Numeric Input

Taking numbers as input and then being able to use this input as a numeric value is not as easy as you might think. Remember when you take input from the keyboard it is saved like this:

    input += k.getkeyChar();

where input is of type String. You can't perform arithmetic on two Strings. You must convert the values to a numeric type such as int, float, or double. Here's a short example:

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class numeric extends Applet{
  String input;
  int output;
  
  public void init(){
    setBackground(Color.blue);
    input = "";
    requestFocus();
    this.addKeyListener(new KeyAdapter(){
      public void keyPressed(KeyEvent k) { 
        if(k.getKeyCode() == KeyEvent.VK_ENTER){
          try{
            output = Integer.parseInt(input);
          }catch(NumberFormatException nfe){
            output = -555; //error code
          }
        }
        else if(k.getKeyCode() == KeyEvent.VK_DELETE){
          input = "";
          output = 0;
        }
        else if(k.getKeyCode() == KeyEvent.VK_BACK_SPACE){
          input = input.substring(0,input.length()-1);
        }
        else{
          input += k.getKeyChar();
        }
        repaint();
      }
    });
  }

public void paint(Graphics g){
    g.setColor(Color.yellow);
    g.drawString("INPUT: "+input, 10, 30);
    g.drawString("OUTPUT: "+output, 10, 50);
  }
}
//<applet code=numeric.class height=80 width=200></applet>

You should try entering decimal numbers and words in as input and see what happens.

The key lines that you should study are these:

          try{
            output = Integer.parseInt(input);
          }catch(NumberFormatException nfe){
            output = -555; //error code
          }

This is an example of exception handling. The Integer.parseInt method throws an exception if it is fed a String value which does not parse into an integer. So, we, as programmers, must handle the situation where a user may enter bad data and the try-catch construct helps us with this. You will notice that anytime a user enters bad input the value of output (an int variable) will be set to -555.
- - - - - - - - - - - - - - - - - - - - - - - - - - - -
Assignment:
You will create a simple calculator that specializes in adding two values. You will have two input strings and one output int variable.