为什么Integer.MAX_VALUE + 1比Integer.MAX_VALUE小？

Question

I am simply trying to add numbers from 1 to Integer.MAX_VALUE, but in output I'm getting nothing. The program is struck in between. Below is the class that I created.

 public class Test {
    public static void main(String args[]) {
        long sum = 0;
        int start_value = 1;
        long end_value = Integer.MAX_VALUE;
        while(start_value <= end_value){
            sum += start_value;
            start_value++;
        }
        System.out.println(sum);
    }
 }

Do anybody have any idea why this is hanging. This program is never completed.

How to solve this type of problem?

Answer 1

It should never complete as you have an infinite loop.

Your loop is effectively

while(start_value <= Integer.MAX_VALUE) {

but the Integer.MAX_VALUE is the largest by definition so it is effective

while (true) {

You can change the loop to do what you need

int start_value = 0;
while(start_value < end_value) {
    start_value++;
    sum += startValue;
}

This way you can catch the problem before it fails.

An obtuse solution might be

for (int i = 1; i > 0; i++)
    sum += i;

This would stop when i overflows.

Answer 2

It's because of something called integer Overflow. When you add 1 to the MAX_VALUE , you get MIN_VALUE if you are using signed integers, or 0 if using unsigned integers.

Briefly explained, when you add 1 to 99 for example, you have to carry the 1 twice to end up on the third digit: 100 . But if you only had a maximum of 2 digits allowed, then you carry the one twice, and end up with 00 . In computers, there is a limited numbers of bits (binary digits) allowed, usually 32 or 64.

You can read more about it here:
Wiki Integer Overflow
Signed vs Unsigned Integers

Answer 3

In addition to the other answers where you're stuck in an infinite loop because the the max integer value is never reached, you should maybe add a print inside the loop so you can see progress:

public class Test {
public static void main(String args[]) {
    long sum = 0;
    int start_value = 1;
    int end_value = Integer.MAX_VALUE - 1;//Just in case
    while(start_value <= end_value){
        sum += start_value;
        start_value++;

        //Print every 100 loops, change this if it prints too often
        if (start_value % 100 == 0){
           System.out.println("integer at: " + start_value + ", sum: " + sum);
        }
    }
    System.out.println(sum + Integer.MAX_VALUE);//Since we only went to Integer.MAX_VALUE -1

    }
}

Answer 4

For the reasons indicated in the other answers, it can be quite tricky to iterate over intervals bounded by maximum or minimum values for a primitive type.

Java 8 allows a new solution to this, because IntStream.rangeClosed and LongStream.rangeClosed can be used.

In your case, you can do

IntStream.rangeClosed(1, Integer.MAX_VALUE).mapToLong(i -> i).sum();

or just

LongStream.rangeClosed(1, Integer.MAX_VALUE).sum();