How can “while (i == i) ;” be a non-infinite loop in a single threaded application?

Question

I just got a question that I can\'t answer.

Suppose you have this loop definition in Java:

while (i == i) ;

What is the type of

Answer 1

i == i is not atomic. Proved by such program:

static volatile boolean i = true;
public static void main(String[] args) throws InterruptedException
{
    new Thread() {
        @Override
        public void run() {
            while (true) {
                i = !i;
            }
        }
    }.start();

    while (i == i) ;
    System.out.println("Not atomic! i: " + i);
}

Update Here is one more example of not-infinite loop (no new threads are created).

public class NoNewThreads {
    public static void main(String[] args) {
        new NoNewThreads();
        System.gc();
        int i = 500;
        System.out.println("Still Running");
        while (i == i) ;
    }

    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        Thread.sleep(1000);
        System.exit(0);
    }
}

Answer 2

The value of i is then Invalid. "Not a Number".

After some googling, i found out that you CAN have NaN ( Not a Number ) in Java! So, a Float Pointing number is the Data Type and the Value is NaN. See here

Answer 3

I was surprised to not see this solution:

while (sin(x) == sin(x)) //probably won't eval to true

In response to a comment, try running this:

double x = 10.5f;
assert (x == asin(sin(x)));

x should always equal the arcsine(sin(x)) in theory, but in practice it doesn't.

Answer 4

double i = Double.NaN;

NaN is not equal to anything, including itself.

Answer 5

Think of Nan as the equivalent of exception but uses a magic value within a calculation. Because a calculation failed - eg square root of a negative, divide by zero etc - it makes no sense in comparing them against anything else. After all if divide by zero is a nan is it equivalent to the square root of -2 or square root of -3 ?

Nan allows a calculation that includes a step that returns an invalid answer to complete without introducing extra exceptions. To verify the answer is value simply test for non nandness ( is that's word if not I bags it) via Float.isNan() o equivalent.

Answer 6

Not infinite loop, one thread :)

import static B.*;
public class A {
    public static void main(String[] args) {
        System.out.println("Still Running");
        while (i == i) ;
    }
}


public class B {

    public static int i;
    static {
        System.exit(0);
    }
}