How do I provide a single button handler object

Question

I am completing a past paper exam question and it asks to create an applet that displays a green square in the center, with three buttons + , - and reset, however, I am trying to make it that when any button is clicked the program should essentially figure out which button was pressed. I know you would use e.getSource() but I am not sure how to go about this.

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

public class Square extends JApplet {

int size = 100;

public void init() {
    JButton increase = new JButton("+");
    JButton reduce = new JButton("-");
    JButton reset = new JButton("reset");

    SquarePanel panel = new SquarePanel(this);
    JPanel butPanel = new JPanel();

    butPanel.add(increase);
    butPanel.add(reduce);
    butPanel.add(reset);

    add(butPanel, BorderLayout.NORTH);
    add(panel, BorderLayout.CENTER);

    ButtonHandler bh1 = new ButtonHandler(this, 0);
    ButtonHandler bh2 = new ButtonHandler(this, 1);
    ButtonHandler bh3 = new ButtonHandler(this, 2);

    increase.addActionListener(bh1);
    reduce.addActionListener(bh2);
    reset.addActionListener(bh3);
}
}

class SquarePanel extends JPanel {
Square theApplet;

SquarePanel(Square app) {
    theApplet = app;
}

public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.setColor(Color.green);
    g.fillRect(10, 10, theApplet.size, theApplet.size);
}
}

class ButtonHandler implements ActionListener {
Square theApplet;
int number;

ButtonHandler(Square app, int num) {
    theApplet = app;
    number = num;
}

public void actionPerformed(ActionEvent e) {
    switch (number) {
        case 0:
            theApplet.size = theApplet.size + 10;
            theApplet.repaint();
            break;
        case 1:
            if (theApplet.size > 10) {
                theApplet.size = theApplet.size - 10;
                theApplet.repaint();
            }
            break;
        case 2:
            theApplet.size = 100;
            theApplet.repaint();
            break;
    }
}

Answer 1

Not the best way to do it, but based on your current code, you could simply compare the object references. You'd need to pass in references to the buttons or access them some other way. e.g.

if(e.getSource() == increase) { \\do something on increase}

Another alternative would be to check the string of the button, e.g.

if(((JButton)e.getSource()).getText().equals("+")){ \\do something on increase}

You can use strings in a switch statement in Java 8, but if you're using Java 7 or lower, it has to be an if statement.