Technical doubt about setBounds() method

Question

Hi all.
I have a little doubt. I don't completely understand the behaviour of setBounds() method of a JComponent extending class.
I prepared a little test application to explain this doubt. Here it is:
import javax.swing.*;
import java.awt.*;
public class TestSetBounds extends JFrame{
    JPanel myPanel = new JPanel();
    MyComponent myComp = new MyComponent();
    public TestSetBounds() {
        super("SetBounds Test");
        setSize(480, 320);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        myPanel.add(myComp);
        setContentPane(myPanel);
        setVisible(true);
    public static void main(String args[]) {
        TestSetBounds tsb = new TestSetBounds();
class MyComponent extends JComponent implements Runnable
    // setting dimension properties
    int compWidth = 25;
    int compHeight = 50;
    // these values are for testing
    int xStart = 490;
    int xEnd = -12;
    Thread t;
    public MyComponent()
        super();
        setOpaque(true);
        setVisible(true);
        if (t == null)
            t = new Thread(this);
            t.start();
    public void paintComponent(Graphics g)
        super.paintComponents(g);
        setBounds(xStart, 160, compWidth, compHeight);
        Graphics2D area2D = (Graphics2D) g;
        area2D.setColor(Color.RED);
        area2D.fillRect(0, 0, compWidth, compHeight);
    public Dimension getPreferredSize() {
      return new Dimension(compWidth, compHeight);
    public Dimension getMaximumSize() {
      return getPreferredSize();
    public void run()
        for(   ; xStart > xEnd; xStart--)
            repaint();
            try
                {t.sleep(10);}
            catch (Exception exc) {}
}   I tested two situations:
a. Setting xStart = 100 and xEnd = -12, the application run how I expected. With negative value of x coordinate, my component is painted starting out of panel. We only can see my component portion which remains on the panel.
b. Setting xStart = 490 and xEnd = 300. My component is initially painted out of panel, like test a. During for-cicle, xStart value decreases, entering the panel width value. At this point I must see the component portion which remain on the panel.But the component seems not be painted.
Why this behaviour?
Thanks,
Massimiliano

Answer 1

Your immediate problem can be solved by making two changes:
a) your component has no initial size:
setSize(25, 50); // this is new
setOpaque(true);
setVisible(true); // not requiredb) use setLocation() to reposition the component
setLocation(xStart, 160);
//repaint(); // not required since changing the location will cause a repaintHowever the code still has many areas of concern.
For example, after the component has stopped moving, resize the frame. Notice how the component jumps to the top and middle of the frame? This is because by default a JPanel uses a LayoutManager. When you resize the frame the LayoutManager is invoked and the component is positioned based on the rules of the LayoutManager.
So, you need to read the Swing tutorial on [url http://java.sun.com/docs/books/tutorial/uiswing/layout/index.html]Using Layout Manager to understand the difference between using a Layout Manager and using "Absolute Positioning" (which is what you are attempting to do).
You should not be altering the "size" or "location" of the component in the paintComponent() method (ie. get rid of the setBounds()). These changes should be done externally as we did by using the setLocation(..) method.
When you fill the rectangle you should just use getSize().width and getSize().height, instead of your hard coded variables. Then you can dynamically change the size of your component by using the setSize() method.
The size of the component should not be hard coded. Once you've made the above change you can simply do:
MyComponent component = new MyComponent();
component.setSize(25, 50);