Do integers in Java have to be initialized?

Question

I was coding on LeetCode and made a program to reverse the order of a number. For example, 123 becomes 321, etc.

I assumed that the integer variable "ans" would be automatically initialized to zero by default. However, when I use the uninitialized variable ans later in the program, I get an error.

Note : this program works perfectly fine if I do initialize ans to zero. But I was just wondering why not initializing ans would get me an error.

class Solution {
    public int reverse(int x) {
        int temp;
        int ans; //uninitialized!

        while(x != 0)
        {
            temp = x % 10;
            ans *= 10; //causes an error!
            ans += temp;
            x /= 10;
        }
        return ans;
    }
}