adding graphic objects to JPanel

[frank van schie]

Dear JavaRanch,

i recently stepped into the world of java programming leaving my short by enjoyable Python roots behind.

it was not long untill i started to bump into difficulties.

like now for instance, i'm trying to make a ball appear on mouseclick that circles around the cursor.
now i got 2 things that i cant stand.

1, the offset of the ball is odd, its not located in the middle and i tried adding/substracting half of its size to the X,Y.

2, how do i make the ball (or a multiplicity of which) appear without inviking errors like NullPointerExeption.

here comes my code, dont know bricks about text formatting on forums yet

sorry for the mess and the lack of commenting.

package taart;

import javax.swing.*;

import taart.Skwer;

import java.awt.*;
import java.awt.event.*;
import java.util.Timer;


public class Bal extends JPanel implements MouseMotionListener,ActionListener,MouseListener{
public int xpo,ypo,size, xpo2,ypo2, xpo3,ypo3,xsin,ycos;
public double x, y;
private JButton links,rechts;
Skwer hoek,hoek2;
public static int a;
private JFrame venster;
private javax.swing.Timer gobble = new javax.swing.Timer(25,this);
Skwer doos;
private char A;
private String aa;
//PointerInfo Q = MouseInfo.getPointerInfo();

//public Graphics g;
	public Bal(){
		JFrame venster = new JFrame();

		venster.addMouseListener(this);
		venster.addMouseMotionListener(this);

		xpo=0;
		xpo=0;
		size = 5;
		hoek =  new Skwer();
//		hoek2 = new Skwer();



//		venster.add(hoek2);
		venster.add(hoek);
//		gobble.start();
//		mouseClicked();
		


		venster.setSize(700,400);
		venster.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		venster.setTitle("kakding");
		venster.setLocation(100,100);
		venster.setVisible(true);
	}



	


	
	@Override
	public void mouseClicked(MouseEvent m0) {
		System.out.print(m0.getX());
		System.out.print("x");
		System.out.print(hoek.getHorPlaats());
		System.out.print("@");
		System.out.print(m0.getY());
		System.out.print("x");
		System.out.print(hoek.getVerPlaats());
		System.out.print("@");

		
		}






	@Override
	public void actionPerformed(ActionEvent e) {
//		hoek.setHorPlaats2((int)(Math.sin(x)));
//		hoek.setVerPlaats2((int)(Math.cos(y)));
//		x+=0.01;
//		y-=0.01;
//		System.out.print(Math.cos(y));
		hoek.repaint();
		
		
	}


	@Override
	public void mouseMoved(MouseEvent m0) {
		hoek.setHorPlaats(m0.getX());
		hoek.setVerPlaats(m0.getY());
		
		hoek.repaint();
//		hoek2.repaint();


	}

	public static void main(String[ ] args) 
	{
		new Bal();
	}

//unused listeners
	@Override
	public void mouseEntered(MouseEvent arg0) {
		// TODO Auto-generated method stub
		
	}

	@Override
	public void mouseExited(MouseEvent arg0) {
		// TODO Auto-generated method stub
		
	}


	@Override
	public void mousePressed(MouseEvent arg0) {
		// TODO Auto-generated method stub
		
	}


	@Override
	public void mouseReleased(MouseEvent arg0) {
		// TODO Auto-generated method stub
		
	}


	@Override
	public void mouseDragged(MouseEvent arg0) {
		// TODO Auto-generated method stub
		
	}
}

and for the Skwer object that i want to make appear.

package taart;

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

public class Skwer extends JPanel
{
	private int hplaats = 0;
	public int getHorPlaats(){
		return hplaats;
	}
	public void setHorPlaats(int nieuw){
		hplaats = nieuw;
	}
	private int vplaats = 0;
	public int getVerPlaats(){
		return vplaats;
	}
	public void setVerPlaats(int nieuw){
		vplaats = nieuw;
	}
	private int hplaats2 =0;
	public int getHorPlaats2(){
		return hplaats2;
	}
	public void setHorPlaats2(int nieuw){
		hplaats2 = nieuw;
	}
	private int vplaats2 =0;
	public int getVerPlaats2(){
		return vplaats2;
	}
	public void setVerPlaats2(int nieuw){
		vplaats2 = nieuw;
	}
	public void paintComponent(Graphics g){
		super.paintComponent(g);
		g.setColor(Color.orange);
		g.fillOval(hplaats, vplaats, 10, 10);
		
	}
}

now where and how do i get n error free ball on my screen by the click of a mouse?

[Unnar Björnsson]

First things first, the reason why your ball isn't at the center of your cursor is because your mouse listeners are registered to a JFrame object, a JFrame is the whole window frame so the 0,0 coords are actually at the top left corner of the frame (above the java icon). Add the mouse listeners to the Skwer object instead

		hoek.addMouseListener(this);
		hoek.addMouseMotionListener(this);

or make a JPanel inside the JFrame and register the listeners to it instead

[Campbell Ritchie]

I shall move this discussion to the GUIs forum, where it would fit better.

And welcome to the Ranch )

[Campbell Ritchie]

Have you considered a MouseAdapter or MouseMotionAdapter which might avoid your having to write seven methods? Make sure to use the @Override annotation with your overridden methods.

[frank van schie]

oh i see.

i shall try these suggestions when i get access to my machine again!
thanks in advance!

[frank van schie]

OK, well that worked.

But i still have no clue on how to add (multiple) Skwer objects on my JPanel.
like uh

for instance i'll make an InvertedSkwer object that moves in the direct opposide direction when i move the mouse
and preferable have it appear when i click the mouse or something.