Testcase
api/javax_swing/JDesktopPane/descriptions.html#getset[JDesktopPane2002]
fails on

Platforms: Linux, Solaris
JDK: 6, 7
L&F: -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel

Methods JDesktopPane.setDesktopManager/getDesktopManager are not consistent under GTK L&F
Please see the following code sample:
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import javax.swing.*;

public class TestJDPane {

    public static void main(String[] args) {
        JDesktopPane desktopPane = new JDesktopPane();
        DesktopManager installed = new DefaultDesktopManager();
        desktopPane.setDesktopManager(installed);
        DesktopManager returned = desktopPane.getDesktopManager();
        if (!returned.equals(installed)) {
            System.out.println("installed = " + installed);
            System.out.println("returned = " + returned);
        }
    }
}
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with -Dswing.defaultlaf=com.sun.java.swing.plaf.gtk.GTKLookAndFeel
the output will be like the following

installed = javax.swing.DefaultDesktopManager@82764b
returned = javax.swing.plaf.synth.SynthDesktopPaneUI$SynthDesktopManager@12452e8