Stopwatch.java

package folioxml.utils;

/**
 * Time the execution of any block of code.
 */
public final class Stopwatch {


    /**
     * Start the stopwatch.
     *
     * @throws IllegalStateException if the stopwatch is already running.
     */
    public Stopwatch start() {
        if (fIsRunning) {
            throw new IllegalStateException("Must stop before calling start again.");
        }
        //reset both start and stop
        fStart = System.currentTimeMillis();
        fStop = 0;
        fIsRunning = true;
        fHasBeenUsedOnce = true;
        return this;
    }

    /**
     * Stop the stopwatch.
     *
     * @throws IllegalStateException if the stopwatch is not already running.
     */
    public Stopwatch stop() {
        if (!fIsRunning) {
            throw new IllegalStateException("Cannot stop if not currently running.");
        }
        fStop = System.currentTimeMillis();
        elapsed += (fStop - fStart);
        fIsRunning = false;
        return this;
    }

    /**
     * Sets the elapsed time to zero.
     *
     * @return
     */
    public Stopwatch reset() {
        elapsed = 0;
        return this;
    }

    /**
     * Express the "reading" on the stopwatch.
     *
     * @throws IllegalStateException if the Stopwatch has never been used,
     *                               or if the stopwatch is still running.
     */
    @Override
    public String toString() {
        validateIsReadable();
        StringBuilder result = new StringBuilder();
        result.append(elapsed);
        result.append(" ms");
        return result.toString();
    }

    /**
     * Express the "reading" on the stopwatch as a numeric type.
     * milliseconds
     *
     * @throws IllegalStateException if the Stopwatch has never been used,
     *                               or if the stopwatch is still running.
     */
    public long toValue() {
        validateIsReadable();
        return fStop - fStart;
    }

    public boolean hasValue() {
        return (fHasBeenUsedOnce && !fIsRunning);
    }

    // PRIVATE ////
    private long fStart;
    private long fStop;
    private long elapsed = 0;

    private boolean fIsRunning;
    private boolean fHasBeenUsedOnce;

    /**
     * Throws IllegalStateException if the watch has never been started,
     * or if the watch is still running.
     */
    private void validateIsReadable() {
        if (fIsRunning) {
            String message = "Cannot read a stopwatch which is still running.";
            throw new IllegalStateException(message);
        }
        if (!fHasBeenUsedOnce) {
            String message = "Cannot read a stopwatch which has never been started.";
            throw new IllegalStateException(message);
        }
    }
}