用於多個按鈕的ActionListener-Swing

Question

public class string implements ActionListener{ 

    JLabel jlab; 

    public void actionPerformed(ActionEvent Ae){ 
     jlab.setText(Ae.getActionCommand()); 
    } 

     public static void main(String args[]){ 
      JFrame j = new JFrame("HP"); 
      j.setSize(300,300); 
      j.setLayout(new FlowLayout()); 
      j.setDefaultCloseOperation(j.EXIT_ON_CLOSE); 
      j.setVisible(true);  

     jlab=new JLabel("Here"); 
     j.add(jlab); 

      JButton j1=new JButton("Button1"); 
      j1.setActionCommand("Your pressed Button 1"); 

      j1.addActionListener(this); 

      j.add(j1); 

      JButton j2=new JButton("Button2"); 
      j2.setActionCommand("Your pressed Button 1"); 
      j2.addActionListener(this); 
      j.add(j2); 
    } 
    } 

我想用同一個EventHandler處理多個按鈕的操作。點擊按鈕時，標籤的文字將相應更改。

此代碼顯示錯誤

Non static variable this cannot be referenced from a static context. 

我怎樣才能糾正這種代碼？

Answer 1

有兩個解決問題的方法：

1. 使用構造

   public class buttons1 implements ActionListener{ 
   
        JLabel jlab; 
   
        buttons1(){ 
   
   
          JFrame j = new JFrame("HP"); 
          j.setSize(300,300); 
          j.setLayout(new FlowLayout()); 
          j.setDefaultCloseOperation(j.EXIT_ON_CLOSE); 
          j.setVisible(true);  
   
          jlab=new JLabel("Here"); 
          j.add(jlab); 
   
          JButton j1=new JButton("Button1"); 
          j1.setActionCommand("Your pressed Button 1"); 
   
          j1.addActionListener(this); 
   
          j.add(j1); 
   
          JButton j2=new JButton("Button2"); 
          j2.setActionCommand("Your pressed Button 2"); 
          j2.addActionListener(this); 
          j.add(j2); 
   
   
        } 
   
        public void actionPerformed(ActionEvent Ae){ 
   
           jlab.setText(Ae.getActionCommand()); 
   
        } 
   
   
   
        public static void main(String args[]){ 
         new buttons1(); 
        } } 
   

2. 或者內的主要創建同一個類的對象，並把它傳遞給addActionListener方法。也使JLabel靜態。

   public class buttons2 implements ActionListener{ 
       static JLabel jlab; 
   
       public void actionPerformed(ActionEvent Ae){ 
   
             jlab.setText(Ae.getActionCommand()); 
   
            } 
   
   
   
       public static void main(String args[]){ 
   
        buttons2 s = new buttons2(); 
   
        JFrame j = new JFrame("HP"); 
         j.setSize(300,300); 
         j.setLayout(new FlowLayout()); 
         j.setDefaultCloseOperation(j.EXIT_ON_CLOSE); 
         j.setVisible(true);  
   
         jlab=new JLabel("Here"); 
         j.add(jlab); 
   
         JButton j1=new JButton("Button1"); 
         j1.setActionCommand("Your pressed Button 1"); 
   
         j1.addActionListener(s); 
   
         j.add(j1); 
   
         JButton j2=new JButton("Button2"); 
         j2.setActionCommand("Your pressed Button 2"); 
         j2.addActionListener(s); 
         j.add(j2); 
        } 
       } 
   

Answer 2

瞭解什麼是static變量以及它的工作原理非常重要。靜態變量不能從非靜態上下文中引用。

import javax.swing.JLabel; 
import javax.swing.JFrame; 
import javax.swing.JButton; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 

import java.awt.FlowLayout; 

public class Stackoverflow implements ActionListener{ 

    private JLabel jlab; /* It's a good practice to make your variables private or protected */ 



    public static void main(String args[]){ 
     new Stackoverflow(); 
    } 
    public Stackoverflow(){ 
      JFrame j = new JFrame("HP"); 
      j.setSize(300,300); 
      j.setLayout(new FlowLayout()); 
      j.setDefaultCloseOperation(j.EXIT_ON_CLOSE); 
      j.setVisible(true);  

      jlab=new JLabel("Here"); 
      j.add(jlab); 

      JButton j1=new JButton("Button1"); 
      j1.setActionCommand("Your pressed Button 1"); 

      j1.addActionListener(this); 

      j.add(j1); 

      JButton j2=new JButton("Button2"); 
      j2.setActionCommand("Your pressed Button 2"); 
      j2.addActionListener(this); 
      j.add(j2); 
    } 
    public void actionPerformed(ActionEvent Ae){ 
     jlab.setText(Ae.getActionCommand()); 
    } 
}