java drawing a circle when mouse clicked

Question

i am writing a program that when the mouse is clicked, a circle will be drawn. The below code i've wrote so far.

import java.awt.*;
import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.event.*;
import java.awt.geom.*;

public class test extends JFrame implements ActionListener, MouseListener {
    Shape circle = new Ellipse2D.Float(10, 10, 10, 10);

    public test () {
        setSize(250,150);
        addMouseListener(this);
    }

    public static void main(String[] args) {
        //TODO code application logic here
        java.awt.EventQueue.invokeLater(new Runnable() {
              public void run() {
                   test frame = new test();
                   frame.setVisible(true);
              }
        });
    }

    public void actionPerformed(ActionEvent ae) {

    }

    public void drawCircle(int x, int y) {
        Graphics g = this.getGraphics();
        g.drawOval(x, y, x, y);
        g.setColor(Color.BLACK);
        g.fillOval(x, y, 2, 2);
    }

    public void mouseClicked(MouseEvent e) {
        drawCircle(e.getX(), e.getY());
        repaint();
    }

    public void mouseExited(MouseEvent e) {

    }

    public void mousePressed(MouseEvent e) {

    }

    public void mouseReleased(MouseEvent e) {

    }

    public void mouseEntered(MouseEvent e) {

    }
}

The code is a 400X400 jframe, when clicked open display a circle at a half seconds, The problem is that, when i release the mouse, the circle disappear. why?

Solution 1

Change your mouseClick(...) to:

int x, y;

public void mouseClicked(MouseEvent e) {
    x = e.getX();
    y = e.getY();

    repaint();
}

Override paint(...):

@Override
public void paint(Graphics g) {
    drawCircle(x, y);
}

Solution 2

When you call repaint(), the component gets painted again from scratch. You're circle is wiped away. You will want to override paintComponent(Graphics) which is called every time the component is painted.

Comments

[kleopatra]

how comes this getGraphics mess is spreading around - a teacher gone wild?

[Matthias Bruns]

You could also save all drawn circles in an array or list and redraw them everytime

[Adam Paynter]

@Traxdata: Yeah, I was thinking about mentioning that, but this question seemed like a homework question, so I left it to him. :)

[Matthias Bruns]

trapped again... :D Anyway overriding paintComponent(Graphics) would be too hard if it's homework

[Andrew Thompson]

Note that the OP is painting in a top level container. JFrame and other TLCs have no paintComponent(Graphics) method. OTOH it is best not to override paint(Graphics) in a TLC, but instead use a JComponent or JPanel - which has the paintComponent(Graphics) method.