abstract classes(no abstract methods or any method implemetations)

hi all,


can anybody explain me why there r some abstract classes with
no methods(no abstract methods or any method implemetations)
for eg:in package
javax.swing.plaf

DesktopIconUI.java
ColorChooserUI.java

here is the source for one more abstract class :  ButtonUI.java

/*
 * @(#)ButtonUI.java 1.15 00/02/02
 *
 * Copyright 1997-2000 Sun Microsystems, Inc. All Rights Reserved.
 *
 * This software is the proprietary information of Sun Microsystems, Inc.
 * Use is subject to license terms.
 *
 */
package javax.swing.plaf;

import javax.swing.AbstractButton;
import java.awt.Insets;

/**
 * Pluggable look and feel interface for JButton.
 *
 * @version 1.15 02/02/00
 * @author Jeff Dinkins
 */
public abstract class ButtonUI extends ComponentUI {
}


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