A calculator is a device that performs arithmetic operations on numbers. This is a simple calculator written in the Java programming language. This calculator can do only addition, subtraction, multiplication, and division. 

import javax.swing.*;
import java.awt.event.*;
public class Calculator extends JFrame {
    private JButton btn0;
    private JButton btn1;
    private JButton btn2;
    private JButton btn3;
    private JButton btn4;
    private JButton btn5;
    private JButton btn6;
    private JButton btn7;
    private JButton btn8;
    private JButton btn9;
    private JButton btndivide;
    private JButton btndot;
    private JButton btnequal;
    private JButton btnminus;
    private JButton btnmultiply;
    private JButton btnplus;
    private JButton ntnclear;
    private JTextField txtbox;  
   float num1 = 0;
   int count = 0;
   float result = 0;

 public Calculator() 
 {
txtbox = new JTextField();
ntnclear = new JButton();
btndivide = new JButton();
btn4 = new JButton();
btnplus = new JButton();
btn9 = new JButton();
btn7 = new JButton();
btn5 = new JButton();
btn6 = new JButton();
btnmultiply = new JButton();
btn1 = new JButton();
btn2 = new JButton();
btn3 = new JButton();
btnminus = new JButton();
btn0 = new JButton();
btndot = new JButton();
btnequal = new JButton();
btn8 = new JButton();
getContentPane().add(txtbox);
txtbox.setBounds(1, 0, 319, 40);
 ntnclear.setText("Clear");
 ntnclear.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
         ntnclearActionPerformed(evt);
     }
 });

 getContentPane().add(ntnclear);
 ntnclear.setBounds(0, 50, 320, 40);
 ntnclear.getAccessibleContext().setAccessibleName("btnClear");
 btndivide.setText("/");

 btndivide.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
         btndivideActionPerformed(evt);
     }
 });

 getContentPane().add(btndivide);
 btndivide.setBounds(240, 90, 80, 60);
 btn4.setText("4");
 
 btn4.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn4ActionPerformed(evt);
     }
 });

 getContentPane().add(btn4);
 btn4.setBounds(0, 150, 80, 60);

 btnplus.setText("+");
 btnplus.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
         btnplusActionPerformed(evt);
     }
 });

 getContentPane().add(btnplus);
 btnplus.setBounds(240, 270, 80, 60);
 btn9.setText("9");
 btn9.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn9ActionPerformed(evt);
     }
 });
 getContentPane().add(btn9);;
 btn9.setBounds(160, 90, 80, 60);
 btn7.setText("7");
 btn7.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn7ActionPerformed(evt);
     }
 });
 
 getContentPane().add(btn7);
 btn7.setBounds(0, 90, 80, 60);

 btn5.setText("5");
 btn5.addActionListener(new java.awt.event.ActionListener() {
  public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn5ActionPerformed(evt);
     }
 });

 getContentPane().add(btn5);
 btn5.setBounds(80, 150, 80, 60);
 btn6.setText("6");
 btn6.addActionListener(new java.awt.event.ActionListener() {
 public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn6ActionPerformed(evt);
     }
 });

 getContentPane().add(btn6);
 btn6.setBounds(160, 150, 80, 60);

 btnmultiply.setText("*");
 btnmultiply.addActionListener(new java.awt.event.ActionListener() {
 public void actionPerformed(java.awt.event.ActionEvent evt) {
         btnmultiplyActionPerformed(evt);
     }
 });

 getContentPane().add(btnmultiply);
 btnmultiply.setBounds(240, 150, 80, 60);
 btn1.setText("1");
 btn1.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn1ActionPerformed(evt);
     }
 });

 getContentPane().add(btn1);
 btn1.setBounds(0, 210, 80, 60);
 btn2.setText("2");
 btn2.addActionListener(new java.awt.event.ActionListener() {

 public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn2ActionPerformed(evt);
     }
 });

 getContentPane().add(btn2);
 btn2.setBounds(80, 210, 80, 60);
 btn3.setText("3");
 btn3.addActionListener(new java.awt.event.ActionListener() {

public void actionPerformed(java.awt.event.ActionEvent evt) {
         btn3ActionPerformed(evt);
     }
 });

 getContentPane().add(btn3);
 btn3.setBounds(160, 210, 80, 60);

btnminus.setText("-");
btnminus.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
         btnminusActionPerformed(evt);
     }
 });

 getContentPane().add(btnminus);
 btnminus.setBounds(240, 210, 80, 60);
 btn0.setText("0");

 btn0.addActionListener(new java.awt.event.ActionListener() {
 public void actionPerformed(java.awt.event.ActionEvent evt) {
  btn0ActionPerformed(evt);

     }
 });

 getContentPane().add(btn0);
 btn0.setBounds(0, 270, 80, 60);
 btndot.setText(".");

 btndot.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
btndotActionPerformed(evt);
}
 });

 getContentPane().add(btndot);
 btndot.setBounds(80, 270, 80, 60);
 btnequal.setText("=");
 btnequal.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
btnequalActionPerformed(evt);
     }
 });

 getContentPane().add(btnequal);
 btnequal.setBounds(160, 270, 80, 60);
 btn8.setText("8");
 btn8.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {

btn8ActionPerformed(evt);
     }
 });

 getContentPane().add(btn8);
 btn8.setBounds(80, 90, 80, 60);
 setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 getContentPane().setLayout(null);
 pack();
 setSize(336,370);
 setVisible(true);
  }                        

 private void ntnclearActionPerformed(java.awt.event.ActionEvent evt) {                                         
  txtbox.setText(""); 
  num1 = 0;
  count = 0;
  }                                        

  private void btn7ActionPerformed(java.awt.event.ActionEvent evt) {                                     
  txtbox.setText(txtbox.getText() + "7");  
  }                                    

  private void btnplusActionPerformed(java.awt.event.ActionEvent evt) { 
  try{
  num1 = Float.parseFloat(txtbox.getText());
  txtbox.setText("");
  count = 2;        
  }
  catch (Exception e){
  }
  }                                       

  private void btnminusActionPerformed
(java.awt.event.ActionEvent evt) {    
  try{
  num1 = Float.parseFloat(txtbox.getText());
  txtbox.setText("");
  count = 1;           
  }
  catch (Exception e){
  }
  }                                        

  private void btnmultiplyActionPerformed

(java.awt.event.ActionEvent evt) {  
  try{
  num1 = Float.parseFloat(txtbox.getText());
  txtbox.setText("");
  count = 3;      

  }
  catch (Exception e){
  }
  }                                           

  private void btndivideActionPerformed(java.awt.event.ActionEvent evt) {  
  try{
  num1 = Float.parseFloat(txtbox.getText());
  txtbox.setText("");
  count = 4;         
  }

  catch (Exception e){
  }
  }

  private void btnequalActionPerformed
(java.awt.event.ActionEvent evt) {                                         
     compute(count);

  }                                        
  private void btndotActionPerformed(java.awt.event.ActionEvent evt) {                                       
          int c = txtbox.getText().length();
          int flag = 0;
          String text = txtbox.getText();

          for (int i = 0; i < c; i++)
          { 
              if (text.charAt(i) == '.') 
              { 
                  flag = 1; break;
              } 
              else 
              { 
                  flag = 0; 
              } 
          }
          if (flag == 0)
          { 
              txtbox.setText(txtbox.getText() + "."); 
          }
  }                                      
  private void btn0ActionPerformed(java.awt.event.ActionEvent evt) {                                     
      txtbox.setText(txtbox.getText() + "0"); 
  }                                    

  private void btn8ActionPerformed(java.awt.event.ActionEvent evt) {                                     
  txtbox.setText(txtbox.getText() + "8");        
  }                                    

  private void btn9ActionPerformed(java.awt.event.ActionEvent evt) {                                     
      txtbox.setText(txtbox.getText() + "9"); 
  }                                    

  private void btn4ActionPerformed(java.awt.event.ActionEvent evt) {                                     
      txtbox.setText(txtbox.getText() + "4"); 
  }                                    
  private void btn5ActionPerformed(java.awt.event.ActionEvent evt) {                                     
      txtbox.setText(txtbox.getText() + "5"); 
  }                                    

  private void btn6ActionPerformed(java.awt.event.ActionEvent evt) {                                     
      txtbox.setText(txtbox.getText() + "6"); 
  }                                    

  private void btn1ActionPerformed(java.awt.event.ActionEvent evt) {                                     
      txtbox.setText(txtbox.getText() + "1"); 
  }                                    

  private void btn2ActionPerformed(java.awt.event.ActionEvent evt) {                                     
     txtbox.setText(txtbox.getText() + "2"); 
  }                                    

  private void btn3ActionPerformed(java.awt.event.ActionEvent evt) {                                     
    txtbox.setText(txtbox.getText() + "3"); 
  }                                    

  public void compute(int count)
      {
          int resultPART2;

          switch (count)
          {
        case 1:

      result = num1 - Float.parseFloat(txtbox.getText());
      resultPART2 = (int)result;

      if (Float.toString(result).endsWith (".0"))
      {
         txtbox.setText(Integer.toString(resultPART2));
      }
      else
      {
         txtbox.setText(Float.toString(result));  
      }    

      break;
  case 2:
      result = num1 + Float.parseFloat(txtbox.getText());
      resultPART2 = (int)result;

      if (Float.toString(result).endsWith (".0"))
      {
         txtbox.setText(Integer.toString(resultPART2));
      }
      else
      {
         txtbox.setText(Float.toString(result));  
      }  
    break;
  case 3:
      result = num1 * Float.parseFloat(txtbox.getText());
      resultPART2 = (int)result;

      if (Float.toString(result).endsWith (".0"))
      {
         txtbox.setText(Integer.toString(resultPART2));
      }
      else
      {
         txtbox.setText(Float.toString(result));  
      }  
      break;
  case 4:

      result = num1 / Float.parseFloat(txtbox.getText());

      resultPART2 = (int)result;

      if (Float.toString(result).endsWith (".0"))
      {
         txtbox.setText(Integer.toString(resultPART2));
      }
      else
      {
         txtbox.setText(Float.toString(result));  
      } 
      break;
  default:
      break;
          }
      }

  public static void main(String args[]) {
      try {

          for (javax.swing.UIManager.LookAndFeelInfo info : 

javax.swing.UIManager.getInstalledLookAndFeels()) {

              if ("Nimbus".equals(info.getName())) {

javax.swing.UIManager.setLookAndFeel(info.getClassName());

                  break;
              }
          }

      } catch (ClassNotFoundException ex) {

 java.util.logging.Logger.getLogger(NewJFrame1.class.getName()).

log(java.util.logging.Level.SEVERE, null, ex);

      } catch (InstantiationException ex) {

  java.util.logging.Logger.getLogger(NewJFrame1.class.getName()).

log(java.util.logging.Level.SEVERE, null, ex);

      } catch (IllegalAccessException ex) {

java.util.logging.Logger.getLogger(NewJFrame1.class.getName()).

log(java.util.logging.Level.SEVERE, null, ex);

      } catch (javax.swing.UnsupportedLookAndFeelException ex) {
java.util.logging.Logger.getLogger(NewJFrame1.class.getName()).
log(java.util.logging.Level.SEVERE, null, ex);
      }

       new Calculator();
  }
}