How is the JVM prevented from 'optimizing away everything' in this piece of example code from Effective Java?

Question

I found this example code for Joshua Bloch's book, Effective Java. It's meant to demonstrate why you should avoid unnecessarily creating objects:

public class Sum {
    private static long sum() {
        Long sum = 0L;
        for (long i = 0; i <= Integer.MAX_VALUE; i++)
            sum += i;
        return sum;
    }

    public static void main(String[] args) {
        int numSets = Integer.parseInt(args[0]);
        long x = 0;

        for (int i = 0; i < numSets; i++) {
            long start = System.nanoTime();
            x += sum();
            long end = System.nanoTime();
            System.out.println((end - start) / 1_000_000. + " ms.");
        }

        // Prevents VM from optimizing away everything.
        if (x == 42)
            System.out.println();
    }
}

What do the last two lines of the main method accomplish here?

Answer 1

That final comparison is the only usage of that variable. Without it, who would care about the value in that variable?

Nobody!

So, the compiler might assume: that value is never used, and the code that writes to it has node side effects. Things that don't get used, why waste time writing to them.

Thus: that final first "read" usages prevents over eager compilerd from "optimizing out" the call to the method you intend to measure!