Helper class to measure execution time in java code

MeasureExecTimeHelper.java

import java.util.concurrent.TimeUnit;

/**
 * Helper class to measure execution time in java code
 *
 * Usage:
 *
 *    public static void main(String[] args) {
 *        MeasureExecTimeHelper.start();
 *        staffToMeasureExecTime();
 *        System.out.println(MeasureExecTimeHelper.stopAndResult_Millis());
 *    }
 *
 * igorok.r<a>gmail.com
 */
public class MeasureExecTimeHelper {

    private static final String TEMPLATE_MS = " %s ms ";
    private static final String TEMPLATE_NS = " %s ns ";

    private static long startTime = 0;

    private static boolean inProgress = false;

    public static void start() {
        inProgress = true;
        startTime = System.nanoTime();
    }

    private static long calcDurationNano() {
        long endTime = System.nanoTime();
        return endTime - startTime;
    }

    private static void markStoppedOrException() {
        if (inProgress) {
            inProgress = false;
        } else {
            throw new IllegalStateException(" Already stopped. Looks like you called result method more than 1 time! ");
        }
    }

    public static long stopAndResultNano() {
        markStoppedOrException();
        return calcDurationNano();
    }

    public static String stopAndResult_Millis() {
        markStoppedOrException();
        return String.format(TEMPLATE_MS, TimeUnit.MILLISECONDS.convert(calcDurationNano(), TimeUnit.NANOSECONDS));
    }

    public static String stopAndResult_Nanos() {
        markStoppedOrException();
        return String.format(TEMPLATE_NS, calcDurationNano());
    }

}