How to check if an integer larger than Integer.MAX_VALUE

Question

Suppose I have an positive int x; I want to double x until it is larger than Integer.MAX_VALUE then stop.

I know MAX_VALUE = 2^31-1 and if x goes beyond this value, overflow occurs, and x becomes negative.

I'm wondering if I can use the condition (x < 0) to check if x goes beyond MAX_VALUE or not, because x is an integer, and it never goes beyond MAX_VALUE so I can't use (x > Integer.MAX_VALUE) to check.

Why not just use a `long` and then check if it's greater than `Integer.MAX_VALUE`? — Jacob G., Mar 21 '19 at 20:30
I know that I can use long, but what if I x is strictly an int and I have to use x to compare? — Louis Tran, Mar 21 '19 at 20:32
Well, you wouldn't be able to store an int larger than the max int. So, how would you know it overflowed vs just being a smaller number? — OneCricketeer, Mar 21 '19 at 20:33
[Integer.compareUnsigned](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Integer.html#compareUnsigned(int,int)). — rgettman, Mar 21 '19 at 20:34

FredK · Answer 1 · 2019-03-23T18:19:28.663

1

Any (positive) number greater than half of the maximum will overflow if doubled, while doubling any number less than or equal to half of the maximum will result in a value less than or equal to the maximum. So This should work:

int sum = startValue;
int halfMax = Integer.MAX_VALUE / 2;
while ( sum <= halfMax ) {
   sum *= 2;
}

The loop starts at the given number, and if it is <= half the maximum, doubles the sum, exiting the loop as soon as sum becomes larger than half of the maximum.

edited Mar 23 '19 at 18:19

answered Mar 21 '19 at 22:08

FredK

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Can you edit your answer to explain how this solves the issue rather than just posting the code? Thanks! – Jonathan Mar 22 '19 at 00:08
@Jonathan Edited with explanation. – FredK Mar 23 '19 at 18:23

score 0 · Answer 2 · answered Mar 21 '19 at 20:54

0

Yes, it will always work because the maximum possible result on the last doubling is (-2^31 + (2^31 - 1)), which is -1. However, it's frowned upon to rely on overflow/underflow, since it's a side effect of how memory works, not an intended effect.

answered Mar 21 '19 at 20:54

Greg Mc

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How to check if an integer larger than Integer.MAX_VALUE

2 Answers2