SpeedConverter.java

import java.util.InputMismatchException;
import java.util.Scanner;


public class SpeedConverter
{
	// only need one conversion factor - use it to
	// multiply for KPH to MPH - divide for M to K
	public static final double CONVERSION = 0.62137;

	public static void main( String[] args ) {
		// use a single scanner to collect input
		Scanner scan = new Scanner( System.in );
		// get boolean that indicates whether the direction
		// of the conversion is to be from KPH to MPH
		Boolean isToMph = getConvertDirection( scan );
		if (null == isToMph) return;	// exit for bad input
		// get speed input
		Double speed = getSpeedInput( scan );
		if (null == speed) return;		// exit for bad input

		// advise user of results
		report( speed, isToMph );

		// NOTE: Boolean and Double wrappers were employed to permit
		// null returns to indicate input exceptions to caller, since
		// exceptions were handled in input methods. The principle is
		// stated as "Input and Validation are the same step."
	}

	public static Boolean getConvertDirection( Scanner scan ) {
		Boolean isToMph = null;	// default for bad input
		System.out.println(
			"Which conversion do you want?\n" +
			"(for KPH to MPH, enter 1 - for MPH to KPH, enter 2)"
		);
		try {
			isToMph = 1 == scan.nextInt();
		}
		catch ( InputMismatchException e ) {
			System.out.println(
				"Invalid input for conversion direction - must be either\n" +
				"1 for KPH to MPH -OR- 2 for MPH to KPH."
			);
		}
		return isToMph;
	}
	public static Double getSpeedInput( Scanner scan ) {
		Double speed = null;	// default speed for bad input
		System.out.println(
			"What speed value do you want converted?\n"
		);
		try {
			speed = scan.nextDouble();
		}
		catch ( InputMismatchException e ) {
			System.out.println(
				"Invalid input for speed - must be numeric."
			);
		}
		return speed;
	}
	public static double convert(double speed, boolean isToMph) {
		return isToMph ? speed * CONVERSION : speed / CONVERSION;
	}
	public static void report( double speed, boolean isToMph ) {
		System.out.println(
			"the speed of " + speed + (isToMph ? " KPH" : " MPH") +
			" is equivalent to " + convert( speed, isToMph ) +
			 speed + (isToMph ? " MPH" : " KPH")
		);
	}
}