如何用for循环创建N个ActionListener？

Question

我用 for 循环创建了 N(unknown SIZE) 个 Jbuttons 并将它们放在框架中。我试图通过单击按钮编号 x 来显示消息 "Button number x"。在第二个 for 循环中，当我尝试创建 N ActionListener 时出现一些错误。

错误

Local variable I defined in an enclosing scope must be final or effectively final

代码

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class program extends JFrame{
    final int N = 10;
    final int JUMPS = 210;
    private JButton[] buttons = new JButton[N];

    public program(){
        super("test");
        setLayout(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        for (int i = 0; i < N; i++) {
            buttons[i] = new JButton("" + i);
            buttons[i].setBounds((i % 4) * JUMPS, (i / 4) *JUMPS , 200, 200);
            add(buttons[i]);
        }

        for (int i = 0; i < N; i++) {
            buttons[i].addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent e) {
                    JOptionPane.showMessageDialog(null, "Button number " + i);
                }
            });
        } 

        pack();
        setVisible(true);
    }

    public static void main(String[] args) {
        program mainGUI = new program();
    }
}

您有解决错误的方法吗？

Answer 1

您不能在 ActionListener 实现中引用局部变量 i，除非 i 是最终的（或实际上是最终的）。但是你可以只创建一个最终变量并引用它。

for (int i = 0; i < N; i++) {
    final int final_i = i;
    buttons[i].addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            JOptionPane.showMessageDialog(null, "Button number " + final_i);
        }
    });
}

而在 Java 8 中，您可以更方便地执行此操作：

for (int i = 0; i < N; i++) {
    final int final_i = i;
    buttons[i].addActionListener(e ->
       JOptionPane.showMessageDialog(null, "Button number " + final_i)
    );
}

Answer 2

你可以

JOptionPane.showMessageDialog(null, "Button number " + e.getActionCommand());

Answer 3

如果匿名 class 是在任何方法中创建的，那么在方法中定义但在匿名 class 主体之外的所有局部变量都应该是最终的，以防需要使用它们匿名 class.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class program extends JFrame{
    final int N = 10;
    final int JUMPS = 210;
    private JButton[] buttons = new JButton[N];

    public program(){
        super("test");
        setLayout(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        for (int i = 0; i < N; i++) {
            buttons[i] = new JButton("" + i);
            buttons[i].setBounds((i % 4) * JUMPS, (i / 4) *JUMPS , 200, 200);
            add(buttons[i]);
        }

        for (int i = 0; i < N; i++) {
        final int buttonNumber = i ;
            buttons[i].addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent e) {
                    JOptionPane.showMessageDialog(null, "Button number " + buttonNumber);
                }
            });
        } 

        pack();
        setVisible(true);
    }

    public static void main(String[] args) {
        program mainGUI = new program();
    }
}

如何用for循环创建N个ActionListener？

How to create N ActionListener with for loop?

java

swing

loops

jbutton

actionlistener