repaint() Method in Java

Question

I am playing around with the Java graphics class a little bit and I was just wondering--when is it necessary to call the method repaint()? I tried commenting it out and it didn't seem to affect my output GUI. I've seen it used a lot in Java GUI code that I've read, though. Would anyone mind explaining when to use it and when not to use it?

Answer 1

The repaint() refreshes the view (component), so whenever you make any change on the component, you must call it. For instance, if you rotate the graphical component, you must make a call to repaint() in order to see the change on the containing component

Answer 2

It's never really necessary in most swing applications, because they handle it automatically (for common operations such as changing text values on buttons and adding data to a list box).

Generally, it's only if you've made some sort of change that swing won't automatically pick up - for example, you're not using a layout manager and are resizing components manually (because normally the layout manager repaints its components when necessary).

Answer 3

When you launch your application, you "paint" your GUI.

You need to call repaint() when you want to re-draw your GUI because you have changed something inside.

If you want to delete a button, you need to remove it (or make it invisible) then you need to call validate() or repaint() to re-calculate (re-draw) the GUI.

Answer 4

The only stuff I can think of:

    new Thread() {
    @Override
    public void run() {
    while (ClassName.this.isVisible()) {
        ClassName.this.updateStatusLabel();
        ClassName.this.validate();
        ClassName.this.repaint(50L);
        try {
        Thread.sleep(1000);
        } catch (final InterruptedException e) {
        Log.log(e);
        }
    }
    }
}.start();

Suppose you have the above code in the constructor of a JDialog. What updateStatusLabel does is checking a boolean variable, either public or settable through a method, and set the icon of a JLabel in base to such boolean. If you don't validate and repaint the GUI, the modification won't be shown until another event, most likely an user-triggered one, is thrown. And, if the user is waiting for the label to show a certain icon because of, let's say, it indicated if a device is reachable through the Internet, he/she'll never interact (or, at least, you're delaying interactions so much).