Java Notes

Kilometers to Miles

This program asks the user for a number of miles and converts it to kilometers. There are two versions: one using dialog I/O and one using console I/O.

Dialog Input / Output

Convert String to a number. We have to convert the input string into a number so that we can do the arithmetic with it. This conversion is done by calling Double.parseDouble. If we had wanted an integer instead of a double, we would have called Integer.parseInt.

Sample dialog boxes from program

Source code

// File   : intro-dialog/KmToMiles.java
// Purpose: Converts kilometers to miles.
// Author : Fred Swartz
// Date   : 24 Aug 2005

import javax.swing.*;  // Package containing JOptionPane

public class KmToMiles {

    //... Static constants
    static final double MILES_PER_KILOMETER = 0.621;                          //Note 1

    public static void main(String[] args) {

        //... Variables
        String kmStr;       // String version of km before conversion to double.
        double kilometers;  // Number of kilometers.
        double miles;       // Number of miles.

        //... Input
        kmStr = JOptionPane.showInputDialog(null, "Enter number of kilometers.");
        kilometers = Double.parseDouble(kmStr);                               //Note 2

        //... Computation
        miles = kilometers * MILES_PER_KILOMETER;

        //... Output
        JOptionPane.showMessageDialog(null, kilometers + " kilometers is "
                                    + miles + " miles.");
    }
}

Notes

Constant values are commonly declared static final before the methods are defined. The 'static' keyword will be explained later. If a variable is defined with the 'final' attribute, it's value can't be changed after an assignment to it, which is exactly what you want to do to prevent a constant from being changed.
You can only do arithmetic operations on numbers in Java, but showInputDialog returns a string. It's necessary to convert the string to a number. Here we convert the string to a double.

Using console I/O

// File   : intro/KmToMilesConsole.java
// Purpose: Converts kilometers to miles.  Used Console I/O.
// Author : Fred Swartz
// Date   : 20 Apr 2006

import java.util.*;  // Package containing Scanner

public class KmToMilesConsole {

    //... Static constants
    static final double MILES_PER_KILOMETER = 0.621;                       //Note 1

    public static void main(String[] args) {

        //... Variables
        double kilometers;  // Number of kilometers.
        double miles;       // Number of miles.

        Scanner in = new Scanner(System.in);                               //Note 2

        //... Input
        System.out.print("Enter number of kilometers: ");
        kilometers = in.nextDouble();

        //... Computation
        miles = kilometers * MILES_PER_KILOMETER;

        //... Output
        System.out.println(kilometers + " kilometers is "
                                    + miles + " miles.");
    }
}

Notes

Constant values are commonly declared static final before the methods are defined. The 'static' keyword will be explained later. If a variable is defined with the 'final' attribute, it's value can't be changed after an assignment to it, which is exactly what you want to do to prevent a constant from being changed.
Creates a new Scanner object called "in". Use this to read all values.