I have a question, sorry if i sound noob on asking it lol. Thought the code works perfectly (thank you a lot, just came in this site for the first time and i must say that i love it congratulations excelent tutorials), i have a doubt. When you click the space bar, you "call" the jump method inside the Robot.java, wich adds 15 to the speedY, and sets jumped to true. Then we have the part that handles virtual gravity, which when jumped is true, subtracts 15 to the speedY. My question is, as the jumped variable is setted to true as soon as the speedY is changed, and that one should be reseted once the jumped variable sets to true, why is it that the speedY variable is not reseted as soon as setted, making it almost instantanly go back to the defaul and preventing te robot from updating and going up in the next loop? sorry if needed ill explain myself better. ty

Oblivious

08/21/2013 12:50pm

I don't understand how the robot appears to be jumping slow either.

DarylZero

08/22/2013 2:35am

I had a problem with the image, too.
Tried all the fixes - did not work.
Then I did the following:
base = getCodeBase();
System.out.println(base);
base = getDocumentBase();
System.out.println(base);
After that I realized I copied the file in the /projectname/src/data folder instead of /projectname/bin/data folder :)
changed the file location, everything was peachy.

As for jumping - you might want to try simple physics equation for movement ( starting from s = s0 + v0*t + 0.5*a*t^2; v = v0*t + a*t ).
I did so in a project I am working in as a game designer - a negative acceleration (a) added to jump/fall movement makes it smoother and actually better. If you find that difficult, try any sin/cos function, it does practically the same.

Griffiin Jarmin

01/09/2014 6:47pm

thank you

03/08/2014 1:12am

How did you change the file location? Bin folder is invisible to me.

Erick

09/01/2013 5:10am

Under Linux (Ubuntu 12.04 LTS) I couldn't get this to work as posted. The window would become unresponsive. After some debugging, I traced the problem to the run() method. Commenting out the while loop resulted in the Robot being drawn once, but obviously no animation was happening.

Based on the code here: http://www.javapractices.com/topic/TopicAction.do?Id=153

I changed the moved the loop to a new private class and got it to work.

This is how my code looks like:

public class StartingClass extends Applet implements KeyListener {

.
.
.

public void start() {
robot = new Robot();
Thread thread = new Thread( new Worker());
thread.start();
}

private final class Worker implements Runnable {

@Override
public void run() {
while (true) {
repaint();
robot.update();
try {
Thread.sleep(17);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

}

Oli

09/08/2013 11:58am

No matter what I do I can't get the robot to move. When I press up and down, it appears in the console but when I press left and right, the robot doesn't move and nothing about the background is displayed in the console as it should. I have checked both classes against the source files and there is no difference :s

Thomas

09/15/2013 6:50am

Just question - Isn't better to have public variables without getters and setters? If there are two methods for each variable, it mess the code pretty much and it does the same as a public variable.

09/19/2013 3:29am

why is it centerx+= speeds

09/19/2013 5:20am

hey its me again could u explain me why we have to put the above code and explanation and my name is really not John i just dont want to reveal my name.

allan

03/06/2014 3:55am

that would be because of
centerX += speedX is just a shorter version of
centerX = centerX + speedX.

centerX is the center of our character. speedX is the speed in which our character moves (6 / -6). in order to actually make our character move, we have to assign new coordinates to centerX. We do that by adding speedX value to centerX.

Saurabh

10/09/2013 10:33pm

I have a question here.
Why are we defining the
================================
Thread thread = new Thread(this);
robot = new Robot();
thread.start();
================================
in start method ?
If we put this same code in init() it still does the purpose ?
I may be wrong here but just want to know why.
Again re-iterating... the tutorials ROCK!!! :-)

Christopher Wolff

10/24/2013 11:31pm

Hello sir,

I think its because we only want to create a new instance of the robot when our game thread starts. That's why it is in the start() method. If we put new Robot() in our init() method the robot will be created before the game starts.

Dushyant

10/10/2013 10:48am

First of all, great tutorials.
I am facing a problem with the movement of the robot character.When I press left or right,only a part of the image moves,the rest of the image part remains still and then when i resize the applet window the remaining part joins the moving part.Please help!
Thanks in advance.

GMan

10/10/2013 10:44pm

Awesome tutorials, man, they're really well done!

A concern I have after this less, however:

I'm a total newbie to programming, just started to try and learn Java maybe a week and a half ago.

In Unit 1 I was able to follow along really well, fully understand what was going, and even take the stuff learned in each lesson and make my own little programs to test my understanding etc.

But after this lesson I just feel lost.. like I can't even comprehend ever being able to take the stuff done today and apply it independently to a program of own. I feel like I was splashing in the kiddy pool and now i've tried to jump in the deep end of the pool haha.

I mean, if I really look at each piece and put my mind to it I understand what's going on basically, but in terms of following it through the big-picture and all the nuances you seem to have to know to get everything to work, it seems so complicated.

I guess what I'm wondering is, after going through Unit 1 and the first few lessons of this Unit should I be able to understand this with ease, or do I need to come back when I have more experience with the fundamentals?

Thanks again for all your hard work!

ginsengjin

02/28/2014 10:40am

I have this exact problem. I understand the code in parts but I don't understand how everything connects to one another as a whole.

sumit

10/24/2013 1:00am

how to create our own robot...???

04/03/2014 3:45am

what do you mean by "our own robot"?
do you mean your own robot image or YourOwnRobot class with YourOwnRobot_speedX , YourOwnRobot_speedY..etc.
plese be clear..

04/03/2014 3:46am

10/24/2013 1:02am

plz help guys.....

10/24/2013 11:28pm

Hey everyone,

Not sure if this has been answered already, but isn't our window already automatically double buffered with the latest version of Java? If so then we wouldn't need an update() method for our Applet correct? Or has automatic double buffering only been added for JFrame?

10/24/2013 11:34pm

Also, is there a reason that we use Applet rather than JApplet?

John Ellis

10/28/2013 1:37am

Don't know if my question will ever be answered but I've got a strange problem. Input seems to read in but my robot will not move at all UNTIL I use the applets drop down menu and hit restart. After the applet restarts he controls just fine. Any idea why that might be?

02/11/2014 8:08am

Great tutorial, but I am also getting this issue. I did replace the robot class with my own class called biker, and the robot image with an image of a bike, but the only changes are to the names, and I made sure they were consistent.

I'm also getting a null pointer exception at the biker.update(); method when the game runs. It does not seem to throw the error on restart though.

Michael J

11/10/2013 1:19am

Great series thus far! I have alot of experience with GML and use a very similar syntax to Java/c# as I code with it. Moving to Java seems to be a breeze since I have experience in c# as well.

However, I have hit a snag. My code looks exactly like yours but I am getting an error before I can even launch the game:

public void moveRight()
public vod moveLeft()
public void stop()

All of those are giving me syntax errors on void, expected @ token.

public void () is throwing EnumBody errors.

Any ideas?

copy/pasted from my IDE
public void moveRight()
{
speedX = 6;
}

public void moveLeft()
{
speedX = -6;
}

public void stop()
{
speedX = 0;
}

public void jump()
{
if(jumped == false)
{
speedY = -15;
jumped = true;
}
}

Forgive my C#/C++ way of formatting. I cannot get used to the javascript/Java way of formatting.

pradeep

11/13/2013 7:12am

When i press space
Please explain why "y-axis" decreases 10 , decreases 9, decreases 8 and so on.....

Claudiu

12/01/2013 3:41pm

I want to know this too. if it goes up he will not suposed to be pozitiv value?

minh

11/30/2013 3:14am

hi guys,

I tried to run the program by creating a new class with main().
public class Main {

public static void main(String[] args) {
System.out.println("******************");
System.out.println("Q-bot starting....");
StartingClass ss = new StartingClass();
ss.init();
}
}

I got a null pointer exception inside StartingClass right at:
Frame frame = (Frame) this.getParent().getParent();

Any idea how it happened? Thanks in advance

Albert

12/05/2013 8:13pm

When I tried to run my project in the applet it says <project root>[in Game] does not exist
Game is my java project if anyone could help me it would be great i am new to programing

gaebolgone

12/06/2013 12:35am

Thanks so much fo this GREAT TUTORIAL !!!
So far, i can understand and implement it (i thinks because i have a little exp. on java :p )
i want to ask,
how to insert image in every moving method???
i mean, i want to insert image a character face right when he move right, face left when he move left, jump pose image when he jump.
can anyone tell me how to do it ??
(sorry if the aswer of my question actually have in this website somewhere, but I am very curious! )

04/03/2014 3:37am

How I did this:
first you should have 3 images in your assets(excluding jumping)...
1. default.png(when your character is not moving or jumping)
2. movinright.png
3. movingleft.png

----------FOR MOVING RIGHT IMAGE----
1. go under "case keyEvent.VK_RIGHT:" in method "public void keyPressed".
add-" robot = getImage(base,"data/movingright.png"); " above "robot.moveright()".
2. add "robot = getImage(base,"data/default.png");" above robot.stopright() in method "public void keyReleased"

same way you do this with moving left and jumping..
Hope it helps

tanmay

12/15/2013 8:48am

my character is flickering from the start of the applet. Should it do so?

12/15/2013 8:12pm

Got it in the startingclass remive the super methods from paint and update classes

12/28/2013 2:26pm

had the same problem of image.
When i prints base value by

System.out.println(base.toString());

it threw

file:/C:/Users/<username>/workspace/KiloboltGame/bin/kiloboltgame.StartingClass1388269291513.html

so it had to go in bin>data not the src>data.

though even being in src>data, code should be changed to "../src/data/character.png"

it worked this way also.

Ethan

01/03/2014 12:34pm

I've followed your code very closely, and i didnt have the robot painted at first, so i was a little alarmed. after fooling around a bit i got half of it to load? and at the top left of the emulator. so I movd his coordinates to the bottom left, going off code a little, but still only half the robot was shown. I've reviewed your code many times and there is nothing really flawed within mine but if i set the coordinates to 0, 0, hes just hanging around in the top corner. any insight would be greatly appreciated.

01/04/2014 3:52pm

So i tried adding a new picture instead of the character.png, to see if it was just a corruption in the image, but it isn't. When i move the image coordinates to the bottom left, 100, 382, only half the image shows up, and if i g to Applet>restart, the image shows up as half, and another image shows up in the top left corner. this also happens if i resize the window. getting frustrated, as i have been working and reviewing this for a couple of days now. any help, still greatly appreciated

BoyKot link

01/25/2014 8:25am

For those who have problem with PNG loading in SDK 1.7 here is my solution:
change
base = getDocumentBase();
to
base = getClass().getClassLoader().getResource("gameone");

01/25/2014 8:30am

Oops, sorry, I've meant your main package name instead of "gameone" ("gameone" is my package name).

So
base = getClass().getClassLoader().getResource("kiloboltgame");
or whatever you've named it.

Vicky

01/30/2014 2:08am

Hello James, thank you for the great tutorial. I do have a small issue. When my robot jump, it goes off from the top of the screen and never comes back to the ground. Is it normal?

03/06/2014 4:06am

Are you sure you got jumped = true; in Robot.java?
============================
public void jump() {
if (jumped == false) {
speedY = -15;
jumped = true;
}
===================
If that doesn't do the trick inspect your update methods IF of jumping.
===================
if (jumped == true) {
speedY += 1;
if (centerY + speedY >= 382) {
centerY = 382;
speedY = 0;
jumped = false;
}
}
===================

04/03/2014 3:07am

Gajendra link

02/04/2014 11:04am

i have the character.png loading problem.. icant resolve it. im using JavaSe 1.6.. plz help. i wnt to continue as soon as possible.. :(

Neeraj Gupta

02/19/2014 10:38am

Hi James,

First of all thanks for these awesome tutorials.

I just wanted to know how you calculated the value when you move the robot left or right or when jumped( -6,6,-15).

moveRight(), which sets the character's horizontal speed (speedX) as 6.
moveLeft(), which sets the character's speedX as -6.
stop(), which sets the speedX as zero.
jump(), which sets the vertical speed as -15.

Edwin

02/23/2014 8:22am

My problem with character.png not loading was that was missing the line below at the end of the update method:

g.drawImage(image, 0, 0, this);

It is working now, hope that helps to others.

Rick

02/23/2014 2:38pm

Thanks so much for this tutorial. Everything worked perfectly for me. That robot pic is really cool =).

sagar

02/24/2014 6:08am

it is showing me only a blank screen :(

02/24/2014 6:12am

sorry i have forget to change the name of my character :s

Karlis link

02/24/2014 12:15pm

I found the problem i had !! :D with robot img! :D
1.Bad code on win7 ultimate ! 1.7 eclise
character = getImage(base, "data/character.png");

2. Fixed code ! all works now ! :) Now the img is seen!
character = getImage(base, "../src/data/character.png");

Rom

03/04/2014 11:54pm

"The Origin (0,0) pixel is at the TOP LEFT. I will talk about this below. This means that if a character has a positive speedY, he is FALLING, not RISING." -- Isn't this supposed to be positive speedY be RISING? In graph upper y axis is positive and lower is negative? I'm a bit confused....

03/06/2014 4:24am

Normally yes, but in this instance no. We just happen to call upper left corner the point of origin (0, 0) instead of lower left corner (as we would normally do in graphs).

You may be confused by the fact that in normal graphs we can have negative values for X and Y axis, which isn't the case now. And we draw it like this:

^positive y
|
|
negative x ____|_________> positive x
|
|
| negative y

when we talk about coordinates in screen resolution, we don't have negative values. We just number pixels starting from upper left corner. So when coordinate is (100, 382), its pixel number 101th counting from left and 383th counting from top (remember starting pixel is 0).

Why we start it from the upper corner instead of lower beats me, but that's just how it is.

03/06/2014 4:46am

i see that textformatting ruined my awesome graph. So here's another, hope you understand.

http://imgur.com/s4lIXKg

On the left is a GRAPH, on the right is SCREEN.

03/08/2014 5:08pm

Thanks Allan for the help!

Sanjay Kumar link

03/07/2014 5:29am

What does creating class Robot in robot mean?
Plz help i am not understanding that robot object method.....

fantastic tutorila for freeee

Valkata

03/08/2014 6:50pm

I didn't quite get what you're trying to understand. It would be helpful if you could copy and paste a bit of the code that's unclear to you.

If you're referring to "robot = new Robot();", then I can try to help.
Robot class is just the general set of rules, values and methods for all future robots. It's like a building plan for a house.

And if you call "new Robot()", you're actually creating a robot that will have all the values and methods defined in Robot. It's like a house that is built and has everything that is in the plan.

This way you can create actual objects that can be in your game. And you can name them any way you like. For example, we can create 3 robots:

robot = new Robot();
another_robot = new Robot();
little_one = new Robot();

Just remember that you have to declare their types first.
private Robot robot, another_robot, little_one;

03/08/2014 6:30pm

A possible solution to the problem of robot not appearing. I believe this wasn't mentioned before:

I accidentally wrote:
createImage(this.getWidth(), this.getHeight());

Whereas it should be:
image = createImage(this.getWidth(), this.getHeight());

Gede Etika

03/11/2014 4:49pm

wow ! thats work ! thank you for tutorial :)

03/30/2014 7:11pm

How come there's no constructor in the Robot class?

04/03/2014 2:50am

it is not compulsory for a class to have a constructor. Since there will only be one instance(robot) of Robot class, I don't think there is a need for constructor though you can add if you want...

KBFan

04/02/2014 8:56am

Can you further dilute the code please. Like the "rubber ducky" method? Because some of us ARE rubber duckies.

Just one example of the line I couldn't understand: Frame frame = (Frame) this.getParent().getParent();

Things like these.

Thanks for the tutorial. It's the best I found on the net so far.

04/03/2014 3:08am

04/03/2014 12:05pm

It is differentiated in the paint method. The variable SpeedX adds/subtracts the centerX variable and variable SpeedY adds/subtracts the centerY variable. Then these variables are called upon by using getCenterX() and getCenterY() in the paint method. See below:

g.drawImage(img, x coordinate, y coordinate, observer)

@Override
public void paint(Graphics g) {
g.drawImage(character, robot.getCenterX() - 61, robot.getCenterY() - 63, this);

}

04/03/2014 9:14pm

Thank you, but still ..
1) how does java know that speedX is meant for moving robot left-right and speedY is for jumping the robot?
2) how does Java know that centerX is meant for X origin and centerY is meant for Y origin?
these are really confusing me since both are just variables without any connection to the game coordinates.
thanks in advance..

04/04/2014 8:34am

1) You are correct, Java does not know what speedX or speedY does, other than that it is a type int.

2) Again, Java does not know that centerX is meant for X origin and that centerY is meant for Y origin either. The way java connects these variables to the game coordinates is through g.drawImage(img, x coordinate, y coordinate, observer), this method takes 4 parameters: image, what the x coordinate should be, what the y coordinate should be, and an observer. In this robot code example we are making the call with this g.drawImage(character, robot.getCenterX() – 61, robot.getCenterY() – 63, this); therefore, we have told this method that centerX is x coordinate and centerY is y coordinate. The drawImage() method differentiates the x and y coordinates and draws the robot where you want it to be drawn based on x and y coordinates you have given. Hope this makes more sense? If it is still confusing try switching the coordinates in g.drawImage like this:
g.drawImage(character, robot.getCenterY() – 63, robot.getCenterX() – 61, this);
What we have done here is tell drawImage() method that center is our x coordinate and centerX is our y coordinate. This is how java knows where to draw the robot.

04/05/2014 1:10am

thank you once again,
got it neat and clean..

ashish

04/06/2014 3:39am

hey!!!
when i right click in robot.java file to select setters and getters i get an error - "the operation is not applicable to the current location.select a field which is not declared as type variable or a type that declares such fields:
help me with ths

jon el sapoman link

04/11/2014 9:16am

For some strange reason, the data folder would not work no matter what I did. I tried everything for about a day or so and only until I just renamed the folder to data1 and added

character = getImage(base, "data1/character.png");

and that fixed it. I realize I just have to zen out when I do this or I will start having anxiety attacks. Eclipse has a mind of it's own.

JIm

04/20/2014 11:02pm

Can you please help me? :)
I'm done with this day 4 tutorial
What happens is that when i run the project
It displays the character but it is sliding down upon the start of the applet
Like this
Character
::::::::::::::::::::::::::::::::::::::::
: :
: picture :
: | slides :
: | down :
: \/ :
::::::::::::::::::::::::::::::::::::::::
As you can see, upon the start-up, the picture is moving downwards and i'm not clicking anything
And also at the console of java eclipse
it displays: Do not scroll the background, like an infinite loop
I've written the codes in a bond paper because i'm planning to study the whole tutorial further more, please help me so I can move on to the next tutorial :) Thanks

pranay

04/23/2014 12:33pm

Please THOROUGHLY recheck your code. I had similar problem where my robot was appearing on the top-left corner and when right was press is slided down dialgonally. It figured that I had somewhere messed up CenterX with CenterY. So check your code thoroughly.

Adink

04/23/2014 8:03am

Hi!

Great tutorials man, i just have one question

how can you declare variables like Robot robot; and Thread thread;

are "Robot" and "Thread" a java keyword or something? because i didn't see you importing them from another class.

Thanks James

04/23/2014 12:26pm

By your query is seems that the concept of using classes and their instances is unclear to you. Here, author creates a 'Robot' class which contains characteristics and methods that it's instance(here, robot) will have. Then by statement-"Robot robot;" you declare(Remember, you are just declaring robot and not initializing it i.e. you are not stating: "Robot robot = new Robot()" in this statement. It means that computer knows that robot is Robot's object but it still can't use it's methods and properties yet) that, robot is an instance of class Robot and it will(if :robot = new Robot() is stated) have all the properties and methods of Robot class.
For "Thread thread" same thing applies, but instead Thread class comes predeclared somewhere in Java library that you don't have to worry about. All you have to do is create an 'Thread' object(here
'thread') and initialize it by " = new Thread(this);" and then you can use its methods and properties .e.g. here in the next line "thread.start()" your Thread object, 'thread' is using method 'start()' defined in Thread class.

For the import part, since your Robot class is in the same package, it does not need any import statement.

If my explanation goes though on you, then i suggest that you should refer Kilobolt's Java basic lessons thoroughly, especially that part which deals with use of classes and their instances or you can mail me at pranaykarani@outlook.com. Thanks for your time.

04/23/2014 11:16pm

Thanks a lot for your explanation, you made it very easy to understand. I just confuse about that Robot class, i didn't realize that i created it within the same package as StartingClass class lol.

As for Thread class then, have we import that one from java library?

Thanks pranay!

04/24/2014 9:11am

I think since StartingClass is extending Applet we don't need any import statement for Thread. But still, I am not completely sure about it, so it will be better if you cross-check it in Kilobolt's earlier tutorials.

Inc

04/23/2014 10:10am

If you don't see a robot (just a black screen) do:
m_Base = getCodeBase();

Now print it to see what path it contains:
System.out.println(m_Base);

Now get the getImage statement right accordingly.
For me it was:
m_Character = getImage(m_Base, "../src/data/character.png");

The second argument is the path relative to the url in the m_Base.

Two dots (..) mean parent directory. You can go up as high as you need: ../../../../../foo/bar.png.
gl hf 0/

04/25/2014 11:54am

hey! ummm I was not getting the robot image just a black screen and "don't scroll the background" in console(infinite loop) but it was because the .png file was named "robot.png".

so my getImage statement is:
character = getImage(base, "data/robot.png");

I can see the robot now in java applet....so everything will be good in the future???

Adrian

04/24/2014 8:31am

Hi! First of all, great tutorials, you did a very good job!

A, also, had the problem with the image not appearing, but managed to solve it, eventually, thanks to the other fellas.

But now, I have another problem, and it really gets me nuts. After i add the moving metods and I compile the StartingClass, it show me the robot very well, but it doesn't move. I really do not know what's wrong, since i've triple checked the program and i have no errors.

If this happened to anybody else, please let me know how you managed the situation.
Thanks a lot!

04/24/2014 9:05am

See, if you don't see an that does not mean that your code does not have bugs. That's the problem with bugs, it is very hard to locate one in your code. The only solution to them is THOROUGHLY cross-checking your code.
And for your situation, Have you called the movement methods properly in your StartingClass.

04/24/2014 9:39am

I found the problem.
thread.start(); - this line was missing, from when i created the thread. Clearly, it wasn't gonna do anything without that. Now it works very well.

Thanks for you answer, anyway, pranay!

sam

04/29/2014 10:08pm

Thanks for this great tutorial. I'm a small problem and im not able to know why its happening. Note that there are no errors in my code and its exactly the same as the publishd code. The problem is that the code is not "Listening" to my keyboard inputs, i.e. character not moving... all the methods are there, i also tried to replace all motion instances by console outputs, so when i press a key, i expect to see what i wrote on the cobsole, but there is not anything... any clues ??

04/29/2014 11:09pm

Since there is no error in your code that does not mean that your code does not have any bugs. I am sure that there must be bug in your code. I suggest you to THOROUGHLY cross-check your code with author's. It helped me every time. Hope it helps you too..

Sam

04/30/2014 5:35pm

Dear Friend,
Honestly, I'm a very experienced programming, though this is the first time I attack video game programming, for that reason I'm there. I checked the code line by line, then i compared it word by word to the author's code, and it is the same. I read today on the net, and it seems that this is a bug with eclipse. To double check, I recopied the authors code, rewrote mine, and still having the problem.

04/30/2014 10:40pm

Did you try downloading and trying author's code?

Akio

04/30/2014 8:48am

Hi,first of all thanks for these incredible tutorials,i started with zero knowledge and still keeping up,because it's fantastic even for absolute beginners.. my question is: what if i want to modify or change character image,should not be i able to do it? i was curious and changed character image with random same pixel ratio "PNG" file,also changed URL to new image name,still i get no picture in Applet,but when i change back to robot i see it,am i doing something wrong,or are we not able to modify this code and it's data?

04/30/2014 9:03am

Sorry to bother you for no reason,i was able to change image,but as i can see it has to do something with transparency ,could you please give me some info about that...

Sorry to bother you for no reason,i was able to change image,but as i can see it has to do something with transparency ,could you please give me some info about that..

Virendra link

05/01/2014 4:59pm

Is the main method missing because its in the Applet Superclass?

05/03/2014 5:09am

Thanks for reply , i think everything's fine now . I can normally add anything i want with or without transparent background.Thanks for help

Rob

05/05/2014 1:08pm

hey guys im super new to this stuff, I am unable to even put the png file in the data folder, its ment to be just a regular folder not a source folder right

Jake

05/09/2014 3:46pm

FYI, just above Figure 2-18, where it talks about the setters and getters, it has this code:

public void setSpeedX(int speedX) {
this.height = speedX;
}

But the local variable is "speedX", not "height"; the auto-generated code has "this.speedX = speedX". Had me confused for a minute.