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Asked: 2026-05-23T19:09:38+00:00

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

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

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    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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1 Answer

  1. Editorial Team

    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. As failures can be obscure, this Q&A posed by @mKorbel examines ways to find EDT violations programmatically.

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