java - setContentPane() 和 addActionListener 收到 NullPointerException

Question

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

public class Form1 {
   private JPanel panel1;
   private JButton button1;


   public Form1() {
       button1.addActionListener(new ActionListener() {
          @Override
          public void actionPerformed(ActionEvent e) {
             System.out.println("Button clicked");
          }
       });
    }

    public static void main(String args[]){
       JFrame frame = new JFrame("Form 1");
       frame.setContentPane(new Form1().panel1);
       frame.pack();
       frame.setVisible(true);
    }
}

该错误与 main 方法中的 setContentPane 以及 actionListener 相关。我已在下面发布了确切的错误。为什么是这样？我使用 IntelliJ IDEA GUI 表单创建器创建了这个。

Exception in thread "main" java.lang.NullPointerException  
    at Form1.<init>(Form1.java:12)  
    at Form1.main(Form1.java:22)

Answer 1

在Java中你不能使用未初始化的变量，否则你将收到一个NPE。请参阅以下代码并请阅读一本Java书籍:)

public class Form1 {
   private JPanel panel1;
   private JButton button1;


   public Form1() {
      panel1 = new JPanel();
      button1 = new JButton1("Press Me");
      panel1.add(button1);
       button1.addActionListener(new ActionListener() {
          @Override
          public void actionPerformed(ActionEvent e) {
             System.out.println("Button clicked");
          }
       });
    }

    public static void main(String args[]){
       JFrame frame = new JFrame("Form 1");
       frame.setContentPane(new Form1().panel1);
       frame.pack();
       frame.setVisible(true);
    }
}