如何在 Java Swing 中使用 repaint 方法

Question

I am very new to the Graphics portion of Java. I have created a frame and on it I have added a panel whose color has been set to Green. Now on clicking that panel I want to draw a circle using a test class's object called Mypanel. But it does not happen. Please guide !

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;

class Mypanel extends JPanel
{
    @Override
    public void paintComponent(Graphics g)
    {
        g.drawOval(15, 15, 5, 5);
    }        
}
public class algo extends javax.swing.JFrame {

    public algo() {
        initComponents();
        jPanel1.setBackground(Color.GREEN);
    }
    Mypanel p = new Mypanel() ;

    private void jPanel1MouseClicked(java.awt.event.MouseEvent evt) {                                     
        p.repaint();
    }                                    

    public static void main(String args[]) {

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new algo().setVisible(true);
            }
        });
    }

}

If I were to guess I would say that I am not supposed to use the repaint method, but I was told that this was to be used.

Answer 1

That code as supplied would not compile. For better help sooner, post a Minimal, Complete, and Verifiable example or Short, Self Contained, Correct Example .

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

class Mypanel extends JPanel {

    boolean clicked = false;

    Mypanel() {
        setBackground(Color.GREEN);
        MouseListener mouseListener = new MouseAdapter() {

            @Override
            public void mouseClicked(MouseEvent e) {
                clicked = true;
                repaint();
            }
        };
        this.addMouseListener(mouseListener);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400, 100);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (clicked) {
            g.drawOval(15, 15, 50, 50);
        }
    }
}

public class algo extends JFrame {

    public algo() {
        initComponents();
        pack();
        //jPanel1.setBackground(Color.GREEN); ?!?
    }

    protected final void initComponents() {
        add(new Mypanel());
    }

    public static void main(String args[]) {

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new algo().setVisible(true);
            }
        });
    }
}

Answer 2

There are a few things to correct in your example...

When you create the frame (ie in the constructor) you'll want to call super(). This is the first thing the constructor has to do. Then, you'll probably want to set an initial width/height, and set the background color of the frame green.

You need to add a mouse listener so that the mouseClicked method is actually called. Then have it add the 'MyPanel' object to the frame, and call repaint.

I think that's roughly what you're going for.