Simple GUI Calculator in Java

Hello world! I'm back. It’s been four years since my last post. I’d been very busy with work in these past few years that I haven’t had time to blog. I missed blogging somehow and this year I figure I’ll get back to blogging again.

We are going to be making simple programming projects starting from now and for our first project, let’s make a simplest calculator using Java. In this project we’ll be using BlueJ as our Java Development Tool.

1. Download BlueJ installer from https://www.bluej.org/ then install it.

2. Click Start>Click BlueJ.

3. Click Project>New Project>For the sake of example, let’s use Java as a project name.



4. Click New Class>Select Class in the class type radio buttons then enter Calc in the Class Name textbox.



5. Double-click the Calc object.



6. A Window containing default BlueJ source code will then appear. Press Ctrl + A to select all the codes, then Press Delete.



7. Enter the following code:

//imports the necessary packages.
//swing for frame, awt for controls, and event for button events
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
//creates a class named Calc and prepares it to accept an event
public class Calc extends JFrame implements ActionListener
{
//creates our controls
  JTextField ansTextField=new JTextField("",18);
  JButton oneButton=new JButton("1");
  JButton twoButton=new JButton("2");
  JButton plusButton=new JButton("+");
  JButton minusButton=new JButton("-");
  JButton equalsButton=new JButton("=");
 //declares three variables
  int intNum1,intNum2;
  String strOperator;
    public Calc()
{
   //creates two panels
   JPanel topPanel=new JPanel();
   JPanel bottomPanel=new JPanel();
   //create two layouts
   BorderLayout border=new BorderLayout(5,5);
   FlowLayout flow=new FlowLayout(FlowLayout.RIGHT,10,10);
   //apply the borderLayout to frame
   setLayout(border);
   //prepare our buttons to accept an event
   oneButton.addActionListener(this);
   twoButton.addActionListener(this);
   plusButton.addActionListener(this);
   minusButton.addActionListener(this);
   equalsButton.addActionListener(this);
   //adds the textbox to the top panel
   topPanel.add(ansTextField);
   //applies flow layout to bottom panel
   bottomPanel.setLayout(flow);
   //add the buttons to the bottom panel
   bottomPanel.add(oneButton);
   bottomPanel.add(twoButton);
   bottomPanel.add(plusButton);
   bottomPanel.add(minusButton);
   bottomPanel.add(equalsButton);
   add(topPanel,BorderLayout.NORTH);
   add(bottomPanel);
   //setup our frame
   setSize(200,200);
   setLocation(0,0);
   setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   setTitle("Calculator");
   setVisible(true);
 
}
public void actionPerformed(ActionEvent e)
{
    //getSource determines the name of the control that recieves an event
    Object source=e.getSource();
    //if the name of the control that recieves an event is oneButton, display 1 in the textfield
    if(source==oneButton)
    {
        ansTextField.setText(ansTextField.getText() + oneButton.getText());
    }
    //if the name of the control is twoButton, display 2 in the textfield
    //you can add else ifs for numbers 3 to 9. I just leave that to you to figure out
    else if(source==twoButton)
    {
         ansTextField.setText(ansTextField.getText() + twoButton.getText());
     
    }
    //if it is plusButton, get the value displayed in the textfield and place it
    //in a variable named intNum1
    //Store "+" in the strOperator variable then clear the text field
    else if(source==plusButton)
    {
        intNum1=Integer.parseInt(ansTextField.getText());
        strOperator="+";
        ansTextField.setText("");
        
    }
   //if it is minusButton, get the value displayed in the textfield and place it
   //in a variable named intNum1
   //store "-" in the strOperator variable then clear the text field
   else if(source==minusButton)
    {
        intNum1=Integer.parseInt(ansTextField.getText());
        strOperator="-";
        ansTextField.setText("");
        
    }
    //if the name of the control that recieves an event is equals button
    //get the current value of the text field and assign it as a value of intNum2
    //if the current value of strOperator is + add the two numbers, if it's value is -
    //subtract the two numbers
    else if(source==equalsButton)
    {
        intNum2=Integer.parseInt(ansTextField.getText());
        if(strOperator=="+")
        {
          ansTextField.setText(Integer.toString(intNum1 + intNum2)); 
        }
          if(strOperator=="-")
        {
          ansTextField.setText(Integer.toString(intNum1 - intNum2)); 
        }
        
    }
}
//Creates an instance of our class so that we can run it
public static void main(String[] args)
{
  Calc c=new Calc();  
}
}

8. Click the compile button.

9. Close the code window.

10. Right-click the class object>Select void main(String[] args) to run your application.



11. You should then see the following output:



You can use this and make a complete basic calculator. Of course I leave that to you to figure out. If you are having a hard time, you can download the source code here Good luck!