Error in adding generic collection sort method

Question

I am trying to call Collections.sort(c); but I am getting an error:

The method sort(List<T>) in the type Collections is not applicable
for the arguments (Collection<capture#1-of ? extends E>)

Here is my code:

import java.lang.*;

public class SortedLinkedList<E extends Comparable<E>> extends LinkedList<E> {
    LinkedList<Object> list = new LinkedList();
    SortedLinkedList()
    {

    }

     SortedLinkedList(Collection<? extends E> c)
    {
        //return sorted linked list of c
        Collections.sort(c);
    }

    public int compareTo(Collection<? extends E> c) {
        // TODO Auto-generated method stub

        return 0;
    }
}

I've declared the class with the generic type extending comparable interface. But this still wouldn't help the error. I followed the post that was mentioned when this was marked duplicate but it couldn't help much. This is my first attempt to learn generics in java. Any help is much appreciated. -Thanks!

Answer 1

The type <E extends Comparable<E>> is fine, but there are several other problems:

• c is a Collection , but you can't sort a Collection unless that Collection is a List , because only List s allow the arranging and rearranging of elements in specific orders. Other types of Collection , such as a Set or bag , do not. You could typecast c to List , but still it would be the wrong object to sort. It looks like you want to put the contents of c into your linked list and then sort that :

   SortedLinkedList(Collection<? extends E> c) { list.addAll(c); Collections.sort(list); } 

• The list is declared LinkedList<Object> but should be LinkedList<E> , ( or maybe List<E> ), so that it is declared to contain sortable objects.

• The assignment of new LinkedList(); should be new LinkedList<E>() , which can be shortened to new LinkedList<>() .

That's enough changes to make the code compile, but let's delve deeper. I infer that what you're trying to do here is create a generic container collection that is a linked list with an invariant that its elements are always maintained in sorted order. In that case, some changes you'll want to make are:

• The list variable should be private to prevent other classes futzing about with it. If you do not want to re-assign the variable after initialization it would also be nice to make it final , which protects against accidental re-assignment and clarifies that that is how you're using it.

• The constructors should be public to allow access from other packages.

• I'm not sure what you intend the compareTo method there for. (How do you define a comparison of one entire collection against another?) Possibly it should be removed.

• Currently you're both encapsulating and extending LinkedList , which doesn't make sense. Each instance of your class has two LinkedList s, one in the list variable, and one inherited from the parent. You need to decide on one or the other.

  • If you want to extend LinkedList then you can get rid of the list variable entirely and call the superclass methods instead. Eg:

     public SortedLinkedList(Collection<? extends E> c) { super.addAll(c); Collections.sort(this); } 

    In this case you will need to override any mutative methods of the parent class to make sure that none of them can be used to subvert the invariant that your class maintains its elements in sorted order. Eg, override add(E element) and make it insert the new element in the correct place. Whereas add(int position, E element) should be overridden to throw an UnsupportedOperationException since inserting an element at a specified index position doesn't make sense, because an element's position in a sorted list is already implied by its value.

    A disadvantage of extending LinkedList is that is possible for new mutative methods to be added to the LinkedList class in future, which could then allow users to subvert your collection's invariant.

  • If you want to encapsulate a LinkedList with your list variable, then you should delete extends LinkedList<E> and instead have implements List<E> .

    In this case you will need to provide an implementation for all the methods of the interface, but you can instantly implement most of them correctly by extending one of the abstract skeletal classes that the Java Collections Framework provides, such as AbstractSequentialList .

  • Third possibility: neither extend nor encapsulate LinkedList but write a linked list from scratch.

• The line import java.lang.*; is unnecessary. Everything in the java.lang package is imported by default .

The following is an example based on the above fixes:

import java.util.*;

public class SortedLinkedList<E extends Comparable<E>>
        extends AbstractSequentialList<E> implements List<E> {
    private final LinkedList<E> list = new LinkedList<>();

    public SortedLinkedList() {}

    public SortedLinkedList(Collection<? extends E> c)
    {
        list.addAll(c);
        Collections.sort(list);
    }

    @Override
    public boolean add(E element) {
        list.add(element);
        Collections.sort(list);
        return true;
    }

    @Override
    public ListIterator<E> listIterator(int index) {
        // Rather than returning list.listIterator(index) directly, we
        // encapsulate it to block the add and set methods:
        return new ListIterator<E>() {
            private final ListIterator<E> base = list.listIterator(index);

            @Override
            public boolean hasNext() {
                return base.hasNext();
            }

            @Override
            public E next() {
                return base.next();
            }

            @Override
            public boolean hasPrevious() {
                return base.hasPrevious();
            }

            @Override
            public E previous() {
                return base.previous();
            }

            @Override
            public int nextIndex() {
                return base.nextIndex();
            }

            @Override
            public int previousIndex() {
                return base.previousIndex();
            }

            @Override
            public void remove() {
                base.remove();
            }

            @Override
            public void set(E e) {
                // prevent unsorting the list
                throw new UnsupportedOperationException();
            }

            @Override
            public void add(E e) {
                // prevent unsorting the list
                throw new UnsupportedOperationException();
            }
        };
    }

    @Override
    public int size() {
        return list.size();
    }
}

The bulk of the List methods get implemented with no effort thanks the magical superclass AbstractSequentialList and its superclasses. However if you check the source you'll find things you can improve if you override those methods because the inherited implementations are designed principally to minimize effort in extending the class. Eg to clear the list it iterates each element and carefully removes them one at a time (via ListIterator.remove() ), whereas deferring to the LinkedList 's clear() method would be faster.

Also, instead of re-sorting the entire list after adding an element, it would be much more efficient to insert it directly in the correct place. You can do this via ListIterator.add , but I'll leave that to you :)

Another nice feature would be to allow your class to be constructed with a custom Comparator to be used for sorting elements. This would allow use of element types that do not implement Comparable , as well as the ability to override the default ordering (eg, users of the class could supply a Comparator for case-insensitive ordering of String s ). TreeMap is an example of a class that supports this sort of feature.

I hope the example is helpful for showing the concepts, anyway.