Math.abs returns wrong value for Integer.Min_VALUE

Solution 1:

Integer.MIN_VALUE is -2147483648, but the highest value a 32 bit integer can contain is +2147483647. Attempting to represent +2147483648 in a 32 bit int will effectively "roll over" to -2147483648. This is because, when using signed integers, the two's complement binary representations of +2147483648 and -2147483648 are identical. This is not a problem, however, as +2147483648 is considered out of range.

For a little more reading on this matter, you might want to check out the Wikipedia article on Two's complement.

Solution 2:

The behaviour you point out is indeed, counter-intuitive. However, this behaviour is the one specified by the javadoc for Math.abs(int):

If the argument is not negative, the argument is returned. If the argument is negative, the negation of the argument is returned.

That is, Math.abs(int) should behave like the following Java code:

public static int abs(int x){
    if (x >= 0) {
        return x;
    }
    return -x;
}

That is, in the negative case, -x.

According to the JLS section 15.15.4, the -x is equal to (~x)+1, where ~ is the bitwise complement operator.

To check whether this sounds right, let's take -1 as example.

The integer value -1 is can be noted as 0xFFFFFFFF in hexadecimal in Java (check this out with a println or any other method). Taking -(-1) thus gives:

-(-1) = (~(0xFFFFFFFF)) + 1 = 0x00000000 + 1 = 0x00000001 = 1

So, it works.

Let us try now with Integer.MIN_VALUE . Knowing that the lowest integer can be represented by 0x80000000, that is, the first bit set to 1 and the 31 remaining bits set to 0, we have:

-(Integer.MIN_VALUE) = (~(0x80000000)) + 1 = 0x7FFFFFFF + 1 
                     = 0x80000000 = Integer.MIN_VALUE

And this is why Math.abs(Integer.MIN_VALUE) returns Integer.MIN_VALUE. Also note that 0x7FFFFFFF is Integer.MAX_VALUE.

That said, how can we avoid problems due to this counter-intuitive return value in the future?

• We could, as pointed out by @Bombe, cast our ints to long before. We, however, must either

  • cast them back into ints, which does not work because Integer.MIN_VALUE == (int) Math.abs((long)Integer.MIN_VALUE).
  • Or continue with longs somehow hoping that we'll never call Math.abs(long) with a value equal to Long.MIN_VALUE, since we also have Math.abs(Long.MIN_VALUE) == Long.MIN_VALUE.

• We can use BigIntegers everywhere, because BigInteger.abs() does indeed always return a positive value. This is a good alternative, though a bit slower than manipulating raw integer types.

• We can write our own wrapper for Math.abs(int), like this:

/**
 * Fail-fast wrapper for {@link Math#abs(int)}
 * @param x
 * @return the absolute value of x
 * @throws ArithmeticException when a negative value would have been returned by {@link Math#abs(int)}
 */
public static int abs(int x) throws ArithmeticException {
    if (x == Integer.MIN_VALUE) {
        // fail instead of returning Integer.MAX_VALUE
        // to prevent the occurrence of incorrect results in later computations
        throw new ArithmeticException("Math.abs(Integer.MIN_VALUE)");
    }
    return Math.abs(x);
}

• Use a integer bitwise AND to clear the high bit, ensuring that the result is non-negative: int positive = value & Integer.MAX_VALUE (essentially overflowing from Integer.MAX_VALUE to 0 instead of Integer.MIN_VALUE)

As a final note, this problem seems to be known for some time. See for example this entry about the corresponding findbugs rule.