Number overflow in Java

Question

I am tring to implement C/C++ atoi function in Java, below is the code snippet

    for (int j = 0; j < s.length(); j++) {

        int digit = Character.digit(s.charAt(j), 10);
        if (sum < limit/10) {
            if (neg) return Integer.MIN_VALUE;
            return Integer.MAX_VALUE;
        }

        sum *= 10;
        if (sum < limit + digit) {
            if (neg) return Integer.MIN_VALUE;
            return Integer.MAX_VALUE;
        }
        sum -= digit;
    }

For line "if (sum < limit + digit) {", which is correct, however, if I use "sum - digit < limit", it will get wrong result, e.g. input "-2147483649", wrong result 2147483647, should be -2147483648.

I figured this out, cuz sum - digit might be overflow, so this come to another question:

    int sum = Integer.MAX_VALUE;
    System.out.println(sum < Integer.MAX_VALUE + 1);

Why this prints false? What is the behind logic?

Ever guessed what would Integer.MAX_VALUE + 1 produce?

devnull
– devnull

2014-04-02 18:05:44 +00:00
Commented Apr 2, 2014 at 18:05 — devnull
– devnull, Commented Apr 2, 2014 at 18:05
I am just using that to test.

thinkman
– thinkman

2014-04-02 18:07:15 +00:00
Commented Apr 2, 2014 at 18:07 — thinkman
– thinkman, Commented Apr 2, 2014 at 18:07

user555045 · Accepted Answer · 2014-04-02 18:16:24Z

6

Integer.MAX_VALUE + 1 is equal to Integer.MIN_VALUE, which will be more obvious if you look at them in hexadecimal:

Integer.MAX_VALUE = 0x7fffffff
                1 = 0x00000001
                    ---------- +
                    0x80000000

And 0x80000000 is also known as Integer.MIN_VALUE.

Obviously there's no int lower than Integer.MIN_VALUE.

Also, attempting to test whether a number has overflowed by seeing if it's bigger than the biggest possible value is fundamentally misguided. It can't be bigger than the biggest possible value, that's what "biggest possible" implies. Also, you can't take a number, look at it, and determine whether it has overflowed or not, because every number can be the result of a non-overflowing computation (and indeed of just writing it down as a constant) and as the result of a computation that overflowed. You need to know how you got that number.

edited Apr 2, 2014 at 18:16

answered Apr 2, 2014 at 18:07

user555045

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2 Comments

user555045 Over a year ago

@MartijnCourteaux no, and I don't understand how you could think that. There's no floating point in sight.

Martijn Courteaux Over a year ago

Sorry, my mistake. I was the one being confused :)

rgettman · Accepted Answer · 2014-04-02 18:05:30Z

1

If you add Integer.MAX_VALUE and 1, then that sum will overflow, so sum won't be less than the resultant "sum", Integer.MIN_VALUE, so it's false.

To get that to work properly, you can cast Integer.MAX_VALUE as a long so overflow won't occur, and the comparison will work properly.

System.out.println(sum < (long) Integer.MAX_VALUE + 1);

Output:

true

answered Apr 2, 2014 at 18:05

rgettman

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2 Comments

Marsh Over a year ago

Or use <= Integer.MAX_VALUE instead of < (long) Integer.MAX_VALUE + 1...though every integer is going to be less than or equal to the max value an integer can hold.

rgettman Over a year ago

@MartinCarney Using <= Integer.MAX_VALUE is useless, because every int value will make that expression true.

Martijn Courteaux · Accepted Answer · 2014-04-02 18:06:02Z

1

FYI: Integer.parseInt(String) does what you are writing.

Why this prints false? What is the behind logic?

System.out.println(sum < Integer.MAX_VALUE + 1);

This prints false, because Integer.MAX_VALUE + 1 == Integer.MIN_VALUE. So this is equivalent with

System.out.println(sum < Integer.MIN_VALUE);

There is nothing smaller than the minimum value.

answered Apr 2, 2014 at 18:06

Martijn Courteaux

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1 Comment

Marsh Over a year ago

Java evaluates Integer.MAX_VALUE + 1 and by default treats it like an int. Adding a positive value to MAX_VALUE will overflow the int, resulting in a negative value. Overflowing MAX_VALUE by 1 will result in a negative number equal to MIN_VALUE. Try printing out MAX_VALUE, 'MIN_VALUE, and MAX_VALUE + 1`.

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