The absolute value of a number is always non-negative. Basically, Math.abs drops the negative sign if there is a negative sign, and leaves the number positive if it already was positive. As you saw in the example you posted, the absolute value of -99 is 99, and the absolute value of 99 is also 99. Absolute value means to make the number non-negative.
I say non-negative instead of positive because the absolute value of 0 is 0, which is neither positive nor negative in pure mathematics. Aside from the 0 case, an absolute value yields a positive number in pure math. In Java, there is one other special case where Math.abs(int) returns a negative number, which is Math.abs(Integer.MIN_VALUE) which returns -2147483648 because +2147483648 isn't a possible value for an int.