Question

注意：这是我的实际代码的缩短版本，但在结构方面几乎相同。我基本上切断了连接到组件代码的面板和连接到框架代码的面板。

在 Display.java 上，我有以下内容。请注意，我没有向targetEnvironmentComboBox添加任何侦听器。不确定这是否有问题：

public class Display extends JFrame {

    private static JButton executeButton;
    private static JComboBox<String> commandOptionsComboBox, targetEnvironmentComboBox;

    //getters
    public static JButton getExecuteButton()    {   return executeButton;               }
    public static JComboBox<String> getCommandOptionsComboBox()    {    return commandOptionsComboBox;      }
    public static JComboBox<String> getTargetEnvironmentComboBox()      {   return targetEnvironmentComboBox;   }

    public Display() {
          super("Display");
          setLayout(new BorderLayout());
          setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

          commandOptionsComboBox = new JComboBox(commandOptions.toArray());
          commandOptionsComboBox.addActionListener(new CommandListener());

          executeButton = new JButton("Execute");
          executeButton.addActionListener(new CommandListener());

          targetEnvironmentComboBox = new JComboBox(targetEnvironments.toArray());


        //main method that gets executed at the start of program
        public static void main(String[] args) {
        new Display();

        }
  }

在单独的 CommandListener.java 上，我有以下内容：

 public class CommandListener implements ActionListener {

JButton executeButton = Display.getExecuteButton();
JComboBox<String> commandOptionsComboBox = Display.getCommandOptionsComboBox();
JComboBox<String> targetEnvironmentComboBox = Display.getTargetEnvironmentComboBox();

@Override
public void actionPerformed(ActionEvent event) {

    if(event.getSource() == executeButton) {
        System.out.println("HGello world");
        executeCommand(event);

    }else if (event.getSource() == commandOptionsComboBox) {
        System.out.println("commandline");
        disableUnusedComponents(event);
    }
}

private void disableUnusedComponents(ActionEvent event) {
     **JComboBox<String> targetEnvironmentComboBox = Display.getTargetEnvironmentComboBox();**
     String command = (String) commandOptionsComboBox.getSelectedItem();
    switch(command) {
          case "-duplicate":
            targetEnvironmentComboBox.setEnabled(false);
            targetEnvironmentComboBox.setVisible(false);
            break;
          default: break;
     }

我的问题是，当我在targetEnvironmentComboBox方法之前的commandListener类中获得actionPerformed()时，它会抛出空指针异常。如果我删除了targetEnvironmentComboBox方法中的disableUsedComponents() **，那么它就能成功获得combobox。

此外，如果我执行executeButton侦听器，它能够获取CommandListener类中所有组件的引用，但如果我执行getCommandOptionsComboBox的侦听器，它将返回null executeButton和targetEnvironmentComboBox。

任何人都可以解释为什么会这样吗？

其次，我知道这可能不是最好的实现。关于我可以改变以遵循更好的做法的任何建议吗？

Answer 1

您正在获得null，因为targetEnvironmentComboBox无法初始化，直到之后您尝试在CommandListener中访问它。创建新的CommandListener时，它会读取targetEnvironmentComboBox并将其存储到本地变量中。并查看您创建CommandListener：

的位置

//Creating CommandListeners, which take the reference to targetEnvironmentComboBox (null)
//Although targetEnvironmentComboBox is later set to a usable value, the local copies still
//have the null reference which is assigned here
commandOptionsComboBox.addActionListener(new CommandListener());
executeButton = new JButton("Execute");
executeButton.addActionListener(new CommandListener());

//initializing targetEnvironmentComboBox, which is still null (the first time at least)
targetEnvironmentComboBox = new JComboBox(targetEnvironments.toArray());

如果您将Display.getTargetEnvironmentComboBox()放在方法中，它就不需要复制参考文件，直到需要它为止，此时它已被正确初始化。

一个简单的修复，虽然不是正确的，但是首先要初始化它：

targetEnvironmentComboBox = new JComboBox(targetEnvironments.toArray());

commandOptionsComboBox.addActionListener(new CommandListener());
executeButton = new JButton("Execute");
executeButton.addActionListener(new CommandListener());

更好的解决方案是使用适当的封装，并将构造函数中正确对象所需的内容传递给CommandListener。我建议修改你的CommandListener课程：

public class CommandListener implements ActionListener {
    JButton executeButton;
    JComboBox<String> commandOptionsComboBox;
    JComboBox<String> targetEnvironmentComboBox;

    public CommandListener(JButton executeButton,
                           JComboBox<String> commandOptionsComboBox,
                           JComboBox<String> targetEnvironmentComboBox){
        this.executeButton = executeButton;
        this.commandOptionsComboBox = commandOptionsComboBox;
        this.targetEnvironmentComboBox = targetEnvironmentComboBox;
    }

    /* The rest of your code */
}

然后传递这些字段（确保你传递非空值），并取消所有静态变量（或重构为更易于管理的东西）。

Answer 2

不使用static方法访问实例，而是使用封装并通过setters and getters访问它。

获得targetEnvironmentComboBox对象的实例的唯一方法是让您的展示实例同时指向＆＃34;在内存中的空间，尝试：

public JComboBox getComobo(){
   return [your_object];
}

而不是：

public static JComboBox getTargetEnvironmentComboBox(){
  // your code . . .
}

并通过Display构造函数传递CommandListener实例。

Answer 3

这是一个简化的例子，说明如何设置这种方式，既可以大大减少样板代码的数量，又基本上不受您遇到的初始化问题的影响。这也是Swing UI代码中一种相当典型的方法，实际上也是引入匿名内部类的驱动因素之一。

public class Display extends JPanel {

    private JButton executeButton;
    private JComboBox<String> commandOptionsComboBox;

    public Display() {

        executeButton = new JButton("Exec");
        commandOptionsComboBox = new JComboBox<>();

        executeButton.addActionListener(new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                System.out.println("Action performed");
                commandOptionsComboBox.setEnabled(false);
            }
        });
    }

为什么我的JComboBox返回null？

3 个答案: