Stopwatch: Record real time taken for an operation, across multiple runs

Stopwatch.java

import java.text.DecimalFormat;

/**
 * The Stopwatch class provides a simple way to record the real-world time taken by an operation,
 * averaged over multiple runs, and report the result easily in logs.
 * 
 * An example use case would be timing JDBC calls to a relational database.
 * 
 * This code does not measure CPU time or pay any attention to whether a thread or process is
 * active or not. For that, you need to use a profiler.
 *  
 * @author mathew <meta@pobox.com>
 * 
 * Placed into the public domain.
 */
public class Stopwatch {

	private static DecimalFormat format = new DecimalFormat("#.##");
	
	private long tstart;
	private long tstop;
	private long total = 0;
	private int runs = 0;

	/**
	 * Start a new run of timing something.
	 * If the Stopwatch is already timing something, that existing run is canceled.
	 */
	public void start() {
		tstart = System.nanoTime();
	}

	/**
	 * Stop the current timing run.
	 * Does nothing if no existing run is active.
	 */
	public void stop() {
		if (tstart != 0) {
			tstop = System.nanoTime();
			total += (tstop - tstart);
			runs += 1;
			tstart = 0;
		}
	}
	
	/**
	 * @return the total number of elapsed seconds recorded by this Stopwatch object in all runs.
	 */
	public double getTotalSeconds() {
		return ((double) total) / 1000000000;
	}
	
	/**
	 * @return the average number of seconds per run recorded by this Stopwatch object.
	 */
	public double getAverageSeconds() {
		if (runs == 0) {
			return 0;
		}
		return ((double) total) / 1000000000 / runs;
	}
	
	/**
	 * @return the number of runs completed.
	 */
	public int getRuns() {
		return runs;
	}
	
	/**
	 * @return a string representation of the Stopwatch object suitable for output to humans.
	 * e.g. "1.46ms (87 runs)"
	 */
	@Override
	public String toString() {
		final double avg = this.getAverageSeconds();
		int pow = (int) Math.round(-Math.log10(avg));
		double mult;
		String unit;
		if (pow < 0) {
			pow = 0;
		}
		if (pow < 2) {
			unit = "s";
			mult = 1.0;
		} else if (pow < 4) {
			unit = "ms";
			mult = 1000.0;
		} else if (pow < 7) {
			unit = "µs";
			mult = 1000000.0;
		} else {
			unit = "ns";
			mult = 1000000000.0;
		}
		return format.format(mult * avg) + unit + " (" + Integer.toString(runs) + " run" + (runs > 1 ? "s" : "") +")";
	}
}