1. Computing

Simple Calculator Example Program

By

Java Code:

The following Java code listing is for a simple calculator. There are nine JButtons to represent the numbers 1 to 9, and three JButtons for addition, subtraction and totaling the result. A JTextField at the top keeps track of the numbers being pressed and the result of the arithmetic operation.

The purpose of this Java listing is to show how to implement an ActionListener interface for handling JButton button event clicks by using the containing class, an inner class and an anonymous inner class.

The article that goes with this program listing is A Simple Calculator Handling Button Events.

//Imports are listed in full to show what's being used
//could just import javax.swing.* and java.awt.* etc..
import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.BorderLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JButton;
import java.awt.Container;

public class SimpleCalc implements ActionListener{
 
    JFrame guiFrame;
    JPanel buttonPanel;
    JTextField numberCalc;
    int calcOperation = 0;
    int currentCalc;
    
    //Note: Typically the main method will be in a
    //separate class. As this is a simple one class
    //example it's all in the one class.
    public static void main(String[] args) {
     
         //Use the event dispatch thread for Swing components
         EventQueue.invokeLater(new Runnable()
         {
             
            @Override
             public void run()
             {
                 
                 new SimpleCalc();         
             }
         });
              
    }
    
    public SimpleCalc()
    {
        guiFrame = new JFrame();
        
        //make sure the program exits when the frame closes
        guiFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        guiFrame.setTitle("Simple Calculator");
        guiFrame.setSize(300,300);
      
        //This will center the JFrame in the middle of the screen
        guiFrame.setLocationRelativeTo(null);
        
        numberCalc = new JTextField();
        numberCalc.setHorizontalAlignment(JTextField.RIGHT);
        numberCalc.setEditable(false);
        
        guiFrame.add(numberCalc, BorderLayout.NORTH);
        
        buttonPanel = new JPanel();
               
        //Make a Grid that has three rows and four columns
        buttonPanel.setLayout(new GridLayout(4,3));   
        guiFrame.add(buttonPanel, BorderLayout.CENTER);
        
        //Add the number buttons
        for (int i=1;i<10;i++)
        {
            addButton(buttonPanel, String.valueOf(i));
        }

        JButton addButton = new JButton("+");
        addButton.setActionCommand("+");
        
        OperatorAction subAction = new OperatorAction(1);
        addButton.addActionListener(subAction);
        
        JButton subButton = new JButton("-");
        subButton.setActionCommand("-");
        
        OperatorAction addAction = new OperatorAction(2);
        subButton.addActionListener(addAction);
        
        JButton equalsButton = new JButton("=");
        equalsButton.setActionCommand("=");
        equalsButton.addActionListener(new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent event)
            {
                if (!numberCalc.getText().isEmpty())
                {
                    int number = Integer.parseInt(numberCalc.getText()); 
                    if (calcOperation == 1)
                    {
                        int calculate = currentCalc  + number;
                        numberCalc.setText(Integer.toString(calculate));
                    }
                    else if (calcOperation == 2)
                    {
                        int calculate = currentCalc  - number;
                        numberCalc.setText(Integer.toString(calculate));
                    }
                }
            }
        });
        
        buttonPanel.add(addButton);
        buttonPanel.add(subButton);
        buttonPanel.add(equalsButton);
        guiFrame.setVisible(true);  
    }
    
    //All the buttons are following the same pattern
    //so create them all in one place.
    private void addButton(Container parent, String name)
    {
        JButton but = new JButton(name);
        but.setActionCommand(name);
        but.addActionListener(this);
        parent.add(but);
    }
    
    //As all the buttons are doing the same thing it's
    //easier to make the class implement the ActionListener
    //interface and control the button clicks from one place
    @Override
    public void actionPerformed(ActionEvent event)
    {
        //get the Action Command text from the button
        String action = event.getActionCommand();
        
        //set the text using the Action Command text
        numberCalc.setText(action);       
    }
    
    private class OperatorAction implements ActionListener
    {
        private int operator;
        
        public OperatorAction(int operation)
        {
            operator = operation;
        }
        
        public void actionPerformed(ActionEvent event)
        {
            currentCalc = Integer.parseInt(numberCalc.getText()); 
            calcOperation = operator;
        }
    }
}


  1. About.com
  2. Computing
  3. Java
  4. Create User Interfaces
  5. Handling Events
  6. Simple Calculator Example Program

©2014 About.com. All rights reserved.