CODEWITHSUNDEEP
javaruntime
Akshaj Kadaveru
Akshaj Kadaveru

Reputation: 135

Why is System.nanoTime not accurate?

import java.math.BigDecimal;
public class testtest {
    public static final BigDecimal TWO = new BigDecimal(2);
    public static final int digits = 1000;
    public static final BigDecimal TOLERANCE = BigDecimal.ONE.scaleByPowerOfTen(-digits);
    public static double MidpointMethod = 0;
    public static long MidpointMethod(int n) {
        BigDecimal k = new BigDecimal(n);
        BigDecimal a = BigDecimal.ONE; // set a to be one
        BigDecimal b = k; // set b to be two  
        long start = System.nanoTime(); // start the timer
        while(a.multiply(a).subtract(k).abs().compareTo(TOLERANCE) >= 0) { // while our decimals aren't close enough to the square root of two
            if(a.multiply(a).subtract(k).abs().compareTo(b.multiply(b).subtract(k).abs()) > 0)  // if a is farther from the square root of two than b
                a = a.add(b).divide(TWO); // set a to be the average of a and b
            else // if a is closer to the square root of two than b
                b = a.add(b).divide(TWO); // set b to be the average of a and b
        }
        return System.nanoTime() - start; // return the time taken
    }
    public static void main(String[] args) {
        System.out.println(MidpointMethod(2)/10e6);
    }

}

This program outputs 6224.5209, but when I ran it it took way, way over 20 seconds to run. Why does it display 6 seconds when it actually took more than 20 seconds?

is the 6 seconds an accurate and precise measure of how long the program took?

Upvotes: 0

Views: 415

Answers (2)

Dennis Hendriks
Dennis Hendriks

Reputation: 86

To convert nanoseconds to milliseconds (which I assume you were trying), divide by 1e6, not 10e6. You are off by a factor of 10.

Upvotes: 4

Alex Botev
Alex Botev

Reputation: 1399

The Syste.nanoTime() is fully accurate given that you work on a decent PC, which I'll assume you are. The problem is any kind of initialisation before you call for the first time the method, including JVM start up, stack set up, heap set up, the Big decimals initialisation takes some time. Also if you are using a lot of your RAM and it is almost full that boot up time can go even more.

Upvotes: 0

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