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    <p>For this answer I'm making a minor assumption that the Nullpointer is not being thrown at the line that you mentioned, but rather when you attempt to add the WindowListener to the parentFrame. This is most likely because you're calling </p> <pre><code>JFrame parentFrame = (JFrame) SwingUtilities.getWindowAncestor(this); </code></pre> <p>before the JPanel has been added to the JFrame hierarchy. </p> <p>Here's a rought code sample on how you could work around this. The thought it to wait for the panel to be notified that it has been attached to the JFrame somewhere in its hierarchy.</p> <pre><code>package test; import java.awt.event.HierarchyEvent; import java.awt.event.HierarchyListener; import java.awt.event.WindowAdapter; import java.awt.event.WindowEvent; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.SwingUtilities; public class HierarchyTest extends JPanel { protected static void loadApp() { HierarchyTest test = new HierarchyTest(); JFrame frame = new JFrame(); frame.add(test); frame.setSize(200, 200); frame.setVisible(true); } /** * @param args */ public static void main(String[] args) { SwingUtilities.invokeLater(new Runnable() { @Override public void run() { loadApp(); } }); } public HierarchyTest() { this.addHierarchyListener(new HierarchyListener() { @Override public void hierarchyChanged(HierarchyEvent e) { // This can be optimized by checking the right flags, but I leave that up to you to look into boolean connected = setupListenersWhenConnected(); if (connected) { HierarchyTest.this.removeHierarchyListener(this); } } }); } protected boolean setupListenersWhenConnected() { JFrame parentFrame = (JFrame) SwingUtilities.getWindowAncestor(this); if (parentFrame == null) { return false; } parentFrame.addWindowListener(new WindowAdapter() { @Override public void windowClosing(WindowEvent e) { // Implementation here System.out.println("This window is closing!"); } }); return true; } } </code></pre>
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    1. POHow to listen for close in a JPanel
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    1. USJeroenWarmerdam
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    1. COMaybe this is a silly question...but do JPanels always have a window ancestor? I went up in the parent tree of this object, and I could not find anything of that type. Perhaps it is hidden?
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    2. CONot a silly question at all, but I do think it illustrates the base of your problem. The JPanel only has a Window (or in this example JPanel) ancestor when it has been inserted somewhere in the hierarchy beneath a JFrame. When the JPanel is created it has no parents, so also no Window ancestors. After you've added it to a container so that you can see it on the screen, then it does.
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    3. COOk - it sounds like I need to hunt around to find where this JPanel is added to the container...eventually. It's legacy code, so it's a matter of finding it in 1000s of classes. Thanks so much!
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