Animation in Java

[Dark Rain]

I was asked to make a hockey game by my Java teacher. I can get the puck to draw, but I'm not sure how to move it. Should I use a thread, or a timer?

Also, how can I check if the puck collides with the outside of the application window? I would like to keep it in the window, and when it hits a window boundary to send it off in another direction.

[Wizard1988]

You should store the (X,Y) position of the puck in variables, and use them for drawing, as in g.drawOval(x,y, height, width) then increment them inside the main loop, to check for collision simply write an if statement checking if it is smaller than 0 or bigger than the height of the window, then compare the y position to 0 and the width. if they collide simply multiply the x, y value by -1 this will reverse the puck

I hope this helps

[Dark Rain]

I have these two classes:

Hockey.class

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Hockey extends JApplet implements ActionListener
{
	private final int WIDTH = 400, HEIGHT = 200;
	
	private JPanel surface = new JPanel();
	private HockeyThread thread;
	
	public void init()
	{	
		getContentPane().setLayout(new BorderLayout());
		getContentPane().add("Center",surface);
		getContentPane().setSize(WIDTH,HEIGHT);

	}// init()
	
	public void actionPerformed(ActionEvent e)
	{
		thread = new HockeyThread(surface);
		Thread hockeyThread = new Thread(thread);
		hockeyThread.start();
	}// actionPerformed()
}//Hockey

HockeyThread.class

import java.awt.*;
import javax.swing.*;

public class HockeyThread implements Runnable 
{
	private int xCoord = 0;
	private int yCoord = 0;
	
	private JPanel surface;
	
	public void run()
	{
		go();
	}
	
	public HockeyThread(JPanel draw)
	{
		surface = draw;
	}
	
	public void go()
	{
		Graphics g = surface.getGraphics();
		
		Dimension size = surface.getSize();
		g.translate(size.width / 2, size.height / 2);
		
		g.fillOval(xCoord,yCoord,20,15);
		g.setColor(Color.black);
	}// go()
}// HockeyThread

There are no syntatical errors, but for some reason the puck won't draw in the center of the screen like it is supposed to. Anyone know what I could be doing wrong here?

[Wizard1988]

I think that your problem is in the run() method, the thread runs but go() is called only once.

I usually do all the drawing inside the paint() method

Try this

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Hockey extends JApplet implements ActionListener
{
	private final int WIDTH = 400, HEIGHT = 200;
	
	private JPanel surface = new JPanel();
	private HockeyThread thread;
	
	public void init()
	{	
		getContentPane().setLayout(new BorderLayout());
		getContentPane().add("Center",surface);
		getContentPane().setSize(WIDTH,HEIGHT);

	}// init()
	
	public void actionPerformed(ActionEvent e)
	{	
	}// actionPerformed()
	
	public void start()
	{
		thread = new HockeyThread(surface);
		Thread hockeyThread = new Thread(thread);
		hockeyThread.start();
	}
}//Hockey

import java.awt.*;
import javax.swing.*;

public class HockeyThread implements Runnable 
{
	private int xCoord = 0;
	private int yCoord = 0;
	
	private JPanel surface;
	
	public void run()
	{
		while(true)
		{
			go();
		}	
	}
	
	public HockeyThread(JPanel draw)
	{
		surface = draw;
	}
	
	public void go()
	{
		Graphics g = surface.getGraphics();
		Dimension size = surface.getSize();
		g.translate(size.width / 2, size.height / 2);
		
		g.fillOval(xCoord,yCoord,20,15);
		g.setColor(Color.black);
	}// go()
}// HockeyThread

[Dark Rain]

I changed some things:

HockeyThread.class

import java.awt.*;
import javax.swing.*;

public class HockeyThread extends HockeyOther implements Runnable 
{	
	private static final int XMIN = 0;
	private static final int XMAX = 420;
	private static final int XSTART = 0;
	private static final int YMIN = 0;
	private static final int YMAX = 345;
	private static final int YSTART = 0;
	private static final int SIDE = 5;
	private static final int DELTA = 5;
	private static final int RANGE = 5;
	
	private Hockey applet;
	private Point location;
	
	public HockeyThread(Hockey app)
	{
		applet = app;
		location = new Point(XSTART,YSTART);
		state = MOVING;
	}
	
	public Point getLocation()
	{
		return location;
	}
	
	public void go()
	{	
		Graphics g = applet.getGraphics();
		
		g.setColor(Color.white);
		g.fillRect(location.x,location.y,SIDE,SIDE);
		
		int dx = (int)(DELTA);
		int dy = (int)(RANGE);
		
		if (location.x + dx >= XMIN && location.x + dx <= XMAX)
		{
			location.x = location.x + dx;
		} else {
			dx *= -1;
		}
		if (location.y + dy >= YMIN && location.y + dy <= YMAX)
		{
			location.y = location.y + dy;
		} else {
			dy *= -1;
		}
		if (location.x + dx <= XMAX && location.x + dx >= XMIN)
		{
			location.x = location.x  + dx;
		} else {
			dx *= -1;
		}
		if (location.y + dy <= YMAX && location.y + dy >= YMIN)
		{
			location.y = location.y + dy;
		} else {
			dy *= -1;
		}
		
		g.setColor(Color.red);
		g.fillRect(location.x,location.y,SIDE,SIDE);
	}// go()

Although, I can't seem to get the boundries right. Either the puck flies out of bounds into nowhere, or starts warping around.

[Wizard1988]

I took some time with MS Paint and explained it a little

[Dark Rain]

Thanks for the tutorial. The only problem is the puck stays at 0,0 now. Do I need a loop somewhere to move it?

[Wizard1988]

You need to do the checking in the main loop, and start with the puck anywhere inbounds

Post your full code again, if ithat doesen't help

[mr1yh1]

DarkRain look at your if blocks.
2 of them are same other 2 . you only need 2...
last one reverse the effect of first.
if (location.x + dx >= XMIN && location.x + dx <= XMAX)
if (location.x + dx <= XMAX && location.x + dx >= XMIN)

if (location.y + dy >= YMIN && location.y + dy <= YMAX)
if (location.y + dy <= YMAX && location.y + dy >= YMIN)

[Dark Rain]

Here is my code:

import java.awt.*;
import javax.swing.*;

public class HockeyThread extends HockeyOther implements Runnable 
{	
	private static final int XMIN = 0;
	private static final int XMAX = 420;
	private static final int YMIN = 0;
	private static final int YMAX = 345;
	private static final int SIDE = 5;
	private static final int DELTA = 5;
	private static final int RANGE = 5;
	
	private static final int XSTART = (int)Math.random() * SIDE;
	private static final int YSTART = (int)Math.random() * SIDE;
	
	private Hockey applet;
	private Point location;
	
	public HockeyThread(Hockey app)
	{
		applet = app;
		location = new Point(XSTART,YSTART);
		state = MOVING;
	}
	
	public Point getLocation()
	{
		return location;
	}
	
	public void go()
	{	
		Graphics g = applet.getGraphics();
		
		g.setColor(Color.white);
		g.fillRect(location.x,location.y,SIDE,SIDE);
		
		int dx = (int)(DELTA);
		int dy = (int)(RANGE);
		
		//	Check x-axis movement
		if ((location.x <= XMIN) | (location.x + SIDE >= XMAX))
		{
			dx *= -1;
		} else {
			dx += dx;
		}
		
		// Check y-axis movement
		if ((location.y <= YMIN) | (location.y + SIDE >= YMAX))
		{
			dy *= -1;
		} else {
			dy += dy;
		}
		
		g.setColor(Color.red);
		g.fillRect(location.x,location.y,SIDE,SIDE);
	}// go()
	
	public synchronized void die()
	{
		state = DEAD;
	}
	
	public void run()
	{
		while (state != DEAD)
		{
			go();
			delay(85);
		}
	}
}// HockeyThread

I call the go() method in the main loop, but it doesn't seem to do any good. Also, I tried setting a random start location with the XSTART,YSTART call but, that also doesn't work.

[Wizard1988]

Can you post the HockeyOther class?

[Dark Rain]

Here you go:

public class HockeyOther 
{
	protected int state;
	public static final int DEAD = -1;
	public static final int MOVING = 0;
	
	public HockeyOther()
	{
		state = MOVING;
	}
	
	protected void delay(int N)
	{
		try {
			Thread.sleep(N);
		} catch (InterruptedException e) {
			System.out.println(e.toString());
		}
	}

}

[Wizard1988]

Is there any other code that you didn't post ? because I can't get this to compile...

[Dark Rain]

Sorry =p. I forgot the GUI part of it:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Hockey extends JApplet
{
	private HockeyThread thread = new HockeyThread(this);
	
	public void init()
	{	
		this.setSize(425,350);
		
		new Thread(thread).start();
	}// init()
	
	public void PaintComponent(Graphics g)
	{
	}
}//Hockey

That's the one you want to compile.

[Wizard1988]

I feel like an idiot... My if statements for the collision detection were wrong, so sorry for misleading you there, but I did get this to work.

I think that your problem might have been that you were declaring dx and dy inside the go() method.

import java.awt.*;
import javax.swing.*;

public class HockeyThread extends HockeyOther implements Runnable 
{	
	private static final int XMIN = 0;
	private static final int XMAX = 400;
	private static final int YMIN = 0;
	private static final int YMAX = 200;
	private static final int SIDE = 10;
	private static final int DELTA = 4;
	private static final int RANGE = 3;
	
	private static final int XSTART = 15;
	private static final int YSTART = 15;
	
	private Hockey applet;
	private Point location;
	
	private int dx;
	private int dy;
	
	public HockeyThread(Hockey app)
	{
		applet = app;
		location = new Point(XSTART,YSTART);
		state = MOVING;
		
		dx = (int)(DELTA);
		dy = (int)(RANGE);
	}
	
	public Point getLocation()
	{
		return location;
	}
	
	public void move()
	{
		location.x += dx;
		location.y += dy;
	}
	
	public void go()
	{	
		Graphics g = applet.getGraphics();
		
		g.setColor(Color.white);
		g.fillRect(location.x,location.y,SIDE,SIDE);
	
		move();
		
		if(location.x + dx >=XMIN && location.x + dx <= XMAX)
		{
			move();
		}
		else{
			dx*=-1;
		}
		if(location.y + dy >= YMIN && location.y +dy <= YMAX)
		{
			move();
		}
		else{
			dy*=-1;
		}
		
		g.setColor(Color.blue);
		g.fillRect(location.x,location.y,SIDE,SIDE);
		
	}// go()
	
	public synchronized void die()
	{
		state = DEAD;
	}
	
	public void run()
	{
		while (state != DEAD)
		{
			go();
			delay(50);
		}
	}
}// HockeyThread

Again sorry for misleading you with the if statements.