Recieving an ActionListener as a constructor parameter and storing it so other methods in the class can add that action listener to buttons?

So I have a NumberPad class, with a class constructor NumberPad(ActionListener listener). I then need to add listener to a button in the addButton() method within the NumberPad class below.
This is specifically how my Professor is asking this to be done. He even says "Modify the constructor in the NumberPad class so that it is now passed an ActionListenerobject that it stores in a private attribute named listener". But I can not figure out how to store an Action Listener object.

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

;/**
 * A numeric keypad
 */
public class NumberPad extends    JPanel
{
    /**
     * Default Constructor
     */
    public NumberPad(ActionListener listener)
    {
        super();
        setupLayout();
    }

    /**
     * Setup and layout this NumberPad
     */
    private void setupLayout()
    {
        // Setup and layout this NumberPad
    	GridLayout pinpad = new GridLayout (4, 3);
    	setLayout(pinpad);
    	addButton("1");
    	addButton("2");
    	addButton("3");
    	addButton("4");
    	addButton("5");
    	addButton("6");
    	addButton("7");
    	addButton("8");
    	addButton("9");
    	addButton("Del");
    	addButton("0");
    	addButton("C");
    }
    
    private void addButton(String anyString)
    {
    	JButton anyButton = new JButton(anyString);
    	add(anyButton);
    }
    
}```

</details>


# 答案1
**得分**: 0

```java
public class NumberPad extends JPanel
{
    private ActionListener listener;

    public NumberPad(ActionListener listener)
    {
        super();
        this.listener = listener;
        setupLayout();
    }

    private void setupLayout()
    {
        GridLayout pinpad = new GridLayout(4, 3);
        setLayout(pinpad);
        addButton("1");
        addButton("2");
        addButton("3");
        addButton("4");
        addButton("5");
        addButton("6");
        addButton("7");
        addButton("8");
        addButton("9");
        addButton("Del");
        addButton("0");
        addButton("C");
    }
    
    private void addButton(String anyString)
    {
        JButton anyButton = new JButton(anyString);
        add(anyButton);
    }
}

public class NumberPad extends    JPanel
{
    private ActionListener listener;
    /**
     * Default Constructor
     */
    public NumberPad(ActionListener listener)
    {
        super();
        // Store as private attribute here
        this.listener = listener;
        setupLayout();
    }

    /**
     * Setup and layout this NumberPad
     */
    private void setupLayout()
    {
        // Setup and layout this NumberPad
        GridLayout pinpad = new GridLayout (4, 3);
        setLayout(pinpad);
        addButton("1");
        addButton("2");
        addButton("3");
        addButton("4");
        addButton("5");
        addButton("6");
        addButton("7");
        addButton("8");
        addButton("9");
        addButton("Del");
        addButton("0");
        addButton("C");
    }
    
    private void addButton(String anyString)
    {
        JButton anyButton = new JButton(anyString);
        add(anyButton);
        // You can use `this.listener` to access the stored private attribute
    }
    
}