Thread: repaint() event does not work.

Hi!
I have a java application with multiple buttons.When pressed,every button shows a different painting.One of the button is supposed to display a moving ball.The problem is,the repinat() event does not work and the ball moves but it is only possible to see it's movement when the button is pressed again. I'm working with a Frame.Why doesn't the repaint() event work?

Does the method that calls repaint() return control to the JVM so that it can call the paint method?
If your code is using the JVM's GUI/EDT thread like in a listener, the JVM will queue the calls to the repaint method and wait until your method exits before it calls the paint method. If your code never gives up control of the GUI's thread, the paint method will never be called.
Your listener should compute the new locations etc, call repaint and exit.

I'm not that advanced with java so could you please explain some of your used terms?what is JVM,what is the GUI's control?

What listener are you referring to?

What listener are you referring to?

buttons.When pressed,

I am assuming that you have listeners for the buttons.

If your code does not use listeners, then I have no idea how it is working.
Can you post your code so we can see how it works?

Please make a small simple program (SSCCE) that compiles, executes and shows your problem.

Please Edit your post and wrap your code with
[code=java]<YOUR CODE HERE>[/code]
to get highlighting and preserve formatting.

Is it really relevant?I thought the Frames are a way to define the window.

Please make a small simple program (SSCCE) that compiles, executes and shows your problem.

Please Edit your post and wrap your code with

<YOUR CODE HERE>

to get highlighting and preserve formatting.

The code for the class with the buttons:

 
import java.awt.*;
import java.awt.event.*;
import java.util.Timer;
import java.util.TimerTask;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
 
public class Menu extends Frame{
private static Picture pic;
 
private static Button b5;
private MovePingPong mpp;
public Menu(String st,MovePingPong mpp)
{
	super(st);
	this.setSize(800, 600);
 
	this.addWindowListener(new ExitListener());
	ButtonListener al=new ButtonListener();
 
	b5=new Button("Ping Pong ball");
	b5.setBackground(Color.gray);
	b5.addActionListener(al);
 
	this.mpp=mpp;
 
    pic=new Picture(this.mpp);
    pic.setBackground(Color.LIGHT_GRAY);
    pic.add(b5);
    this.add(pic);
    this.setFocusable(true);
    this.setVisible(true);
 
}
 
public class ButtonListener implements ActionListener{
 
	public void actionPerformed(ActionEvent e){
		Button b=(Button)e.getSource();
		Graphics g=pic.getGraphics();
 
		if(b.equals(Menu.b5))
				pic.paintFramePingPong(mpp, g);
	}
 
}
 
public class ExitListener extends WindowAdapter{
	@Override
	public void windowClosing(WindowEvent e){
		System.exit(0);
	}
}
 
public static void main(String[] args)
{
	MovePingPong mpp=new MovePingPong(5);
	new Menu("This is an Applicaion Biiaaaaacch!!",mpp);
}
 
}

The code for the class using the repaint() event which doesn't work.

 
import java.awt.Color;
import java.awt.Graphics;
import java.util.Timer;
import java.util.TimerTask;
import java.awt.*;
public class MovePingPong extends Frame{
	int x_speed=5,y_speed=5;
	Ball b=new Ball(200,300,Color.black);
	Timer timer;
 public MovePingPong(int seconds){
	 timer=new Timer();
	 timer.schedule(new RemindTask(), 5*100,10*5);
 }
 class RemindTask extends TimerTask {
     public void run() {
     	if(b.GetX()!=1335 && b.GetX()!=10 && b.GetY()!=10 && b.GetY()!=730)
     	{
     		 b.Move(x_speed, y_speed);
     		 repaint();
     	}
     	if(b.GetX()==1335 || b.GetX()==10){
     		x_speed=-x_speed;
     		b.Move(x_speed,y_speed);
     		repaint();
     	}
     	if(b.GetY()==730 || b.GetY()==10)
     	{
     		y_speed=-y_speed;
     		b.Move(x_speed,y_speed);
     		repaint();
     	}
 }
}
 public void drawPingPong(Graphics g)
 {
 
	 b.DrawCircle(g);
 }
}

This is my SSCCE.

The code you just posted does not compile. I get these errors:


TestMenu.java:12: cannot find symbol
symbol : class Picture
location: class TestMenu
private static Picture pic;
^
TestMenu.java:74: cannot find symbol
symbol : class Ball
location: class TestMenu.MovePingPong
Ball b=new Ball(200,300,Color.black);
^
TestMenu.java:30: cannot find symbol
symbol : class Picture
location: class TestMenu
pic=new Picture(this.mpp);
^
TestMenu.java:45: cannot find symbol
symbol : variable b5
location: class java.awt.Menu
if(b.equals(Menu.b5))
^
TestMenu.java:60: non-static variable this cannot be referenced from a static context
MovePingPong mpp=new MovePingPong(5);
^
TestMenu.java:74: cannot find symbol
symbol : class Ball
location: class TestMenu.MovePingPong
Ball b=new Ball(200,300,Color.black);
^
TestMenu.java:72: warning: [serial] serializable class TestMenu.MovePingPong has no definition of serialVersionUID
public class MovePingPong extends Frame {
^
TestMenu.java:11: warning: [serial] serializable class TestMenu has no definition of serialVersionUID
public class TestMenu extends Frame{
^
6 errors
2 warnings

8 error(s)

There are classes I use here which I didn't think that are relevant.
Picture:

 
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Panel;
 
public class Picture extends Panel{
	public Picture(MovePingPong mpp)
	{
		super();
		this.setLocation(0,85);
		this.setLayout(new FlowLayout());
		this.setSize(800,600);
	}
 
public void paintFramePingPong(MovePingPong mpp,Graphics g){
	g.setColor(Color.black);
	g.clearRect(0, 0, 1500, 1000);
	mpp.drawPingPong(g);
}
}

Ball:

 
import java.awt.Color;
import java.awt.Graphics;
 
public class Ball {
  private double x,y,radius;
  Color c;
 
  public Ball(Color c){
	  this.x=300;
	  this.y=300;
	  this.radius=20;
	  this.c=c;
  }
  public Ball(double x,double y,Color c)
  {
	  this.x=x;
	  this.y=y;
	  this.radius=20;
	  this.c=c;
  }
  public double GetX()
  {
	  return this.x;
  }
  public double GetY()
  {
	  return this.y;
  }
  public void Move(double dx,double dy)
  {
	  x+=dx;
	  y+=dy;
  }
  public void DrawCircle(Graphics g){
	  g.drawOval((int)x, (int)y, 2*(int)radius, 2*(int)radius);
  }
}

Can you make ONE source that compiles and executes and shows the problem. You have posted at least 4 so far. One source that shows the problem would be bettter.

You do not override any paint methods. That is the method that the JVM will can after you call repaint.

I've never seen your technique for animation. There are dozens of code samples on the forum on how to draw moving shapes. None look like your code. Do a Search for paintComponent and repaint and see how others have done it.

First suggestion would be to use Swing components instead of AWT components.

I don't override any paint methods because my drawing method is in a Panel class.If repaint() only works with the overrided paint method then what should I do to update the screen and create an animation?

what should I do to update the screen and create an animation

A bit of rewriting. Use the timer to call you code that sets the new/next position of the shape, calls repaint and exits. In the paint method draw the shape at the new location.
Next timer call to your method which changes the location and calls repaint. The paint method is called shortly afterwards and can draw the shape at the new location.

Isn't that exactly what I did in MovePingPong class?

No. I don't see an override of the paint method in that class.
The JFrame that MovePingPong extends is never set visible nor does it have any components added to it.
Add a paint method to MovePingPong and put a println statement in it to see if it is called following a call to repaint. Also set the size of the MovePingPong frame and make it visible.

I changed the drawPingPong method to an Override paint method.The method is entered when compiled but the same affect,the screen is only updated when the button is pressed again,the repaint() does not work.

I made the changes I suggested in post #20 and added this one extra line to the new paint method:
b.DrawCircle(g);

This causes a circle to move around in the MovePingPong frame.

I don't understand,I thought I did it as well.
Does the changed class look like this?

 
import java.awt.Color;
import java.awt.Graphics;
import java.util.Timer;
import java.util.TimerTask;
import java.awt.*;
public class MovePingPong extends Frame{
	int x_speed=5,y_speed=5;
	Ball b=new Ball(200,300,Color.black);
	Timer timer;
 public MovePingPong(int seconds){
	 timer=new Timer();
	 timer.schedule(new RemindTask(), 5*100,10*5);
 }
 class RemindTask extends TimerTask {
     public void run() {
     	if(b.GetX()!=1335 && b.GetX()!=10 && b.GetY()!=10 && b.GetY()!=730)
     	{
     		 b.Move(x_speed, y_speed);
     		 repaint();
     	}
     	if(b.GetX()==1335 || b.GetX()==10){
     		x_speed=-x_speed;
     		b.Move(x_speed,y_speed);
     		repaint();
     	}
     	if(b.GetY()==730 || b.GetY()==10)
     	{
     		y_speed=-y_speed;
     		b.Move(x_speed,y_speed);
     		repaint();
     	}
 }
}
@Override
public void paint(Graphics g){
	b.DrawCircle(g);
}
}

Where did you do this:
set the size of the MovePingPong frame and make it visible.

I did it in the MovePingPong class.