StopwatchCPU.java

Below is the syntax highlighted version of StopwatchCPU.java from §1.4 Analysis of Algorithms.

/******************************************************************************
 *  Compilation:  javac StopwatchCPU.java
 *  Execution:    java StopwatchCPU n
 *  Dependencies: none
 *
 *  A version of Stopwatch.java that measures CPU time on a single
 *  core or processor (instead of wall clock time).
 *
 *  % java8 StopwatchCPU 100000000
 *  6.666667e+11 (1.05 seconds)
 *  6.666667e+11 (7.50 seconds)
 *
 ******************************************************************************/

import java.lang.management.ThreadMXBean;
import java.lang.management.ManagementFactory;

/**
 *  The {@code StopwatchCPU} data type is for measuring
 *  the CPU time used during a programming task.
 *
 *  See {@link Stopwatch} for a version that measures wall-clock time
 *  (the real time that elapses).
 *
 *  @author Josh Hug
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */

public class StopwatchCPU {
    private static final double NANOSECONDS_PER_SECOND = 1000000000;

    private final ThreadMXBean threadTimer;
    private final long start;

    /**
     * Initializes a new stopwatch.
     */
    public StopwatchCPU() {
        threadTimer = ManagementFactory.getThreadMXBean();
        start = threadTimer.getCurrentThreadCpuTime();
    }

    /**
     * Returns the elapsed CPU time (in seconds) since the stopwatch was created.
     *
     * @return elapsed CPU time (in seconds) since the stopwatch was created
     */
    public double elapsedTime() {
        long now = threadTimer.getCurrentThreadCpuTime();
        return (now - start) / NANOSECONDS_PER_SECOND;
    }

    /**
     * Unit tests the {@code StopwatchCPU} data type.
     * Takes a command-line argument {@code n} and computes the
     * sum of the square roots of the first {@code n} positive integers,
     * first using {@code Math.sqrt()}, then using {@code Math.pow()}.
     * It prints to standard output the sum and the amount of time to
     * compute the sum. Note that the discrete sum can be approximated by
     * an integral - the sum should be approximately 2/3 * (n^(3/2) - 1).
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]);

        // sum of square roots of integers from 1 to n using Math.sqrt(x).
        StopwatchCPU timer1 = new StopwatchCPU();
        double sum1 = 0.0;
        for (int i = 1; i <= n; i++) {
            sum1 += Math.sqrt(i);
        }
        double time1 = timer1.elapsedTime();
        StdOut.printf("%e (%.2f seconds)\n", sum1, time1);

        // sum of square roots of integers from 1 to n using Math.pow(x, 0.5).
        StopwatchCPU timer2 = new StopwatchCPU();
        double sum2 = 0.0;
        for (int i = 1; i <= n; i++) {
            sum2 += Math.pow(i, 0.5);
        }
        double time2 = timer2.elapsedTime();
        StdOut.printf("%e (%.2f seconds)\n", sum2, time2);
    }
}