如何使用JList操作JList？

Question

我有两个JLists，另一个有武器类别(我正在做角色扮演角色创建软件)，另一个有特定类别的物品。我想让商品根据类别变化。

目前，每次用户更改类别(每个DefaultListModel变量都有不同的项集)时，带有项目的jList都会交换它的DefaultListModel，这会在用户更改类别后单击某个项目时以某种方式生成NullPointerException。

我试着在网上搜索它，我也读过JList的例子，但是我还没能解决这个问题。

下面是相关的代码

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();");
        }
}

下面是我在NetBeans集成开发环境下运行我的程序时得到的错误消息(第一行包含了我的try/catch块打印的异常：

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)

Answer 1

可能的错误在此行中

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

jList1.getSelectedValue()可以是空值，因此当toString()函数对空值进行调用时，它会抛出异常

而应使用此方法来避免异常

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

这样，如果选择为空，您将只返回null的字符串值；