Selection Sort is one of many sorting algorithms, but unlike a few others it is easy to understand and implement as we’re going to show here!

The sorting algorithm consists of 3 elements:

1. Iteration over given elements.
2. Finding index of the smallest number (selection), starting from the current position up to the end of the given array.
3. Swapping the smallest element with the one at current position.

Notice that the algorithm goes through the input array only once (linear execution time), but the selection part is executed for each step and also goes through whole input array, but -1 in each loop. Swap is executed values.length times.

• the running time is ϴ(n²) (Big Theta n²), which means that it won’t run faster nor slower than n²,
• doesn’t have worst nor best case and always run in ϴ(n²),
• selection (indexOfMinimum method) will be executed n²/2 + n/2 times,
• memory complexity in O(1), because the algorithm reuses the same array.

package com.farenda.tutorials.algorithms.sorting;

public class SelectionSorter {

    public void sort(int[] values) {
        for (int i = 0; i < values.length; i++) {
            int minPos = indexOfMinimum(values, i);
            swap(values, i, minPos);
        }
    }

    private int indexOfMinimum(int[] values, int i) {
        int minPos = i;
        for (; i < values.length; ++i) {
            if (values[i] < values[minPos]) {
                minPos = i;
            }
        }
        return minPos;
    }

    private void swap(int[] values, int first, int second) {
        int tmp = values[first];
        values[first] = values[second];
        values[second] = tmp;
    }
}

Notice that all parts of the algorithm have been implemented in separate methods. This make code more readable and is good style. If you are worried about the cost of calling methods in a loop, then no need to. JVM Hot-Spot is taking care of that and will optimize (e.g. inline methods) code where needed.

Let’s write a couple of JUnit tests to make sure that our implementation of Selection Sort algorithm behaves correctly:

package com.farenda.tutorials.algorithms.sorting;

import org.junit.Test;

import static org.junit.Assert.assertArrayEquals;

public class SelectionSortTest {

    private SelectionSorter sorter = new SelectionSorter();

    @Test
    public void shouldDoNothingWithEmptyArray() {
        int[] values = {};

        sorter.sort(values);
    }

    @Test
    public void shouldDoNothingWithOneElementArray() {
        int[] values = {42};

        sorter.sort(values);

        assertArrayEquals(new int[] {42}, values);
    }

    @Test
    public void shouldSortValues() {
        int[] values = { 9, -3, 5, 0, 1};
        int[] expectedOrder = { -3, 0, 1, 5, 9};

        sorter.sort(values);

        assertArrayEquals(expectedOrder, values);
    }
}

• Binary Search is expecting to receive sorted data