Why JFrame redefine the "EXIT_ON_CLOSE" which it inherits from interface "WindowConstants"?

Question

WindowConstants is defined as this:

public interface WindowConstants
{
    public static final int DO_NOTHING_ON_CLOSE = 0;

    public static final int HIDE_ON_CLOSE = 1;

    public static final int DISPOSE_ON_CLOSE = 2;

    public static final int EXIT_ON_CLOSE = 3;

}

JFrame is defined as this:

public class JFrame  extends Frame implements WindowConstants,
                                              Accessible,
                                              RootPaneContainer,
                              TransferHandler.HasGetTransferHandler
{
    /**
     * The exit application default window close operation. If a window
     * has this set as the close operation and is closed in an applet,
     * a <code>SecurityException</code> may be thrown.
     * It is recommended you only use this in an application.
     * <p>
     * @since 1.3
     */
    public static final int EXIT_ON_CLOSE = 3;

Why the EXIT_ON_CLOSE is redefined? And since it is final in the WindowConstants interface, how could it be redefined?

Answer 1

In Java 1.3, when this was all added, EXIT_ON_CLOSE was relevant only to JFrame and not to other implementations of WindowConstants. As such - it was not present in WindowConstants and was defined in JFrame. The 3 other XXX_ON_CLOSE options were in the interface. (English Javadoc is not online anymore, though still downloadable, so no reference here. If you search for "WindowConstants Java 1.3" you'll get a Japanese version of the Javadoc - but since the page structure is the same, you can still see the point)

It was later (1.4) moved to WindowConstants, however the field was not removed from JFrame due to compatibility issues.

There is no redefining there. What's happening is shadowing. I.e. the JFrame field hides (but does not eliminate) the WindowConstants field.