You are creating an array of zero length (no slots to put anything in)

 int array[]={/*nothing in here = array with no slots*/};

and then trying to assign values to array slots (which you don't have, because there are none)

array[i] = number; //array[i] = element i in the array of length 0

You need to define a larger array to fit your needs

 int array[] = new int[4]; //Create an array with 4 elements [0],[1],[2] and [3] each containing an int value

You cannot make an empty array and then let it grow dynamically whenever the user enters a number in the command line. You should read the numbers and put them in an ArrayList instead. An ArrayList does not require a initial size and does grow dynamically. Something like this:

public void main(String[] args) {
    ArrayList<Integer> numbers = new ArrayList<Integer>();
    BufferedReader console = 
            new BufferedReader(new InputStreamReader(System.in));
    while(true) {
        String word = console.readLine();
        if (word.equalsIgnoreCase("end") {
            break;
        } else {
            numbers.add(Integer.parseInt(word);
        }
    }

Ofcourse you won't use while(true)and you won't put this in main, but it's just for the sake of the example

the rand() function is based on a complex algorithm, in which each iteration returns a number that is seemingly unrelated to the last one. However, it's still an algorithm, and every time we run it, will run the same algorithm, thus giving us the same sequence than last time. Now, that doesn't mean it will always output the same number. The sequence is the one that is constant. Actually, no computer can generate a truly TRULY random number, as randomness depends on probability, while computes are deterministic ( meaning that for the same input, it will return the same output). The difference between rand() and srand() is that the first one doesn't depend on any variable, so is a constant sequence. The second one will take a number as an input (called seed) and will use it inside an algorithm to generate a seemingly random sequence. Again, this is pseudo randomness, since if you give it the same seed, it will return the same sequence. Hope this makes sense to anyone.

int[] sum = new int[method(x)]