Question

In the following code, I see no way in which T1 and T3 are different. Certainly my calculator says they are not.

public class longTest {
    public static final long T1 = 24 * 60 * 60 * 1000 * 30;
    public static final long T2 = 24 * 60 * 60 * 1000;
    public static final long T3 = T2 * 30;

    public static void main(String[] args) {

        System.out.println(T1);
        System.out.println(T2);
        System.out.println(T3);
    }
}

So why do I get output of:

-1702967296
86400000
2592000000

It’s not just System.out.println in this sample program either. When I have T1 in eclipse and mouse over the variable I get a gloss showing the same values.

java version “1.6.0_33” OSX

Answer 1

As paranoid-android says, it’s an int overflow issue. To understand why T3 is different, see below.

public static final long T1 = 24 * 60 * 60 * 1000 * 30;

All the numbers here are ints so this is the same as

public static final long T1 = (long) (24 * 60 * 60 * 1000 * 30);

The int part overflows, so the implicit cast to long arrives too late to avoid loss of precision.
In

public static final long T2 = 24 * 60 * 60 * 1000;

you have the same but its a smaller number that fits in both a 32 bit int (31 bits for positive values) and in a 64 bit long.

public static final long T3 = T2 * 30;

This multiplies a long by an int so does 64 bit arithmetic which is why it has a different value.
It’s equivalent to

public static final long T3 = T2 * (long) 30;

The implicit long here is early enough to prevent loss of precision.