Java Collections Interfaces Hierarchy

Collection Hierarchy

After knowing the meaning and purpose of Collections framework, let us know the basic interfaces, derived classes and their properties. Java data structures are nothing but subclasses of Collection and Map interfaces. Following hierarchy gives all the interfaces of Collections framework.

 

Fig: Basic interfaces of collections framework

 

Map and SortedMap form a separate hierarchy and not connected with Collection interface. Bu still, they are part of collections framework. It is done so to get interoperability with collections and legacy classes. Observe, interfaces Map and SortedMap are not derived from Collection interface. Collection being at the top of the hierarchy, it gives generic methods for all the derived classes.

Note: Framework name is collections and the interface name is Collection. Do not get confused; later, we get one more class called Collections.

Description of fundamental interfaces

A table is given showing the core interfaces with their implemented classes with brief description.

Interface	Brief Description	Derived Classes
Collection	It forms the root interface and comes with methods that define basic operations and makes all DS as one unit (called by a common name)	Majority of all DS come under this interface
Set	Subclasses of this interface have a common feature of not allowing duplicate elements	HashSet, LinkedHashSet
SortedSet	Subclasses of this interface come with a common feature of printing the elements in ascending order implicitly. Earlier feature derived from Set is allowing only unique elements	TreeSet
List	Subclasses allow duplicate elements	LinkedList, ArrayList, Vector
Map	Allows the subclasses to store key/value pairs. Does not allow duplicate keys.	HashMap, LinkedHashMap, Hashtable
SortedMap	Prints elements in ascending order of keys. A feature derived from Map is it does not allow duplicate keys.	TreeMap

 

Table: Description of Main interfaces

 

The elements of List can be accessed with their index numbers.

Collection classes store elements of single entities, but Map stores key/value pairs; to store a value, a key must be supplied along. SortedSet and SortedMap print their stored elements in ascending order (known as natural order) by default. The order can be customized explicitly using Comparator interface.

Following figure displays the main interfaces with relation to their derived classes.

As it can be observed, there comes some abstract classes in between interfaces and concrete classes (DS classes). They add some extra features to subclasses other than inherited from interfaces.

Following table gives the properties of derived classes.

Data Structure	Interface	Duplicates	Methods available	DS that inherits
HashSet	Set	Unique elements	equals(), hashCode()	Hashtable
LinkedHashSet	Set	Unique elements	equals(), hashCode()	Hashtable, doubly-linked list
TreeSet	SortedSet	Unique elements	equals(), compareTo()	Balanced Tree
ArrayList	List	Duplicates allowed	equals()	Resizable array
LinkedList	List	Duplicate allowed	equals()	Linked list
Vector	List	Duplicates allowed	equals()	Resizable array
HashMap	Map	Unique keys	equals() and hashCode()	Hash table
LinkedHashMap	Map	Unique keys	equals() and hashCode()	Hash table and doubly-linked list
Hashtable	Map	Unique keys	equals(), hashCode()	Hash table
TreeMap	SortedMap	Unique keys	equals(), compareTo()	Tree Map

 

Figure: Collections Features

 

There are two distinct hierarchies monitored by Collection and Map. Map is also placed in collections framework to have interoperability.

Basic Features of Main Interfaces

Core collection interfaces are the base to Java Collections framework. They include the interfaces Collection, Set, List, Queue and Map on which all the data structures are built.

1. Collection is the root interface for all the hierarchy (except Map).
2. Set interface unique feature is that it does not accept duplicate elements. That is, no two elements will be the same.
3. SortedSet interface is derived from Set interface and adds one more feature that the elements are arranged in sorted order by default.
4. List interface permits duplicate elements.
5. Queue interface holds elements and returns in FIFO order.
6. Map adds the elements in key/value pairs. Duplicate keys are not allowed, but duplicate values are allowed. One key can map one value only.
7. SortedMap interface is a particular case of Map. Keys are sorted by default.