为什么循环迭代超出 Integer.MAX_VALUE？

Question

Can someone explain why this code is an infinite loop on Java 7,8? The loop should stop when i = Integer.MAX_VALUE + 1 but it seems to deny the limit. Why does this happen?

public static void main(String[] args) {
    for (int i = Integer.MAX_VALUE - 100; i <= Integer.MAX_VALUE; i++);
}

Answer 1

• The value Integer.MAX_VALUE changes from 2147483647 to -2147483648 ie Integer.MIN_VALUE when it overflows.
• Integer overflow occurs when the arithmetic operation tries to create value that is beyond reach of data type value range.
• In java integers have range from -2147483648 to 2147483647 .
• In for loop when i becomes 2147483647 then i++ operation will try to increment its value by 1 which is not possible because range is only till 2147483647 .
• So instead of having its value as 2147483648 it will go back to starting value ie -2147483648
• Hence i <= Integer.MAX_VALUE always becomes true .
• As condition never becomes false , loop will continue till infinity.

You can check value overflow by simply writing your code as below.

for (int i = Integer.MAX_VALUE - 100; i <= Integer.MAX_VALUE; i++){
        System.out.println(i);
}

You will see below output.

2147483645
2147483646
2147483647
-2147483648
-2147483647
-2147483646

Answer 2

Look at

 i <= Integer.MAX_VALUE

This is always true. If i = Integer.MAX_VALUE+1 then it will overflow, and become negative.

Do this:

 System.out.println(Integer.MAX_VALUE+1)

Answer 3

Because when i reaches being equal to Integer.MAX_VALUE then it becomes Integer.MAX_VALUE + 1 which becomes negative

Better would be

for (long i = Integer.MAX_VALUE - 100; i <= Integer.MAX_VALUE; i++)

Answer 4

You expect the loop to run till i <= Integer.MAX_VALUE and stop as soon as it becomes Integer.MAX_VALUE + 1 , ie, 2147483648 . However, THAT DOES NOT HAPPEN because whenever you add something to Integer.MAX_VALUE, say x , the calculation takes the form:

Integer.MIN_VALUE + x - 1;

Since int cannot store beyond 2147483647 (and of course beyond -2147483648 ), this thing is done to make sure that there's no overflow. So, after 2147483647 , i becomes -2147483648 , which is obviously less than +2147483647 and thus, it is an infinite loop .

If you want the loop to iterate in that range (ie, you just wanna use the values), you can make use of a type cast between long and int like this:

for(long i = (long)Integer.MAX_VALUE - 100; i<= (long)Integer.MAX_VALUE; i++){
    //to use i as an int, type cast from (long) to (int)
}