Code

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

public class Calculadora extends JFrame implements ActionListener {

  private JLabel label1, label2, label3;
  private JTextField valor1, valor2;
  private JButton suma, resta, div, mul;

  public Calculadora() {

    setLayout(null);

    label1 = new JLabel("Numero 1: ");
    label1.setBounds(10, 10, 100, 20);
    add(label1);
  
    label2 = new JLabel("Numero 2: ");
    label2.setBounds(10, 50, 100, 20);
    add(label2);

    label3 = new JLabel("Resultado: ");
    label3.setBounds(10, 140, 120, 20);
    add(label3);

    valor1 = new JTextField();
    valor1.setBounds(100, 10, 100, 20);
    add(valor1);

    valor2 = new JTextField();
    valor2.setBounds(100, 50, 100, 20);
    add(valor2);

    suma = new JButton("SUMAR");
    suma.setBounds(230, 10, 90, 20);
    add(suma);
    suma.addActionListener(this);

    resta = new JButton("RESTAR");
    resta.setBounds(230, 50, 90, 20);
    add(resta);
    resta.addActionListener(this);

    mul = new JButton("MULTI");
    mul.setBounds(230, 80, 90, 20);
    add(mul);
    mul.addActionListener(this);

    div = new JButton("DIV");
    div.setBounds(230, 110, 90, 20);
    add(div);
    div.addActionListener(this);

  }

  public void actionPerformed(ActionEvent e) {
    if(e.getSource() == suma) {

       int n1 = 0, n2 = 0, total = 0;
       n1 = Integer.parseInt(valor1.getText());
       n2 = Integer.parseInt(valor2.getText());
       total = n1 + n2;

       label3.setText("Resultado: " + total);
    }

    if(e.getSource() == resta) {

       int n1 = 0, n2 = 0, total = 0;
       n1 = Integer.parseInt(valor1.getText());
       n2 = Integer.parseInt(valor2.getText());
       total = n1 - n2;

       label3.setText("Resultado: " + total);
    }

    if(e.getSource() == mul) {

       int n1 = 0, n2 = 0, total = 0;
       n1 = Integer.parseInt(valor1.getText());
       n2 = Integer.parseInt(valor2.getText());
       total = n1 * n2;

       label3.setText("Resultado: " + total);
    }

    if(e.getSource() == div) {

       int n1 = 0, n2 = 0, total = 0;
       n1 = Integer.parseInt(valor1.getText());
       n2 = Integer.parseInt(valor2.getText());
       total = n1 / n2;

       label3.setText("Resultado: " + total);
    }

  } 

  public static void main(String args[]) {

    Calculadora ventana = new Calculadora();

    ventana.setBounds(0, 0, 360,200);
    ventana.setVisible(true);
    ventana.setResizable(false);
    ventana.setLocationRelativeTo(null);
    ventana.setTitle("Calculadora");
  }
}