Here is the Calculator code we wrote (5th period, but the classes were similar):

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
     
public class Calculator {
         
    private JLabel label;  //where we display the current value
    private int num;       //number shown on the display
    
    private class NumberListener implements ActionListener {
        int val;  //value of this button
        
        public NumberListener(int val) {
            this.val = val;
        }
        
        public void actionPerformed(ActionEvent e) {
            num = num*10 + val;
            label.setText(num + "");
        }
    }
    
    public Calculator() {
        JFrame frame = new JFrame("Calculator");
        frame.setLayout(new BorderLayout());
        
        JPanel bPanel = new JPanel();  //panel with all the buttons
        bPanel.setLayout(new GridLayout(4,5));
        
        JButton button7 = new JButton("7");
        button7.addActionListener(new NumberListener(7));
        bPanel.add(button7);
        JButton button8 = new JButton("8");
        button8.addActionListener(new NumberListener(8));
        bPanel.add(button8);
        JButton button9 = new JButton("9");
        button9.addActionListener(new NumberListener(9));
        bPanel.add(button9);
        JButton buttonDiv = new JButton("/");
        bPanel.add(buttonDiv);
        
        JButton button4 = new JButton("4");
        button4.addActionListener(new NumberListener(4));
        bPanel.add(button4);
        JButton button5 = new JButton("5");
        button5.addActionListener(new NumberListener(5));
        bPanel.add(button5);
        JButton button6 = new JButton("6");
        button6.addActionListener(new NumberListener(6));
        bPanel.add(button6);
        JButton buttonMul = new JButton("*");
        bPanel.add(buttonMul);
        
        JButton button1 = new JButton("1");
        button1.addActionListener(new NumberListener(1));
        bPanel.add(button1);
        JButton button2 = new JButton("2");
        button2.addActionListener(new NumberListener(2));
        bPanel.add(button2);
        JButton button3 = new JButton("3");
        button3.addActionListener(new NumberListener(3));
        bPanel.add(button3);
        JButton buttonMin = new JButton("-");
        bPanel.add(buttonMin);
        
        JButton button0 = new JButton("0");
        button0.addActionListener(new NumberListener(0));
        bPanel.add(button0);
        JButton buttonPlus = new JButton("+");
        bPanel.add(buttonPlus);
        
        frame.add(bPanel, BorderLayout.CENTER);
        
        label = new JLabel("0");  
        frame.add(label, BorderLayout.NORTH);
        num = 0;
        
        frame.pack();
        frame.setVisible(true);
    }
    
    public static void main(String[] args) {
        new Calculator();
    }
}