Question

During a programming class, the professor was teaching us about x++ and ++x, with x being an integer.

He said that in the scenario we are able to just put either x++ or ++x, ++x is more efficient (by little, but still, in theory, more efficient nonetheless).

But I forgot why. Anyone knows? This was with Java.

Answer 1

It’s not more efficient in Java. It can be more efficient in languages where the increment/decrement operators can be overloaded, but otherwise the performance is exactly the same.

The difference between x++ and ++x is that x++ returns the value of x before it was incremented, and ++x returns the value of x after it was incremented. In terms of code generation, both make up for the exact same number of instructions, at least when you can use either interchangeably (if you can’t use them interchangeably, you shouldn’t be worrying about which one is faster, you should be picking the one you need). The only difference is where the increment instruction is placed.

In C++, classes can overload both the prefix (++x) and postfix (x++) operators. When dealing with types that overload them, it is almost universally faster to use the prefix operator because the semantics of the postfix operator will return a copy of the object as it was before the increment, even when you wouldn’t use it, while the prefix operator can simply return a reference to the modified object (and God knows C++ developers prefer to return references rather than copies). This could be a reason to consider ++x superior to x++: if you gain the habit of using ++x you could save yourself some slight performance trouble when/if you switch to C++. But in the context of Java only, both are absolutely equivalent.

Much like pst in the comments above, I never use the return value of x++ or ++x, and if you never do either, you should probably just stick to the one you prefer.