303. Range Sum Query - Immutable

Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.
Example:
Given nums = [-2, 0, 3, -5, 2, -1]
sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

Note:
You may assume that the array does not change.
There are many calls to sumRange function.

public class NumArray {

    int sum[] = null;
    
    public NumArray(int[] nums) {
        if(nums.length == 0){
            return;
        }    
        sum = new int[nums.length];
        sum[0] = nums[0];
        for(int i = 1; i<nums.length; i++){
            sum[i] = sum[i-1] + nums[i];
        }
    }

    public int sumRange(int i, int j) {
        if(sum == null){
            return 0;
        }
        return sum[j] - (i==0?0:sum[i-1]);
    }
}