java - JComboBox 下拉时出现错误 - Java

Question

从 GUI 访问列表时，我的 jcombo 框出现问题。

组合框是使用 netbeans 设计器来放置字符串项目的。

然后我像这样填充组合框:

private javax.swing.JComboBox<String> userInfoCombo;

public List getSchoolList() throws IOException {

    Map<String, List<String>> schoolMap = new HashMap<String, List<String>>();

    schoolMap = schoolMapCreator.updateList();

    List<String> schoolNameList = new ArrayList<String>();

    for (List<String> value : schoolMap.values()) {
        String schoolName = value.get(1);
        schoolNameList.add(schoolName);
    }

    userInfoCombo.removeAllItems();

    //for (String schoolName : schoolNameList) {
        //userInfoCombo.addItem(schoolName);
    //}

    //with SO user suggestion update--same error comes up
    userInfoCombo.setModel(new DefaultComboBoxModel(schoolNameList.toArray()));

    System.out.println("Get School Name List : -----------> " + schoolNameList);
    return schoolNameList;
}

当启动 gui 并调用 getSchoolList 时，我看到列表中的第一个值是组合框中的一个选项。但是，当按箭头查看其他选项时，出现以下异常:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at com.apple.laf.AquaMenuPainter.paintSelectedMenuItemBackground(AquaMenuPainter.java:153)
    at com.apple.laf.AquaComboBoxRendererInternal.paintComponent(AquaComboBoxRendererInternal.java:143)
    at javax.swing.JComponent.paint(JComponent.java:1056)
    at javax.swing.CellRendererPane.paintComponent(CellRendererPane.java:151)
    at javax.swing.plaf.basic.BasicListUI.paintCell(BasicListUI.java:227)
    at javax.swing.plaf.basic.BasicListUI.paintImpl(BasicListUI.java:317)
    at javax.swing.plaf.basic.BasicListUI.paint(BasicListUI.java:240)
    at javax.swing.plaf.ComponentUI.update(ComponentUI.java:161)
    at javax.swing.JComponent.paintComponent(JComponent.java:780)
    at javax.swing.JComponent.paint(JComponent.java:1056)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JViewport.paint(JViewport.java:728)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JLayeredPane.paint(JLayeredPane.java:586)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5210)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1579)
    at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1502)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5158)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4969)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:831)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:814)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
    at javax.swing.RepaintManager.access$1200(RepaintManager.java:64)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1732)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at com.apple.laf.AquaMenuPainter.paintSelectedMenuItemBackground(AquaMenuPainter.java:153)
    at com.apple.laf.AquaComboBoxRendererInternal.paintComponent(AquaComboBoxRendererInternal.java:143)
    at javax.swing.JComponent.paint(JComponent.java:1056)
    at javax.swing.CellRendererPane.paintComponent(CellRendererPane.java:151)
    at javax.swing.plaf.basic.BasicListUI.paintCell(BasicListUI.java:227)
    at javax.swing.plaf.basic.BasicListUI.paintImpl(BasicListUI.java:317)
    at javax.swing.plaf.basic.BasicListUI.paint(BasicListUI.java:240)
    at javax.swing.plaf.ComponentUI.update(ComponentUI.java:161)
    at javax.swing.JComponent.paintComponent(JComponent.java:780)
    at javax.swing.JComponent.paint(JComponent.java:1056)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JViewport.paint(JViewport.java:728)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JLayeredPane.paint(JLayeredPane.java:586)
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5210)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1579)
    at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1502)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5158)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4969)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:831)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:814)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
    at javax.swing.RepaintManager.access$1200(RepaintManager.java:64)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1732)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at com.apple.laf.AquaMenuPainter.paintSelectedMenuItemBackground(AquaMenuPainter.java:153)
    at com.apple.laf.AquaComboBoxRendererInternal.paintComponent(AquaComboBoxRendererInternal.java:143)
    at javax.swing.JComponent.paint(JComponent.java:1056)
    at javax.swing.CellRendererPane.paintComponent(CellRendererPane.java:151)
    at javax.swing.plaf.basic.BasicListUI.paintCell(BasicListUI.java:227)
    at javax.swing.plaf.basic.BasicListUI.paintImpl(BasicListUI.java:317)
    at javax.swing.plaf.basic.BasicListUI.paint(BasicListUI.java:240)
    at javax.swing.plaf.ComponentUI.update(ComponentUI.java:161)
    at javax.swing.JComponent.paintComponent(JComponent.java:780)
    at javax.swing.JComponent.paint(JComponent.java:1056)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JViewport.paint(JViewport.java:728)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JComponent.paintChildren(JComponent.java:889)
    at javax.swing.JComponent.paint(JComponent.java:1065)
    at javax.swing.JLayeredPane.paint(JLayeredPane.java:586)
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5210)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1579)
    at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1502)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5158)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4969)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:831)
    at javax.swing.RepaintManager$4.run(RepaintManager.java:814)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
    at javax.swing.RepaintManager.access$1200(RepaintManager.java:64)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1732)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

更新 当我尝试使用简单的字符串列表填充组合框时，也会发生此错误，因此我很确定这与我之前向列表添加值的方式无关。如果我添加组合并使用 Gui 模型填充它，也会发生此错误。

Answer 1

for (List<String> value : schoolMap.values()) {
    //String schoolName = value.get(1);
    schoolNameList.add(value.get(1));
   }

     //userInfoCombo.removeAllItems(); no need to remove all

    //for (String schoolName : schoolNameList) {
       // userInfoCombo.addItem(schoolName);
    //}

   userInfoCombo.setModel(new DefaultComboBoxModel(schoolNameList.toArray()));

您不必迭代列表即可使用默认组合框模型将值设置为组合框