Inconsistencies with && and divide-by-zero error in Java

Question

I'm starting out Java, and I am experiencing some inconsistencies. Why does this work:

if ((d != 0) && (n / d < 3)) {
    compute(a, d);
}

But if I do this:

if ((n / d < 3) && (d != 0)) {
    compute(a, d);
}

I get this error:

Exception in thread "main" java.lang.ArithmeticException: / by zero
    at compute.main(compute.java:63)

Answer 1

Java evaluates expressions using Short-Circuit Logic. What this means is that for &&, if the left expression is false, then Java does not bother evaluating the right expression (since the whole expression is guaranteed to be false already).

In your first example, if d == 0, then d != 0 is false, so it will not evaluate n / d. In your second example, if d == 0, then evaluating n / d < 3 will give you the divide by zero error.

Answer 2

In your first example, because of short circuit evaluation, when d == 0, the compiler figures out it doesn't need to evaluate n/d, since false && true would still be false.

If you turn the conditions around, you end up evaluating n/d first, before knowing if d is zero. What you cause is an error because of a division by zero.