Multithread Quick Sort slower than normal Quick Sort in Java?

Question

Im comparing Quick Sort (normal, with recursivity) and Threaded Quick Sort (Using threads). And i got that Normal-QuickSort is faster than Threaded-QuickSort. Is this right? have a feeling that im doing something wrong.

Here is the code:

public class QuickSort implements SortAlgorithm {
    public void Sort(Integer[] array) {
        Sort(array, 0, array.length-1);
    }

    private void Sort(Integer[] array, Integer low, Integer high) {
        if (low < high) {
            Integer pivot = partition(array, low, high);
            Sort(array, low, pivot-1);
            Sort(array, pivot+1, high);
        }
    }

    private Integer partition(Integer[] array, Integer low, Integer high) {
        Integer pivot = array[high];  
        Integer i = (low-1);
        for (Integer j=low; j<high; j++) 
            if (array[j] < pivot) { 
                i++; 
                Integer temp = array[i]; 
                array[i] = array[j]; 
                array[j] = temp; 
            } 

        Integer temp = array[i+1]; 
        array[i+1] = array[high]; 
        array[high] = temp; 

        return i+1; 
    }
}

public class ThreadsQuickSort extends Thread implements SortAlgorithm {
    private Integer low, high;
    private Integer[] array;

    public ThreadsQuickSort() {
        ;
    }
    
    public ThreadsQuickSort(Integer[] array, Integer low, Integer high) {
        this.array = array;
        this.low = low;
        this.high = high;
    }

    public void run() {
        Sort(array, low, high);
    }

    public void Sort(Integer[] array) {
        Sort(array, 0, array.length - 1);
    }

    private void Sort(Integer[] array, Integer low, Integer high) {
        if (low < high) {
            Integer pivot = partition(array, low, high);
            ThreadsQuickSort lowQSort = new ThreadsQuickSort(array, low, pivot - 1);
            lowQSort.start();
            ThreadsQuickSort highQSort = new ThreadsQuickSort(array, pivot + 1, high);
            highQSort.start();
            try {
                lowQSort.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            try {
                highQSort.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    private Integer partition(Integer[] array, Integer low, Integer high) {
        Integer pivot = array[high];  
        Integer i = (low-1);
        for (Integer j=low; j<high; j++) 
            if (array[j] < pivot) { 
                i++; 
                Integer temp = array[i]; 
                array[i] = array[j]; 
                array[j] = temp; 
            } 

        Integer temp = array[i+1]; 
        array[i+1] = array[high]; 
        array[high] = temp; 

        return i+1; 
    }
}

The main function sorts an array of 10,000 items with both algorithms and get their execution time. Got this output:

Single array sorted in 11 ms
The array was successfully sorted
Threaded array sorted in 6378 ms
The array was successfully sorted

My question is if this is well-coded and the output was expected. This is not about the implementation of Threaded-QuickSort.

Answer 1

~70 micro creation and switch per thread.

The sum of the threads should be around 20000-1 Which gives 20000 * 70us = 1400000us = 1400ms = 1.4s

So it is nearly the expected.