Collections Framework

What we’ll cover

What is a collection?

The Collection Interface

The Iterator Interface

The Iterable Interface

Iterator and Collection interface

What is a Collection?

• Collection is an interface which ensures a class has the ability to hold a series of objects.
• Often, we consider Map objects to be a Collection, although they do not implement the Collection interface.

Collections and Maps

List	Set	Map
ArrayList	HashSet	TreeMap
LinkedList	TreeSet	HashMap
ArrayDeque	PriorityQueue	WeakHashMap
	EnumSet	IdentityHashMap
	LinkedHashSet	LinkedHashMap

Collection Interface

• Fundamental interface for Collection classes in java.

public interface Collection<E> extends Iterable<E> {
    boolean add(E element);
    boolean addAll(Collection<? extends E> collection);
    void clear();
    boolean contains(Object object);
    boolean containsAll(Collection<?> collection);
    boolean isEmpty();
    Iterator<E> iterator();
    boolean remove(Object object);
    boolean removeAll(Collection<?> collection);
    boolean retainAll(Collection<?> collection);
    int size();
    Object[] toArray();
    <T> T[] toArray(T[] array);
}

Collection Interface

boolean add(E element)

• Attempts to add an element to the Collection
• returns true if adding the element changes the Collection, else false.
• Adding an already-present-object to a Set collection will return false.

public void demo() {
	Collection<String> set = new HashSet<>();
	String valueToBeAdded = "Hedjet";
	set.add(valueToBeAdded);
	System.out.println(set.add(valueToBeAdded)); // prints false
}

Collection Interface

boolean addAll(Collection)

• Attempts to add a collection of elements to the Collection
• returns true if adding the elements changes the Collection, else false.

public void demo() {
  Collection<String> set = new HashSet<>();
  String[] valuesToBeAdded = {"Froilan", "Wilhem", "Leon", "Nhu", "Kris"};
  Collection<String> valuesAsList = Arrays.asList(valueToBeAdded);
  System.out.println(set.addAll(list)); // prints true
}

Collection Interface

boolean remove(Object)

• Attempts to remove an object from the Collection
• returns true if removing the element changes the Collection, else returns false
• Removing an element that is not present in an ArrayList will return false.

public void demo() {
  // prints false
  System.out.println(new ArrayList().remove(new Object()));
}

Collection Interface

boolean removeAll(Collection)

• Attempts to remove a collection of elements from the Collection
• returns true if removing the elements changes the Collection, else returns false

public void demo() {
  String[] elementsAsArray = {"The", "Quick", "Brown"};
  Collection<String> originalCollection = new ArrayList<>();
  Collection<String> elementsAsList = Arrays.asList(elementsAsArray);

  // prints false
  System.out.println(originalCollection.removeAll(elementsAsList));
}

Collection Interface

boolean retainAll(Collection)

• Retains only the elements in this collection that are contained in the specified collection.
• returns true if retaining the elements changes the Collection, else returns false

public void demo() {
  String[] originalArray = {"The", "Quick", "Brown"};
  String[] elementsToBeRetained = {"The", "Quick"};
  List<String> originalList = new ArrayList<>(Arrays.asList(originalArray));
  List<String> retentionList = Arrays.asList(elementsToBeRetained);

  // prints true  
  System.out.println(originalList.retainAll(retentionList)));
}

Collection Interface

boolean isEmpty()

• returns true if the size of the Collection is 0, else returns false.

public void demo() {
  String[] elementsAsArray = {"The", "Quick", "Brown"};
  Collection<String> elementsAsList = Arrays.asList(arrayOfStrings);
  System.out.println(elementsAsList.isEmpty()); // prints false
}

Collection Interface

int size()

• Returns the number of elements in the Collection.

public void demo() {
  String[] elementsAsArray = {"The", "Quick", "Brown"};
  Collection<String> elementsAsList = Arrays.asList(arrayOfStrings);
  System.out.println(elementsAsList.size()); // prints 3
}

Collection Interface

void clear()

• Removes all elements from the Collection.

public void demo() {
  String[] elementsAsArray = {"The", "Quick", "Brown"};
  List<String> elementsAsList = new ArrayList<>(Arrays.asList(arrayOfStrings));
  elementsAsList.clear();
  System.out.println(elementsAsList.isEmpty()); // prints true
}

Collection Interface

Object[] toArray()

• Populates a new Object[] with the elements from this Collection

public void demo() {
  String[] elementsToAdd = {"The", "Quick", "Brown"};
  List<String> elementList = new ArrayList<>(Arrays.asList(elementsToAdd));
  Object[] listAsObjectArray = elementList.toArray();
}

Collection Interface

E[] toArray(E[])

• Populates a new array of the respective type with the elements from this Collection

public void demo() {
  String[] elementsToAdd = {"The", "Quick", "Brown"};
  List<String> elementList = new ArrayList<>(Arrays.asList(elementsToAdd));

  int newArrayLength = elementList.size();
  String[] arrayToBePopulated = new String[newArrayLength];
  String[] listAsStringArray = elementList.toArray(arrayToBePopulated);
}

Iterator interface

• Iterator is used to visit the elements in the Collection, one by one.

public interface Iterator<E> {
  E next();
  boolean hasNext();
  void remove();
  default void forEachRemaining(Consumer<? super E> action);
}

Collection Interface
Iterator<E> iterator()

• Returns an object that implements the Iterator interface

Iterable Interface

• Iterable ensures the implementing class is a valid candidate for the foreach loop
• Collection extends Iterable, therefore all Collection types are valid candidates for the foreach loop.
• All Iterables must provide an implementation for Iterator<E> iterator().
• Is NOT the same as the Iterator interface.

Iterable Interface

public interface Iterator<E> {
  Iterator<E> iterator();
  forEach(Consumer<? super E> E);
}

Iterator Interface

• Repeatedly calling the next() method enables you to visit each element from the collection, one by one.
• NoSuchElementException is thrown upon invoking next() on an Iterator that has reached the end of the collection.
  • This can be prevented by evaluating the hasNext() method before calling next().
  • The compiler translates foreach loops into a loop with an iterator.

public static void printIterable(Iterable<Object> iterable) {
    Iterator iterator = iterable.iterator();
    while(iterator.hasNext()) {
        System.out.println("Current Element = " + iterator.next());
    }
}

Iterator Interface

• As of Java8, you can call the forEachRemaining method with a Consumer lambda expression.
• The lambda expression is invoked with each element of the iterator, until there are none left.

public static void printIterable(Iterable<Object> iterable) {
  Iterator iterator = iterable.iterator();
  iterator.forEachRemaining((element) -> System.out.println(element));
}

Iterator Interface
next()

• Think of Java iterators as being between elements.
• When you call next, the iterator jumps over the next element, and it returns a reference to the element that it just passed.

Iterator Interface
remove()

• removes the element that was returned by the last call to next()
• Often, you may need to view an element before deciding to delete it.
• It is illegal to call remove() if it wasn’t preceded by a call to next().

public void deleteFirstElement(Iterator<String> iterator) {
	iterator.next(); // skip first element
	iterator.remove(); // remove first element
}

AbstractCollection Class

• The Collection interface declares 18 methods.
• To avoid implementing a lot of the fundamental methods, the Collection library developers created an AbstractCollection class.
• AbstractCollection has a concrete implementation of all Collection methods except size() and iterator()

Sample AbstractCollection implementation

public class MyCollection<E> extends AbstractCollection<E>  {
    private final Iterable<E> iterable;
    public MyCollection(Iterable<E> iterable) {
    	this.iterable = iterable;
    }

    @Override
    public Iterator<E> iterator() {  return iterable.iterator(); }

    @Override
    public int size() {
        List<E> list = new ArrayList<>();
        iterator().forEachRemaining(list::add);
        return list.size();
    }
}