Random return number according to weight.java

Random return number according to weight
// http://www.geeksforgeeks.org/random-number-generator-in-arbitrary-probability-distribution-fashion/
return a random number from (0,...,n-1) with given weights


private static int randomNumber(int[] weights) {
    if (weights == null || weights.length == 0)    return 0;
    int n = weights.length;
    for (int i = 1; i < n; i++) 
        weights[i] += weights[i - 1];//[1,2,4,5,1,3] -> [1,3,7,12,13,16]
    Random rand = new Random();
    int num = rand.nextInt(weights[n - 1]);//num is from 0 to 15
    return binarySearch(weights, 0, n - 1, num);//returned value is from 0 to n-1
}
//find the leftmost mid that target < weights[mid]; eg.target=10,weight[mid]=12
private static int binarySearch(int[] weights, int start, int end, int target) {
    while (start < end) {
        int mid = start + (end - start) / 2;
        if (target <= weights[mid]) 	end = mid;
        else 	start = mid + 1;
    }
    return start;
}