Collection Framework – Class Hierarchy

Why Collection Framework?

Collections are nothing but group of objects stored in well-defined manner. Earlier, Arrays are used to represent these group of objects. But, arrays are not re-sizable. size of the arrays are fixed. Size of the arrays cannot be changed once they are defined. This causes lots of problem while handling group of objects. To overcome this drawback of arrays, Collection framework or simply collections are introduced in java from JDK 1.2.

What is Collection Framework in Java?

Collection Framework is representing a unified architecture for storing and manipulating the group of objects. Using collection framework, you can store the objects as a list or as a set or as a queue or as a map and perform operations like adding an object or removing an object or sorting the objects without much hard work.

Class Hierarchy of Collection Framework:

The entire collection framework is divided into four interfaces.

1) List —> It handles sequential list of objects. ArrayList, Vector and LinkedList classes implement this interface.

2) Queue —> It handles special list of objects in which elements are removed only from the head. LinkedList and PriorityQueue classes implement this interface.

3) Set —> It handles list of objects which must contain unique element. This interface is implemented by HashSet and LinkedHashSet classes and extended by SortedSet interface which in turn, is implemented by TreeSet.

4) Map —> This is the one interface in Collection Framework which is not inherited from Collection interface. It handles group of objects as Key/Value pairs. It is implemented by HashMap and HashTable classes and extended by SortedMap interface which in turn is implemented by TreeMap.

Three of above interfaces (List, Queue and Set) inherit from Collection interface. Although, Map is included in collection framework it does not inherit from Collection interface.

Collection interface is the root level interface in the collection framework. List, Queue and Set are all sub interfaces of Collection interface. JDK does not provide any direct implementations of this interface. But, JDK provides direct implementations of its sub interfaces.

Collection interface extends Iterable interface which is a member of java.lang package. Iterable interface has only one method called iterator(). It returns an Iterator object, using that object you can iterate over the elements of Collection. Here is the class diagram of Collection interface.

 

Collection interface contains total 15 abstract methods. 14 of its own and one is inherited from Iterable interface. Here is the list and descriptions of those methods.

No.	Method	Description
1	public boolean add(Object element)	is used to insert an element in this collection.
2	public boolean addAll(Collection c)	is used to insert the specified collection elements in the invoking collection.
3	public boolean remove(Object element)	is used to delete an element from this collection.
4	public boolean removeAll(Collection c)	is used to delete all the elements of specified collection from the invoking collection.
5	public boolean retainAll(Collection c)	is used to delete all the elements of invoking collection except the specified collection.
6	public int size()	return the total number of elements in the collection.
7	public void clear()	removes the total no of element from the collection.
8	public boolean contains(Object element)	is used to search an element.
9	public boolean containsAll(Collection c)	is used to search the specified collection in this collection.
10	public Iterator iterator()	returns an iterator.
11	public Object[] toArray()	converts collection into array.
12	public boolean isEmpty()	checks if collection is empty.
13	public boolean equals(Object element)	matches two collection.
14	public int hashCode()	returns the hashcode number for collection.

Iterator interface

Iterator interface provides the facility of iterating the elements in forward direction only.

Methods of Iterator interface

There are only three methods in the Iterator interface. They are:

1. public boolean hasNext() it returns true if iterator has more elements.
2. public object next() it returns the element and moves the cursor pointer to the next element.
3. public void remove() it removes the last elements returned by the iterator. It is rarely used.

 Note : 

equals() and hashcode() methods in the Collection interface are not the methods of java.lang.Object class. Because, interfaces do not inherit from Object class. Only classes in java are sub classes of Object class. Any classes implementing Collection interface must provide their own version of equals() and hashcode() methods or they can retain default version inherited from Object class.