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。我基本上剪掉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
添加任何监听器。不确定这是否会成为问题:
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 actionPerformed()
方法之前的 commandListener
类中获取 targetEnvironmentComboBox
时,它会抛出一个 空指针异常。如果我删除它,我会在 disableUsedComponents()
方法中获取 targetEnvironmentComboBox
,其中 **,它能够成功获取组合框
。
此外,如果我执行 executeButton
监听器,它就能够获取对 CommandListener
类中所有组件的引用,但是如果我执行 >getCommandOptionsComboBox
,它为 executeButton
和 targetEnvironmentComboBox
返回 null。
谁能解释一下为什么会这样?
其次,我知道这可能不是最好的实现。对于我可以改变以遵循更好的实践的事情有什么建议吗?
最佳答案
您收到 null
,因为 targetEnvironmentComboBox
直到您尝试在 CommandListener 中访问之后后才会初始化
。当创建新的 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());
如果将 Display.getTargetEnvironmentComboBox()
放入该方法内,则在需要时不会获取引用的副本,此时它已正确初始化。
一个简单的解决方法,虽然不是正确的方法,但首先初始化它:
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,并将构造函数中访问正确对象所需的内容传递给 CommandListener
。我建议修改您的 CommandListener
类,如下所示:
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(确保传递非空值),并消除所有静态变量(或重构为更易于管理的内容)。
关于java - 为什么我的 JComboBox 返回 null?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/34550917/