FocusListener不能像其他Listener一样添加？

Question

在我的代码中，当我尝试将 FocusListener 添加到 JTextField 时，它说

ChatClient.java:30: error: incompatible types: ChatClient cannot be converted to FocusListener
    text.addFocusListener(this);

但是添加 MouseListener 效果很好。为什么会这样？用 FocusListener 创建另一个 class 是可行的。我想知道添加 MouseListener 和 FocusListener 之间的区别。有没有其他方法可以简单地添加 FocusListener 而无需为其编写单独的 class？

public void makeUI(){
    text = new JTextField(defaultMessage);
    text.setBounds(10,620,295,40);
    text.addFocusListener(this);
    add(text);
    button = new JButton("SEND");
    button.setBounds(310,620,80,40);
    button.setForeground(Color.WHITE);
    button.setBackground(Color.decode("#11A458"));
    button.setFocusPainted(false);
    button.addActionListener(this);
    add(button);
    setSize(400,700);
    setLayout(null);
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }
  public void focusGained(FocusEvent ae){
    if(text.getText().equals(defaultMessage)){
        text.setText("");
    }
  }
  public void focusLost(FocusEvent ae){
    if(text.getText().isEmpty()){
      text.setText(defaultMessage);
    }
  }

Answer 1

我看到您添加了 focusGained 和 focusLost 方法，但是您的 class 是否实现了 FocusListener 接口？

所以，代码应该是这样的：

/**
 * 1. this implements FocusListener is important
 * 2. another interfaces you had here before also should be present
 */
public class YourClass implements FocusListener {  

        public void makeUI(){
            text = new JTextField(defaultMessage);
            text.setBounds(10,620,295,40);
            text.addFocusListener(this);
            add(text);
            button = new JButton("SEND");
            button.setBounds(310,620,80,40);
            button.setForeground(Color.WHITE);
            button.setBackground(Color.decode("#11A458"));
            button.setFocusPainted(false);
            button.addActionListener(this);
            add(button);
            setSize(400,700);
            setLayout(null);
            setVisible(true);
            setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        }

        @Override
        public void focusGained(FocusEvent ae){
            if(text.getText().equals(defaultMessage)){
                text.setText("");
            }
        }

        @Override
        public void focusLost(FocusEvent ae){
            if(text.getText().isEmpty()){
                text.setText(defaultMessage);
            }
        }

}

FocusListener不能像其他Listener一样添加？

FocusListener cannot be added like other Listeners?

java

swing

actionlistener

mouselistener

focuslistener