如何使用 JList 操作 JList？答案

【问题标题】：how do I manipulate JList with JList?如何使用 JList 操作 JList？
【发布时间】：2012-10-17 18:42:46
【问题描述】：

我有两个 JList，另一个有武器类别（我正在做角色扮演角色创建软件），另一个有特定类别中的项目。我希望根据类别更改项目

目前，每次用户更改类别时，带有项目的 jList 都会交换其 DefaultListModel（每个 DefaultListModel 变量都有不同的项目集），每当用户在更改类别后单击项目时，都会以某种方式产生 NullPointerException。

我尝试在网上搜索此内容，并阅读了 JList 的示例，但我无法解决问题。

这里是相关代码

private void jList1ValueChanged(javax.swing.event.ListSelectionEvent evt) { //Kategory
    String selection = jList1.getSelectedValue().toString();

    if(selection.equals(settings.weapon_classes[0])){
        jList2.clearSelection();
        jList2.setModel(model_l_pistols);
        jList2.validate();

    }
    if(selection.equals(settings.weapon_classes[1])){
        jList2.clearSelection();
        jList2.setModel(model_m_pistols);
        jList2.validate();
    }
}

private void wpn_available_valuechanged_listener(javax.swing.event.ListSelectionEvent evt) {                                                     

     Weapon wpn = new Weapon(); //The details of selected weapon are stored here for desplaying
        try{
            String wpn_name = jList2.getSelectedValue().toString();
            if(jList1.getSelectedValue().equals("Light pistols")){
                wpn = controller.get_Weapon(wpn_name, settings.light_pistols); //Stores right item details from array in settings class
            }
            if(jList1.getSelectedValue().equals("Medium pistols")){
                wpn = controller.get_Weapon(wpn_name, settings.medium_pistols);
            }

        }catch(Exception e){
            System.out.println(e + "String wpn_name = jList2.getSelectedValue().toString();");
        }
}

这是我在使用 NetBeans IDE 运行程序时收到的错误消息（第一行包括我的 try/catch 块打印异常：

java.lang.NullPointerExceptionString wpn_name = jList2.getSelectedValue().toString();
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at charcreation.GUI.wpn_available_valuechanged_listener(Unknown Source)
    at charcreation.GUI.access$6900(Unknown Source)
    at charcreation.GUI$67.valueChanged(Unknown Source)
    at javax.swing.JList.fireSelectionValueChanged(JList.java:1765)
    at javax.swing.JList$ListSelectionHandler.valueChanged(JList.java:1779)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:167)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:147)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:194)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:388)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:398)
    at javax.swing.DefaultListSelectionModel.removeSelectionIntervalImpl(DefaultListSelectionModel.java:559)
    at javax.swing.DefaultListSelectionModel.clearSelection(DefaultListSelectionModel.java:403)
    at javax.swing.JList.clearSelection(JList.java:2013)
    at charcreation.GUI.jList1ValueChanged(Unknown Source)
    at charcreation.GUI.access$6800(Unknown Source)
    at charcreation.GUI$66.valueChanged(Unknown Source)
    at javax.swing.JList.fireSelectionValueChanged(JList.java:1765)
    at javax.swing.JList$ListSelectionHandler.valueChanged(JList.java:1779)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:167)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:147)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:194)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:388)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:398)
    at javax.swing.DefaultListSelectionModel.setSelectionInterval(DefaultListSelectionModel.java:442)
    at javax.swing.JList.setSelectionInterval(JList.java:2035)
    at javax.swing.plaf.basic.BasicListUI$Handler.adjustSelection(BasicListUI.java:2731)
    at javax.swing.plaf.basic.BasicListUI$Handler.mousePressed(BasicListUI.java:2687)
    at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:263)
    at java.awt.Component.processMouseEvent(Component.java:6260)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6028)
    at java.awt.Container.processEvent(Container.java:2041)
    at java.awt.Component.dispatchEventImpl(Component.java:4630)
    at java.awt.Container.dispatchEventImpl(Container.java:2099)
    at java.awt.Component.dispatchEvent(Component.java:4460)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4574)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4235)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
    at java.awt.Container.dispatchEventImpl(Container.java:2085)
    at java.awt.Window.dispatchEventImpl(Window.java:2475)
    at java.awt.Component.dispatchEvent(Component.java:4460)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    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)

【问题讨论】：

标签： nullpointerexception jlist defaultlistmodel

【解决方案1】：

此行可能有错误

String selection = jList1.getSelectedValue().toString();

jList1.getSelectedValue() 可以是空值，因此当toString() 函数调用空值时，它会引发异常

改用这个来避免异常

String selection = String.valueOf(jList1.getSelectedValue());

这样，如果它是一个空选择，你只会得到null的字符串值；

【讨论】：