Reading a double from the Keyboard

The scheme used to represent integers is completely different from the scheme used to represent floating point. Even though you might regard 221 and 221.0 as equivalent, the bit patterns that represent each one are completely different.

Floating point input using java.io is similar to integer input. A Scanner object is used to scan through a stream of input characters and convert them into a float or a double. The methods that do this are nextFloat() and nextDouble().

Here is a program that converts a String of characters into primitive type double, and then prints out the value and twice the value:

// This program requires Java 1.5 or higher
//
import java.io.*;
import java.util.Scanner;

class DoubleDouble
{
  public static void main (String[] args)
  {
    double value;
    Scanner scan = new Scanner( System.in );
 
    System.out.print("Enter a double:");
    value = scan.nextDouble();

    System.out.println("value: " + value +" twice value: " + 2.0*value );
  }
}

Running the program writes the following to the screen:

C:\temp>java DoubleDouble
Enter a double: 3.14
value: 3.14 twice value: 6.28

It would be worth your effort to copy this program to a file and to compile and run it.