Why is my multi threading not efficient?

I've designed a class that fills an array with integers using a various number of threads, in order to see the power of multi threading. But according to my result, there is none...

The idea: The idea was too fill an array of 100000000 integers with the value "1". Starting with 1 thread (one threads fills the whole array) and incrementing it until 100 threads (each thread fills a sub array of size 100000000/nbThreads)

Example: With 10 threads, I create 10 threads and each is filling an array of 10000000 integers.

Here is my code:

public class ThreadedArrayFilling extends Thread{
    private int start;
    private int partitionSize;
    public static int[] data;
    public static final int SIZE = 100000000;
    public static final int NB_THREADS_MAX = 100;


    public static void main(String[] args){
        data = new int[SIZE];
        long startTime, endTime;
        int partition, startIndex, j;
        ThreadedArrayLookup[] threads;

        for(int i = 1; i <= NB_THREADS_MAX; i++){       
            startTime = System.currentTimeMillis();
            partition = SIZE / i;
            startIndex = 0;
                threads = new ThreadedArrayLookup[i];
            for(j = 0; j < i; j++){         
                threads[j] = new ThreadedArrayLookup(startIndex, partition);
                startIndex += partition;
            }
            for(j = 0; j < i; j++){
                try {
                    threads[j].join();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            endTime = System.currentTimeMillis();       
            System.out.println(i + " THREADS: " + (endTime - startTime) + "ms");
        }
    }

    public ThreadedArrayFilling(int start, int size){
        this.start = start;
        this.partitionSize = size;
        thi