Question

Can anyone confirm if this is true?

Java will turn a long % int into a long value. However it can never be greater than the modulus to it is always safe to cast it to an int.

long a = 
int b =
int c = (int) (a % b); // cast is always safe.

Similarly a long % short will always be safe to cast to a short.

If true, does any one know why Java has a longer type for % than needed?

Additionally, there is a similar case for long & int (if you ignore sign extension)

Answer 1

For most (if not all) arithmetic operations, Java will assume you want the maximum defined precision. Imagine if you did this:

long a = ...;
int b = ...;

long c = a % b + Integer.MAX_VALUE;

If Java automatically down-casted a % b to an int, then the above code would cause an int overflow rather than setting c to a perfectly reasonable long value.

This is the same reason that performing operations with a double and an int will produce a double. It’s much safer to up-cast the least-accurate value to a more accurate one. Then if the programmer knows more than the compiler and wants to down-cast, he can do it explicitly.

Update

Also, after thinking more about this, I’m guessing most CPU architectures don’t have operations that combine 32-bit and 64-bit values. So the 32-bit value would need to be promoted to a 64-bit value just to use it as an argument to the CPU’s mod operation, and the result of that operation would be a 64-bit value natively. A down-cast to an int would add an operation, which has performance implications. Combining that fact with the idea that you might actually want to keep a long value for other operations (as I mention above), it really wouldn’t make sense to force the result into an int unless the developer explicitly wants it to be one.