Segment Tree (Implementation in Java)

Test.java

class SegmentTree{
    int[] tree;

    SegmentTree(int n){
        tree = new int[n];
    }

    void build(int[] arr, int node, int start, int end){
        if(start == end){
            tree[node] = arr[start];
        }
        else{
            int mid = (start + end)/2;
            build(arr, 2*node + 1, start, mid);
            build(arr, 2*node + 2, mid + 1, end);
            tree[node] = tree[2*node + 1] + tree[2*node + 2];
        }
    }

    void update(int[] arr, int node, int index, int val, int start, int end){
        if(start == end){
            tree[node] += val;
            arr[start] += val;
        }
        else{
            int mid = (start + end)/2;
            if(start <= index && index <= mid){
                update(arr, 2*node + 1, index, val, start, mid);
            }
            else{
                update(arr, 2*node + 2, index, val, mid + 1, end);
            }
            tree[node] = tree[2*node + 1] + tree[2*node + 2];
        }
    }

    int query(int node, int start, int end, int left, int right){
        if(right < start || end < left){
            return 0;
        }
        if(left <= start && end <= right){
            return tree[node];
        }
        int mid = (start + end)/2;
        int p1 = query(2*node + 1, start, mid, left, right);
        int p2 = query(2*node + 2, mid + 1, end, left, right);
        return p1 + p2;
    }
}

public class Test{
    public static void main(String[] args){
        int[] arr = {1, 2, 3, 4, 5, 6, 7, 8};
        int n = arr.length;
        int height = (int)(Math.log(n)/Math.log(2)) + 1;
        int tree_nodes = (int) Math.pow(2, height + 1);
        SegmentTree ob = new SegmentTree(tree_nodes);
        ob.build(arr, 0, 0, n - 1);
        for(int i = 0; i < tree_nodes; i++){
            System.out.print(ob.tree[i] + " ");
        }
        System.out.println();
        System.out.println(ob.query(0, 0, n - 1, 0, 5));
    }
}

Great implementation, there is the one optimization that can be done in this code
int height = (int) (Math.log(n) / Math.log(2)) + 1;
int treeNodes = (int) Math.pow(2, height + 1);
treeNodes = n * 4
Here is why:
(Math.log(n) / Math.log(2)) + 1 = log2(n) + 1
2 ^ (log2(n) + 2) = 2 ^ (log2(n)) * 2 ^ 2 = n * 4