StopWatch.java

/**
 * A stopwatch accumulates time when it is running. You can repeatedly start and
 * stop the stopwatch. You can use a stopwatch to measure the running time of a
 * program.
 */
public class StopWatch {
    private long elapsedTime;
    private long startTime;
    private boolean isRunning;

    /**
     * Constructs a stopwatch that is in the stopped state and has no time
     * accumulated.
     */
    public StopWatch() {
        reset();
    }

    /**
     * Starts the stopwatch. Time starts accumulating now.
     */
    public void start() {
        if (isRunning)
            return;
        isRunning = true;
        startTime = System.nanoTime();
    }

    /**
     * Stops the stopwatch. Time stops accumulating and is is added to the
     * elapsed time.
     */
    public void stop() {
        if (!isRunning)
            return;
        isRunning = false;
        long endTime = System.nanoTime();
        elapsedTime = elapsedTime + endTime - startTime;
    }

    /**
     * Returns the total elapsed time.
     *
     * @return the total elapsed time
     */
    public long getElapsedTime() {
        if (isRunning) {
            long endTime = System.nanoTime();
            elapsedTime = elapsedTime + endTime - startTime;
            startTime = endTime;
        }
        return elapsedTime;
    }

    /**
     * Stops the watch and resets the elapsed time to 0.
     */
    public void reset() {
        elapsedTime = 0;
        isRunning = false;
    }
}