Make a negative number positive

You're looking for absolute value, mate. Math.abs(-5) returns 5...

The concept you are describing is called "absolute value", and Java has a function called Math.abs to do it for you. Or you could avoid the function call and do it yourself:

number = (number < 0 ? -number : number);

or

if (number < 0)
    number = -number;

Just call Math.abs. For example:

int x = Math.abs(-5);

Which will set x to 5.

Note that if you pass Integer.MIN_VALUE, the same value (still negative) will be returned, as the range of int does not allow the positive equivalent to be represented.