Java power of number calculation problem with big numbers

Question

I am new to Java and when I was trying to find power of number without Math.pow method I realized the answer was not right. And I want to learn why?

public class Main() {

int x = 1919;
int y = x*x*x*x;

public static void main(String[] args) {

System.out.println(y);
}
}

Output: 2043765249

But normally answer is: 13561255518721

@Joe: That question is about floating point and the fact that `pow` returns a floating point number. There are no floating point numbers here, and neither is there usage of `pow`. — Jörg W Mittag, Oct 25 '20 at 09:36
I don't understand why would anybody downvote this, even if it's an absolute beginner question. The author did provide a proper code and expected result. — Alex Shesterov, Oct 25 '20 at 09:39
@ExDet, the problem here is [Integer Overflow](https://en.wikipedia.org/wiki/Integer_overflow). The result simply doesn't fit into an `int`. You need to use a "bigger" type like `long` or `BigInteger`. — Alex Shesterov, Oct 25 '20 at 09:41
@JörgWMittag so it is reaching to end point of integer again and again till to sum exact same number with my value. -2,147,483,647 , 2,147,483,647 ? — ExDet, Oct 25 '20 at 09:41

score 3 · Answer 1 · answered Oct 25 '20 at 09:41

You're using an int for the answer, you're getting a numeric overflow.

Use double or long or BigInteger for y and you'll get the right answer.

public class Main {

    public static void main(String[] args) {
        System.out.println(Math.pow(1919, 4));
        System.out.println((double) 1919*1919*1919*1919);
    }
}

Outputs the same value.

score 3 · Accepted Answer · answered Oct 25 '20 at 09:41

If you go step by step, you'll see that at a moment the value becomes negative, that's because you reach Integer.MAX_VALUE which is 2^31 -1

int x = 1919;
int y = 1;
for (int i = 0; i < 4; i++) {
    y *= x;
    System.out.println(i + "> " + y);
}

0> 1919
1> 3682561
2> -1523100033
3> 2043765249

You may use a bigger type like double or BigInteger

double x = 1919;

double y = x * x * x * x;
System.out.println(y); // 1.3561255518721E13

BigInteger b = BigInteger.valueOf(1919).pow(4);
System.out.println(b); // 13561255518721

score 0 · Answer 3 · answered Oct 25 '20 at 09:41

Use long instead of int

public class Main{
    public static void main(String[] args) {
        long x = 1919;
        long y = x*x*x*x;
        System.out.println(y);
    }
}

Read about variables in Java.

Java power of number calculation problem with big numbers

3 Answers3