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In the last example you saw how to read a string. Now I will show you how to read a number. It is necessary to read a number if you want to perform any sort of mathematical operation on what the user has entered.

Probably the easiest way to read a number is to first read a String, and then convert it to a number. Java provides several methods to convert from a string to a numeric type. Which method you use depends on the type of number you want.

• byte: Byte.parseByte(str)
• double: Double.parseDouble(str)
• float: Float.parseFloat(str)
• int: Integer.parseInt(str)
• long: Long.parseLong(str)
• short: Short.parseShort(str)

For this example we will input a number, in miles, and convert that number into kilometers. Listing 4.2 shows this example program.

Listing 4.2: Input Numbers (InputNumbers.java)

import java.io.*;

public class InputNumbers
{
  public static void main(String args[])
  {
    try
    {
      InputStreamReader inputStreamReader = 
        new InputStreamReader ( System.in );
      BufferedReader in = 
        new BufferedReader ( inputStreamReader );
      System.out.print("Enter a length in miles? ");
      String miles = in.readLine();
      double dMiles = Double.parseDouble(miles);
      double dKilometers = 1.609344 *dMiles;
      System.out.println("That is " + dKilometers + 
        " kilometers.");
    }
    catch(IOException e)
    {
    }
  }
}

As you can see from the above program it is very similar to the previous example where a string was entered into the program.

However, once the string has been read into the variable “miles”, the string is converted into a double. The string is converted into a double, rather than an long, because doubles can have decimal places.

To convert the string into a double, the following line of code is used.

double dMiles = Double.parseDouble(miles);

To convert the number of miles into Kilometers the miles are multiplied by 1.609344. You may want to define a constant, using the final keyword, to hold this number.