java - 如何通过内部类itemListener修改类变量？

Question

我这里有以下代码:

public class A extends Runnable {
  ArrayList<String> choosenFiles;

  run(){
  /*graphic stuff such as JFrame,JLabel,JButtons*/
  }

  private ButtonListener implements ActionListener {
  /*options*/
  }

  public class cbListener implements ItemListener {
    @Override
    public void itemStateChanged(ItemEvent e) {
        Checkbox cb = (Checkbox)e.getItemSelectable();
        String cbl = cb.getLabel();

        if (e.getStateChange() == ItemEvent.SELECTED) {
            JOptionPane.showMessageDialog(null, "selected:\n" + cbl);
            choosenFiles.add(cbl);
        } else if (e.getStateChange() == ItemEvent.DESELECTED) {
            JOptionPane.showMessageDialog(null, "deselected:\n" + cbl);
            choosenFiles.remove(cbl);
        }
    }
}  

问题似乎是 CBListener 无法访问 choosenFile，因为出现 AWT-EventQueue-0 java.lang.NullPointerException 。

这是堆栈跟踪:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at FileDownloader.Client.User$cbListener.itemStateChanged(Unknown Source)
at java.awt.Checkbox.processItemEvent(Checkbox.java:563)
at java.awt.Checkbox.processEvent(Checkbox.java:530)
at java.awt.Component.dispatchEventImpl(Component.java:4660)
at java.awt.Component.dispatchEvent(Component.java:4488)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:674)
at java.awt.EventQueue.access$400(EventQueue.java:81)
at java.awt.EventQueue$2.run(EventQueue.java:633)
at java.awt.EventQueue$2.run(EventQueue.java:631)
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$3.run(EventQueue.java:647)
at java.awt.EventQueue$3.run(EventQueue.java:645)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege
(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:644)
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)

有办法解决这个问题吗？

Answer 1

看起来像choosenFiles尚未初始化；这应该尽早发生，例如在构造函数或初始化方法中。此外，尽可能使用最通用的界面也是一种很好的做法，因此如果没有充分的理由 choosenFiles确实被定义为 ArrayList<String> ，将其设为 List<String> .