How to Create A Basic Calculator In Java Applet In Advanced Way

In my last post I have shown How to Create A Basic Calculator In Java Applet.
That was too basic. The objective was to gain more knowledge on..

• how to add buttons
• how to add controls
• how to work with action listener

In this post we are going in but advanced way..


For creating a calculator, we need to define the requirement first.. These requirements will keep on adding whenever required. We will be cleared about how prototype model works.
So will create a prototype first and keep implementing the calculator.

The requirements are ..

• There will be buttons with number 1 to 9
• There will be simple arithmetic operation button like-‘+’,’-‘,’*’ and ‘/’
• There will an equal(‘=’) button
• The resultant will be displayed in a text box.

 How to Create A Basic Calculator In Java Applet Here I have told the basics. Now in this post I will show how to code in a compact manner.
For adding button 0 to 9.

for(int i=0;i<10;i++  )
{
button[i]=new Button(""+i);
//Naming of the button
add(button[i]);
//Adding the buttons
button[i].addActionListener(this);
//adding action listeners to those buttons
} 

Now on the action performed section-

//This section is bit tricky here we are finding from which button 
//we are getting the request
String output=event.getActionCommand();
char firstCharacter= output.charAt(0);
if(Character.isDigit(firstCharacter))
text1.setText(text1.getText()+ output);
//if the incoming character is digit we need to append 
//with the existing digits 
   if(output.equals("+ "))
   {
   m=Integer.parseInt(text1.getText());
   //taking the number provided as first number
   key1="add";
   //Setting the key based on that later we will 
   //perform operations
   text1.setText("");
   //Performing clean up activity
   }
   else if(output.equals("-"))
   {
   m=Integer.parseInt(text1.getText());
   key1="min";
   text1.setText("");
   }
   else if(output.equals("*"))
   {
    m=Integer.parseInt(text1.getText());
   key1="mul";
   text1.setText("");
   }
   else if(output.equals("/"))
   {
   m=Integer.parseInt(text1.getText());
   key1="div";
   text1.setText("");
   }
   else if(output.equals("C"))
   {
   //This is for clearing the screen.
   text1.setText("");
   }
 

The working process of equals button.

• It will take the second number
• based on the key1  value it will determine the operation
• It should also check the divide rule..the second number should not be zero.It should print error if second number is zero.
• Later it should setup the resultant error.

if(output.equals("="))
{
  n=Integer.parseInt(text1.getText());
  //Getting the second number
  if(key1=="add")
  {
  //if key is add then add operation will be performed 
  //on first number and second number
  res=m+n;
  }
  if(key1=="min")
  {
                res=m-n;
  }
  if(key1=="mul")
  {
  res=m*n;
  }
  if(key1=="div")
  {
  //Checking if the second number is 0 
  //or not to avoid divided by zero error
   if(n!=0)
    res=m/n;
   else
   //if second number is zero then it will print error
   res1="Error";
  }
    if(res1.equals(""))
    //Setting up the result
    text1.setText("" +res);
    else
    text1.setText("" +res1);
}
} 

Let me write the complete code here..

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class calculator_updated extends Applet implements ActionListener
{
//Initialize the text field
TextField text1;
//Create array of buttons
Button button[]=new Button[10];
//Create the buttons for special programs
Button add,sub,mul,div,equalto,clr;
String key1="",res1="";
int m=0,res=0,n=0;
public void init()
{
text1=new TextField(20);
//craete a layout
GridLayout layout=new GridLayout(4,5);
setLayout(layout);
add(text1);
//adding the textfield
for(int i=0;i<10;i++  )
{
button[i]=new Button("" +i);
//Naming of the button
add(button[i]);
//Adding the buttons
button[i].addActionListener(this);
//adding action listeners to those buttons
}
add=new Button("+" " + ");
sub=new Button("+" "-");
mul=new Button("+" "*");
div=new Button("+" "/");
equalto=new Button("+" "=");
clr=new Button("+" "C");
//Adding the special buttons and adding action listeners to it
add(add);
add.addActionListener(this);
add(sub);
sub.addActionListener(this);
add(mul);
mul.addActionListener(this);
add(div);
div.addActionListener(this);
add(equalto);
equalto.addActionListener(this);
add(clr);
clr.addActionListener(this);

}
public void actionPerformed(ActionEvent event)
{
//This section is bit tricky here we are finding from which button 
//we are getting the request
String output=event.getActionCommand();
char firstCharacter= output.charAt(0);
if(Character.isDigit(firstCharacter))
text1.setText(text1.getText() +output);
//if the incoming character is digit we need to append 
//with the existing digits 
   if(output.equals("+ "))
   {
   m=Integer.parseInt(text1.getText());
   //taking the number provided as first number
   key1="add";
   //Setting the key based on that later we will 
   //perform operations
   text1.setText("");
   //Performing clean up activity
   }
   else if(output.equals("-"))
   {
   m=Integer.parseInt(text1.getText());
   key1="min";
   text1.setText("");
   }
   else if(output.equals("*"))
   {
    m=Integer.parseInt(text1.getText());
   key1="mul";
   text1.setText("");
   }
   else if(output.equals("/"))
   {
   m=Integer.parseInt(text1.getText());
   key1="div";
   text1.setText("");
   }
   else if(output.equals("C"))
   {
   //This is for clearing the screen.
   text1.setText("");
   }


if(output.equals("="))
{
  n=Integer.parseInt(text1.getText());
  //Getting the second number
  if(key1=="add")
  {
  //if key is add then add operation will be performed 
  //on first number and second number
  res=m+n;
  }
  if(key1=="min")
  {
                res=m-n;
  }
  if(key1=="mul")
  {
  res=m*n;
  }
  if(key1=="div")
  {
  //Checking if the second number is 0 
  //or not to avoid divided by zero error
   if(n!=0)
    res=m/n;
   else
   //if second number is zero then it will print error
   res1="Error";
  }
    if(res1.equals(""))
    //Setting up the result
    text1.setText("" +res);
    else
    text1.setText("" +res1);
}
}
}

Well I have already explained this in my last post.Hence not going that much deep.
The HTML code will look like-

<HTML>
<HEAD>
</HEAD>
<BODY>
<div >
<applet width=300 height=300 code="calculator_updated.class"> </applet>
</APPLET>
</div>
</BODY>
</HTML> 

I tried to perform 2+3 by clicking on the 2 , then 3 and lastly =
I have got 5.