Creating a simple custom JComponent in Java?

Question

I want to start building my own customized JComponent's for a project at work. I have a simple example below that should just create a ball on the screen. (I found most of it on the internet) but it does provide a decent starting point. My question is why does this code not show the ball in my form? What have I done wrong?

Also what would be all of the basic methods that SHOULD be provided for a custom JComponent?

Code:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.GridBagLayout;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

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

    public testBall() {
        JPanel testPane = new JPanel();
        testPane.setBackground(Color.white);
        testPane.setLayout(new GridBagLayout());
        testPane.add(new MyBall(30,30,10));

        JFrame frame = new JFrame("Testing");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new BorderLayout());
        frame.add(testPane);
        frame.pack();
        frame.setSize(500, 300); 
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

class MyBall extends JComponent
{
    private static final long serialVersionUID = 1L;
    public MyBall() { }
    public MyBall(int x, int y, int diameter)
    {
        super();
        this.setLocation(x, y);
        this.setSize(diameter, diameter);
    }

    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.setColor(Color.red);
        g.fillOval(0, 0, 100, 100);
    }
}

Where I could find a list of all of the methods that should be overridden in a JComponent class? (I know there are ones that should always be included in JComponent.)

If I make an instance of this component in a class and need to change the color of the circle would I just call there repaint() method from that class?

Answer 1

You are adding the MyBall to testPane (which has a GridBagLayout) without specifying any constraints. (That is, your call to add() has no second parameter.) The default constraints are most likely not what you want. Try using BorderLayout for your test pane, as this uses BorderLayout.CENTER as the default, which is probably reasonable in your case:

testPane.setLayout(new BorderLayout());

This causes the ball to show up for me.

As for your second question, I think your class is fine as defined. The main method you want to implement is paintComponent() as you have. Sometimes it becomes necessary to override the get min/max/preferred size methods, but it really just depends on what you want the component to do. JComponent is not abstract, so you don't have to override anything if you don't want to. It provides a lot of functionality out of the box such as keyboard focus, pluggable look-and-feel, accessibility, etc. As long as you don't want to change any of that stuff, just leave it as is. Implement paintComponent() and the various get*Size() methods and be done with it. (You just kind of have to pick through the methods in the JavaDoc to see what is appropriate to override.)

The other option is to extend a subclass of JComponent, if there as a class that does something similar to what you want to do. For example, JPanel is often a good starting point for impelmenting your own container.

You were probably looking for something more concrete than 'it depends', but right now, if all you want is to draw a ball, then simply override the methods that deal with the rendering of the JComponent (paintCompentent(), and the get*Size() methods).

As a side note, you really should be using SwingUtilities.invokeLater() to create your Swing components on the Swing thread. See the section entitled "Swing's Threading Policy" at http://docs.oracle.com/javase/6/docs/api/javax/swing/package-summary.html.