Question

I can not access public method of my class MyPanel which extends JPanel. In MyPanel I previously had private method called moveSquare, but in order to make it work for external requests, I changed it to public, but it still do not work? How to make it working?

import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.BorderFactory;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseMotionListener;
import java.awt.event.MouseMotionAdapter;

public class mull {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JPanel p = createAndShowGUI();
                iterate(p);
                // "The method iterate(MyPanel) in the type mull is not applicable for the arguments (JPanel)"
            }
        });
    }

    private static JPanel createAndShowGUI() {
        System.out.println("Created GUI on EDT? "+
        SwingUtilities.isEventDispatchThread());
        JFrame f = new JFrame("Swing Paint Demo");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
        // how to grab link to this panel - in order to use it in iteration loop ?
        MyPanel p = new MyPanel();
        f.add(p);
//        f.add(new MyPanel());
        f.pack();
        f.setVisible(true);
        return p;
    } 
    private static void iterate(JPanel p){
        // the loop should change square position on each iteration
        // how to implement ?

        for (int i  = 0; i < 999; i++){
            ((MyPanel) p).moveSquare(100 + i*10, 200 + i*10); // here is problem: 
            //"Cannot make a static reference to the non-static method moveSquare(int, int) from the type MyPanel"
        }


    }
}

class MyPanel extends JPanel {

    private int squareX = 50;
    private int squareY = 50;
    private int squareW = 200;
    private int squareH = 200;

    public MyPanel() {

        setBorder(BorderFactory.createLineBorder(Color.black));

        addMouseListener(new MouseAdapter() {
            public void mousePressed(MouseEvent e) {
                moveSquare(e.getX(),e.getY());
            }
        });

        addMouseMotionListener(new MouseAdapter() {
            public void mouseDragged(MouseEvent e) {
                moveSquare(e.getX(),e.getY());
            }
        });

    }
    // originally this method was private - in orger to access it within mull, it vas changed to public
    public void moveSquare(int x, int y) {
        int OFFSET = 1;
        if ((squareX!=x) || (squareY!=y)) {
            repaint(squareX,squareY,squareW+OFFSET,squareH+OFFSET);
            squareX=x;
            squareY=y;
            repaint(squareX,squareY,squareW+OFFSET,squareH+OFFSET);
        } 
    }


    public Dimension getPreferredSize() {
        return new Dimension(900,700);
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);       
        g.drawString("This is my custom Panel!",10,20);
        g.setColor(Color.RED);
        g.fillRect(squareX,squareY,squareW,squareH);
        g.setColor(Color.BLACK);
        g.drawRect(squareX,squareY,squareW,squareH);
    }  
}

What does this syntax do ((MyPanel) p).moveSquare(100 + i*10, 200 + i*10); ?
How to paint custom shapes (ovals) in iterate(p)?

Answer 1

It doesn’t seem like you’re even calling iterate().

But in any case, your moveSquare method is not static, and thus expects a specific MyPanel instance to work on. So you’ll probably want to keep a reference to your panel. Say, instead of f.add(new MyPanel()), you could use:

MyPanel p = new MyPanel();
f.add(p);

and then pass p to iterate and call p.moveSquare(100,200) in there.