NullPointerException in BoxLayout

Does anybody have an idea how I could start debugging this error in Java Swing?

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerEx开发者_StackOverflow中文版ception
    at javax.swing.BoxLayout.preferredLayoutSize(BoxLayout.java:282)
    at java.awt.Container.preferredSize(Container.java:1599)
    at java.awt.Container.getPreferredSize(Container.java:1584)
    at javax.swing.JComponent.getPreferredSize(JComponent.java:1636)
    at java.awt.BorderLayout.layoutContainer(BorderLayout.java:804)
    at java.awt.Container.layout(Container.java:1421)
    at java.awt.Container.doLayout(Container.java:1410)
    at java.awt.Container.validateTree(Container.java:1507)
    at java.awt.Container.validateTree(Container.java:1513)
    at java.awt.Container.validate(Container.java:1480)
    at javax.swing.plaf.basic.BasicTabbedPaneUI.ensureCurrentLayout(BasicTabbedPaneUI.java:1429)
    at javax.swing.plaf.basic.BasicTabbedPaneUI.getTabBounds(BasicTabbedPaneUI.java:1449)
    at javax.swing.plaf.synth.SynthTabbedPaneUI.setRolloverTab(SynthTabbedPaneUI.java:491)
    at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneLayout.layoutContainer(BasicTabbedPaneUI.java:2384)
    at java.awt.Container.layout(Container.java:1421)
    at java.awt.Container.doLayout(Container.java:1410)
    at java.awt.Container.validateTree(Container.java:1507)
    at java.awt.Container.validate(Container.java:1480)
    at javax.swing.plaf.basic.BasicTabbedPaneUI.ensureCurrentLayout(BasicTabbedPaneUI.java:1429)
    at javax.swing.plaf.basic.BasicTabbedPaneUI.getTabBounds(BasicTabbedPaneUI.java:1449)
    at javax.swing.plaf.synth.SynthTabbedPaneUI.setRolloverTab(SynthTabbedPaneUI.java:498)
    at javax.swing.plaf.basic.BasicTabbedPaneUI.setRolloverTab(BasicTabbedPaneUI.java:558)
    at javax.swing.plaf.basic.BasicTabbedPaneUI.access$2000(BasicTabbedPaneUI.java:37)
    at javax.swing.plaf.basic.BasicTabbedPaneUI$Handler.mouseMoved(BasicTabbedPaneUI.java:3645)
    at java.awt.Component.processMouseMotionEvent(Component.java:6333)
    at javax.swing.JComponent.processMouseMotionEvent(JComponent.java:3285)
    at java.awt.Component.processEvent(Component.java:6057)
    at java.awt.Container.processEvent(Container.java:2041)
    at java.awt.Component.dispatchEventImpl(Component.java:4651)
    at java.awt.Container.dispatchEventImpl(Container.java:2099)
    at java.awt.Component.dispatchEvent(Component.java:4481)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4251)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
    at java.awt.Container.dispatchEventImpl(Container.java:2085)
    at java.awt.Window.dispatchEventImpl(Window.java:2478)
    at java.awt.Component.dispatchEvent(Component.java:4481)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:643)
    at java.awt.EventQueue.access$000(EventQueue.java:84)
    at java.awt.EventQueue$1.run(EventQueue.java:602)
    at java.awt.EventQueue$1.run(EventQueue.java:600)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
    at java.awt.EventQueue$2.run(EventQueue.java:616)
    at java.awt.EventQueue$2.run(EventQueue.java:614)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:613)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

It is thrown from this method in BoxLayout (line size = ...)

public Dimension preferredLayoutSize(Container target) {
Dimension size;
synchronized(this) {
    checkContainer(target);
    checkRequests();
    size = new Dimension(xTotal.preferred, yTotal.preferred);
}

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Extending a layout manager to provide synchronization is unlikely to be effective. Verify that all GUI components are constructed on the the event dispatch thread. Likewise, verify that all data models are updated on the the event dispatch thread, using either invokeLater() or SwingWorker.

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From the amount of code I can see, I would guess that you are trying to do stuff with swing coponents without using an event dispatch thread..

This causes ugly exceptions like the one above. I had this issue in a project and I realized it too late to be able to fix it so I just put every thing in try cat and did not print the entire stack trace...

If this is indeed the problem you can possibly catch and ignore null pointer exceptions..

Best approach is to use the event dispatch threads

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Because the error happens "within" Swing (only java* classes on the stack trace), I'd take a very close look at my code to make sure all my Swing calls happen on the EDT.

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If you can get to that line, then it is likely that xTotal.preferred is null, or yTotal.preferred is null.