Button zuweisen (Java) - Frage.

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

public class Hausaufgabe extends JFrame {
    JButton button_1 = new JButton("Addieren");
    JButton button_2 = new JButton("Subtrahieren");
    JLabel label_1 = new JLabel("0");
    FlowLayout layout_1 = new FlowLayout();
    int zaehler = 0;
    
    ActionListener addierer= new ActionListener(){
        public void actionPerformed(ActionEvent e){
        addieren();
        }};
     ActionListener subtrahierer= new ActionListener(){
        public void actionPerformed(ActionEvent e){
        subtrahieren();
        }};


public Hausaufgabe(){
    this.getContentPane().add(button_1);
    this.getContentPane().add(label_1);
    this.getContentPane().add(button_2);
    setLayout(new FlowLayout());
    this.setTitle("Hausaufgabe");
    this.setBounds(300,350,250,100);
    this.setVisible(true);
    
    button_1.addActionListener(addierer);
    button_1.addActionListener(subtrahierer);
    
}

private void addieren(){
    zaehler++;
    label_1.setText(""+zaehler);
}

private void subtrahieren(){
    zaehler--;
    label_1.setText(""+zaehler);
}}