MouseEvent on JPanel - wrong coordinate

Question

I have written the following micro-paintbrush program in Java:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;


class AuxClass1 extends JFrame implements MouseListener, MouseMotionListener{

    private JPanel panel1 = new JPanel();
    private JPanel panel2 = new JPanel();
    private JLabel label1_x = new JLabel();
    private JLabel label1_y = new JLabel();
    private JLabel label1_x_info = new JLabel("");
    private JLabel label1_y_info = new JLabel("");
    //add a container keep panels with widgets 
    private Container con1 = getContentPane();

    private int xval1;
    private int yval1;

    private GridLayout layout1 = new GridLayout(2,2,2,2);

    private JOptionPane info1 = new JOptionPane();

    //get the class that controls the mouse
    public AuxClass1(){
        super("Mouse Experiment");
        panel1.setBackground(Color.WHITE);      
        panel1.setLayout(layout1);
        label1_x.setText("X Location");
        label1_x.setBorder(BorderFactory.createLineBorder(Color.BLUE, 2));
        label1_y.setBorder(BorderFactory.createLineBorder(Color.BLUE, 2));
        label1_x_info.setBorder(BorderFactory.createLineBorder(Color.RED, 2));
        label1_y_info.setBorder(BorderFactory.createLineBorder(Color.RED, 2));
        label1_y.setText("Y Location");     
        panel1.add(label1_x);
        panel1.add(label1_y);
        panel1.add(label1_x_info);
        panel1.add(label1_y_info);
        con1.add(panel1, BorderLayout.NORTH);
        panel2.setBackground(new Color(100,200,200));
        panel2.setBorder(BorderFactory.createLineBorder(new Color(255,255,0), 2));
        panel2.addMouseListener(this);
        panel2.addMouseMotionListener(this);
        con1.add(panel2, BorderLayout.CENTER);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(500, 500);
        setLocationRelativeTo(null);
        setVisible(true);


    }

    @Override
    public void mouseClicked(MouseEvent arg0) {


    }

    @Override
    public void mouseMoved(MouseEvent arg0) {
        // TODO Auto-generated method stub
        if (arg0.getSource()==panel2){
            x_var = arg0.getX();
            y_var = arg0.getY();
            label1_x_info.setText(Integer.toString(x_var));
            label1_y_info.setText(Integer.toString(y_var));
        }

    }

    @Override
    public void mouseDragged(MouseEvent e) {
        // TODO Auto-generated method stub
        if (e.getSource()==panel2){
            //info1.showMessageDialog(this, "This is an awesome Mouse toolbox!");
            xval1 = e.getX();
            yval1= e.getY();
            AuxClass2 Inst2 = new AuxClass2(xval1, yval1);
            Inst2.paintComponent(getGraphics());
            }

    }

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

    }

    @Override
    public void mouseExited(MouseEvent arg0) {
        if (arg0.getSource()==panel2){
            label1_x_info.setText("");
            label1_y_info.setText("");
        }
        // 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

    }


}

class AuxClass2 extends JPanel{

    //JOptionPane info2 = new JOptionPane();
    private int xval2;
    private int yval2;

    public AuxClass2(int input1, int input2){

        xval2 = input1;
        yval2 = input2;
        setSize(500,500);

    }

    @Override
    public void paintComponent(Graphics g){
        super.paintComponents(g);
        g.setColor(Color.BLUE);
        g.fillRect(xval2, yval2+70, 5, 5);  

    }

}

public class MainClass{

    private static AuxClass1 Inst1;

    public static void main(String args[]){

        Inst1 = new AuxClass1();


    }

}

It works alright except for the Y coordinate of the mouseDragged method (see paintComponent method in class t3_aux2). For some reason the Y coordinate used by the method is ~70 pixels less than the actual one in panel2. I suspect this is something to do with the inherited JPanel method in t3_aux2 class but not too sure.

If someone could clarify this point, it would be cool. Thanks.

UPD: If anyone has suggestions on how to improve style and/or optimize code, that would be massively appreciated too.

UPD2: Changed the names to comply with Java naming conventions.

Answer 1

I tried your code. I think your problem comes from the fact that you paint on the t3_aux1 using t3_aux2 coordinates. I'll try something to confirm that, and I come back here ...

EDIT: OK, that's it.

in t3_aux1 constructor, if you write

System.out.println("panel1 height = " + panel1.getHeight());
System.out.println("label1 height = " + label1_x.getHeight())

it prints

panel1 height = 42
label1 height = 20

So your offset is 42 + 20 + 4*2 = 70

4*2 comes from your lines borders with a thickness of 2.

Since it's possible to calculate the exact offset, you can dynamically fix it.

EDIT 2 :

In fact, the coordinates you uses come from panel2 since the mouseListener is attached to panel2. But you draw on the JFrame Graphics, not on panel2 graphics.

writing this should fix your coordinates problem.

inst2.paintComponent(panel2.getGraphics());

But as Kleopatra said, you're not doing it the right way. You should never call getGraphics() or paintComponent(). I think you should consider using java.awt.Canvas as a super class for your "panel2" object.

One more advice : Be aware that your drawings aren't memorized, so, if you reduce the window or hide it behind another one, everything drawn will be lost.