Java Collection Framework: Complete Guide with Examples

The Java Collection Framework (JCF) is a set of classes and interfaces used to store and manipulate groups of objects dynamically. It provides efficient data structures and algorithms.

What is Collection Framework?

A collection is a group of individual objects treated as a single entity. Java provides Collection Framework to overcome limitations of arrays.

• Dynamic size (growable)
• Can store heterogeneous objects
• Provides built-in methods

Collection Interface

The Collection interface is the root interface of Collection Framework.

Important Methods

• add()
• remove()
• size()
• clear()

List Interface

List is an ordered collection that allows duplicates.

• Insertion order preserved
• Duplicates allowed

Implementations

• ArrayList
• LinkedList
• Vector

Example

import java.util.*;

List list = new ArrayList();
list.add(10);
list.add("Java");

System.out.println(list);

Set Interface

Set does not allow duplicate elements.

• No duplicates
• Insertion order not guaranteed

Implementations

• HashSet
• LinkedHashSet
• TreeSet

Example

Set set = new HashSet();
set.add(10);
set.add(10);

System.out.println(set); // duplicate removed

Queue Interface

Queue follows FIFO (First In First Out).

• PriorityQueue

Example

Queue q = new PriorityQueue();
q.add(10);
q.add(5);

System.out.println(q);

Map Interface

Map stores data in key-value pairs.

• Keys → unique
• Values → duplicates allowed

Implementations

• HashMap
• LinkedHashMap
• TreeMap
• Hashtable

Example

Map map = new HashMap();
map.put(1, "Java");
map.put(2, "Spring");

System.out.println(map);

Cursors in Java

Cursors are used to retrieve objects one by one from collections.

Types

• Enumeration (legacy)
• Iterator
• ListIterator

Example

Iterator it = list.iterator();

while(it.hasNext()) {
    System.out.println(it.next());
}

Collection Framework Summary

• Collection → root interface
• List → ordered, duplicates allowed
• Set → no duplicates
• Queue → FIFO
• Map → key-value pairs

Collection Framework FAQ

What is Collection Framework?

A set of classes and interfaces to manage data efficiently.

Difference between List and Set?

List allows duplicates, Set does not.

Difference between HashMap and Hashtable?

HashMap is not synchronized, Hashtable is thread-safe.