1. Introduction to the sorting exercises

To sort items, we need to be able to compare them. In Java, we express such a requirement by defining a (functional) interface, that includes the method for comparison called int compare(T a, T b). In this exercise we require that any class defining the items to be sorted, will be accompanied by a Comparator<T> instance. The sort method can than use this comparator objects to do the sorting. You can and even are advised to use lambda expressions or method references. See PRC2 for that topic.

To make the student implementation and teachers tests mutually independent, we define an intermediate API, that both will use or implement.

sorters cd 2 0
Figure 1. sorting service class diagram
Typical test interaction with queue and sorter.
    Queue<TestType> queue = factory.createPreferredQueue( sortKind );
    queue = fillRandom( queue, 100 );
    Comparator<TestType> comp = new CountingComparator<>( ( a, b ) -> a.compareTo( b ) );
    Sorter<String> sorter = factory.createSorter( sortKind,  comp );
    Queue<String> sortedQueue = sorter.sort( queue );
    assertThat( sortedQueue ).isSameAs( queue );
    assertThat( sortedQueue).isOrderedAccoridingTo( comp);
    // more tests.
sorting service makes your implementations available to be tested.
public class SortingServices implements SortingServiceFactory {

    Map<SortKind, Supplier<Queue>> queueMap = Map.of(
            SortKind.SELECTION, () -> new HQueue() // add more when ready

    Map<SortKind, Function<Comparator, Sorter>> sorterMap = Map.of(
            SortKind.SELECTION, ( c ) -> new SelectionSorter( c ) // add more when ready

    public <T> Queue<T> createPreferredQueue( SortKind forSorter ) {
        return queueMap.get( forSorter ).get();

    public <T> Sorter<T> createSorter( SortKind kind, Comparator<T> comparator ) {
        return sorterMap.get( kind ).apply( comparator );

     * Add your sorter kinds here.
     * @return the sorters you support.
    public SortKind[] supportedSorters() {
        return new SortKind[]{
            SortKind.SELECTION // add more when ready
insertion navigator
Figure 2. Example InsertionSorter

In order to sort the objects in a queue, one first chooses a comparator, constructs the Sorter with it and then sorts the queue by calling the Queue sort(Queue q) method. If you use another comparator, you should create a fresh sorter object with this comparator, which will then sort the queue with the ordering rules imposed by this new comparator.
The sorter instance itself is very lightweight, since it only needs to keep a reference to the comparator (in its only field), this imposes no performance penalty. In a typical implementation, the sorter instances could do with just two members:

  1. The sort method and the final comparator instance, and

  2. one constructor taking the comparator.

As an example one of our sorter implementations looks like this in the NetBeans navigator view on the right.

Select-sort with Gypsy folk dance

1.1. Tasks and Explanation

  1. The implementing classes of the Sorter<T> interface must have a constructor taking a Comparator<T> as only argument.

  2. The sort method in the sort interface takes a Queue argument, see diagram.

  3. A basic sorter object, i.e. an InsertionSorter or a SelectionSorter, should be able to sort all kinds of objects, as long as there is a Comparator object available for those objects. Sorting the objects is possible by using and of course implementing the Java Comparator interface. The Sorter interface must be implemented by the XXXSorter; this interface demands the following method to be implemented: Queue<E> sort( Queue<E> q ).

  4. The Sorter interface, the Queue interface as well as the enum SortKind defining the sorts of Sorters.[1] reside in the api package. See the class diagram above.
    How you organise your own implementation is up to you. The only requirement is to allow the teacher test classes to find your implementing classes by providing the necessary data for a ServiceLoader to find your sorting services. For that we introduce a abstract factory called sortingservice.SortingServiceFactory which is able to create sorter and queue instances dependent on the SorterKind enum value that is passed to the factor methods. Remember to state the fully qualified name of your factory implementation in the file
    src/main/resources/META-INF/services/sortingservice.SortingServiceFactory inside your project.
    For instance johnniessorterservice.ServiceFactory, if your factory implementation listens to that fully qualified name.

  5. The constructor of the XXXSorter classes takes the comparator.

  6. The sort method takes a Queue as input, which is to be sorted. This queue is the one that is created in your factory and is up to you.
    Make sure your factory produces the correct queue implementation for the SortKind. Of course you are allowed to re use queue implementations where appropriate. However, using a e.g. doubly linked queue where the 2nd link provides no benefit to the sorter is not a good idea, because your queue internal node would carry around and have to initialise and update a useless reference if you do. The teacher tests will have filled that queue using its put(T e) method before passing it to the sort method.
    The sort method should NOT use arrays. The teachers tests will check for that by forbidding array access in the Sorter and Queue implementation.
    This also implies that fields of some array types are not allowed.

  7. The sort method sorts the queue and is supposed to return the same instance. Try to be frugal, both time (think big-O) and space wise, because that is the objective of this exercise. The less garbage you make the better your implementation will perform in the long run.

  8. The teachers tests invoke the sort method with different queue content, and measure the number of compare operations you do, because this is a good estimator for the big-O performance of the sort implementation.

  9. You are supposed to work test driven. Maybe start at a queue implementation and then continue with say selection sort using that queue. So write a test, write code and test that code, making the test pass, then rinse and repeat.

  10. How you re-arrange the items in the queue or if you use intermediate queues is up to you, as long as you return the same queue instance.

You are not supposed to use arrays in your Sorter or queue implementations.
Our tests will assert for that and will either not be executed or will fail your code if your do.
The same goes for abusing a java.util.List.sort or any subclass method to do your sorting.

1.2. Teacher Non Functional Validity Checks

The teacher tests will test according to the following non functional requirements in Queue and Sorter implementations.

  • Use of java.util.List or any of its implementing classes is not allowed.

  • Use of the utility class java.util.Arrays is not allowed.

  • Array access is not allowed in any of the methods.

  • The method in the Queue and Sorter implementing classes should be package private or private with the exception of the API methods; the method stipulated by the interfaces should be public.

  • The sorter and queue implementations should have the required package private constructors so that the factory can create instances, without exposing the constructor outside the package.

  • The sorter implementation is allowed one private final field of type Comparator<E>, and as many methods as you like.

  • The Node-s of the queue may be inner classes or outer classes, up to your discretion.

    • If you use inner classes for your Nodes, use static inner classes, otherwise each node will have a reference to the outer class which in this case will be a waste of space. Using non-static inner classes will also disallow you to move Nodes from one queue instance to another, which might otherwise be useful in a sorting implementation.

  • The queue implementation may use fields of the Node and of primitive types. No other types fields are required nor allowed.

1.3. Teacher functional and non functional assertions.

  • The test count the number of compare invocations by using a specially wrapper comparator instance. These tests will provide such comparator to the Sorter. The count is an indicator of the efficiency of the implementation. We expect \(c \times O(n^2)\) for selection insertion and insertion sort, where c is some constant like \(\frac{1}{2} \text{or} \frac{1}{4}\).

  • The tests should complete in reasonable time. For that we use a JUnit timeout rule set to 2000 milliseconds.

  • As sorted queue is deemed sorted if for all q.get() invocations, the value returned is not less than that of the previous q.get(). This property is called monotonic ascending order.

  • All tests for all students will be done with the same random data. The type of data is to the test’s discretion, since the implementations should be generic.

  • The Sorter will be tested with some a-typical queue fillings. Think of empty, one element, two elements, already sorted, all elements equal, reversely sorted, etc.

  • For the 'performance' tests, the tests will use queue sizes of different orders of magnitude.

From the Non-funtional junit test javadoc:

Inspect that the api implementing classes/instances provide by the sorting service factory comply with the alda 2020 rules.

  • The Sorter class should have exactly one final field of type Comparator<T>. Rationale: No more are needed and makes the Sorter thread-safe as in reusable by all threads in an application.

  • The Queue class is only allowed to have fields of the same package or primitive fields. The fields from the same package are expected to be link Nodes and are expected to have one or two fields of the Node type, the next and optional prev links, and some reference type field (the item or payload). The Node instances should NOT have primitive fields, nor a reference to an outer outer class if there is one.
    In short: if it is an inner class it should be a static inner class.

  • The number of methods in Sorter and Queue implementations is, other than by the implemented interfaces, unspecified.

1.4. Tasks week 8 and 9, Selection and Insertion sort.

  • Implement the sorting services using the selection and insertion sort approach.

Insert-sort with Romanian folk dance

1.5. Tasks week 10 and 11: QuickSort

As a sequel to the basic sorting assignments, we now proceed with the more sophisticated sorting routines. The basic sorting methods: Selection- and Insertion-sort are of complexity \(O(n^{2})\). The sorting method in this excercise has complexity \(O(n \times log(n))\). You should be aware of this and check that your implementation meets that expectation.


  1. Implement the quicksort algorithm using Bentley-McIlroy three-way partitioning, an optimization for the situation in which there are many duplicates.

  2. Consider using Medianof-three-partitioning. The underlying data structure has to be a (double) linked queue.

  3. Internally you may traverse the queue by using node.next and node.previous, but get(int i) (element by index), which is what a java.util.List provides, should not be used nor supported. It is also quite inefficient by itself, because this operation already has an \(O(n)\).[2] complexity.

Quick-sort with Hungarian folk dance

1.6. Tasks week 12, 13, and 14: Heap Sort

We finish our sorting project by implementing HeapSort. The sorting method in this exercise has complexity \(O(N ∗ log(N))\) and is expected to have better behavior than QuickSort on almost all, in particular on completely sorted lists. You should be aware of this and check that your implementation is not less efficient.

  • Implement the heapsort algorithm, using TreeNodes as underlying data structure. This data structure can still behave as a queue with minimal put and get operations.

  • Make sure that you have a Queue implementation that can be used for input and output as per teachers tests and the class diagram above.

  • Some helper methods that might be useful are (exact signature is dependent on your own implementation):

    • private TNode buildTree(Queue<T> q);

    • private TNode heapify();

    • private void sink(TNode parent);

    • private boolean less(TNode n1, TNode n2);

    • private void heapSortStep(TNode lastNode);

    • private void removeLastNode();

    • private void exchangeItems(TNode n1, TNode n2);

To build a tree, it might be useful to use an itemsQueue with all Nodes to be added, and an empty availableParentsQueue. The first itemNode of the itemsQueue is put as first availableParent. As long as the itemsQueue is not empty, get the next availableParent, assign the next itemNode to the left of the parentNode and put this itemNode as availableParent, continue by assigning the next itemNode to the right of the parentNode and put this itemNode as availableParent as well.

Quick-sort with Hungarian folk dance

1. Pun intended
2. That goes for the linked lists variants in java.util too.