Java math function to convert positive int to negative and negative to positive?

Just use the unary minus operator:

int x = 5;
...
x = -x; // Here's the mystery library function - the single character "-"

Java has two minus operators:

• the familiar arithmetic version (eg 0 - x), and
• the unary minus operation (used here), which negates the (single) operand

This compiles and works as expected.

x = -x;

This is probably the most trivial question I have ever seen anywhere.

... and why you would call this trivial function 'reverse()' is another mystery.

What about x *= -1; ? Do you really want a library function for this?

Another method (2's complement):

public int reverse(int x){
    x~=x;
    x++;
    return x;
}

It does a one's complement first (by complementing all the bits) and then adds 1 to x. This method does the job as well.

Note: This method is written in Java, and will be similar to a lot of other languages