Collection Framework in Java: Complete Guide with Examples (2026)

Collection Framework in Java: Complete Guide with Examples

Q: What is the difference between List and Set in Java?

A List allows duplicate elements and is ordered, whereas a Set does not allow duplicates. List is implemented by ArrayList, LinkedList, and Vector; Set is implemented by HashSet and TreeSet.

Q: What is the difference between Queue and Deque in Java?

A Queue follows FIFO (First In, First Out) order, while a Deque (double-ended queue) allows insertion and removal at both ends. Queue is implemented by PriorityQueue; Deque is implemented by ArrayDeque.

Q: What is the Collections class in Java?

The Collections class is a utility class in java.util that contains only static methods to operate on collections, such as sort(), reverse(), shuffle(), binarySearch(), max(), min(), and synchronized variants.

Collections are used in almost every programming language. Most languages support various types of collections such as List, Set, Queue, and Stack. In this guide, you will learn the Java Collection Framework in detail — its interfaces, classes, methods, and advantages.

What is a Collection?

A Collection is a group of objects.

A Framework is a pre-defined architecture where we can manipulate a group of objects.

Java 1.2 provided a Collection Framework that is used to provide sorting, searching, and manipulation of a group of objects. The Collection Framework provides a set of interfaces and classes and also provides implementations of algorithms.

All classes and interfaces of the Collection Framework exist in the java.util package.

Why Collection Framework?

The Java Collections Framework has the following advantages:

Reduced development effort by using core collection classes rather than implementing your own collection classes.
Code quality is enhanced with the use of well-tested Collections Framework classes.
Reduced effort for code maintenance by using collection classes shipped with the JDK.
Re-usability and interoperability.

Forms of Collection

There are two forms of a collection:

Generic Collection
Non-Generic Collection

Generic Collection

A group of similar or homogeneous type of objects is called a Generic Collection.

For example: List<String> list = new ArrayList<String>();

In the above collection, we can have String type of objects only.

Non-Generic Collection

A group of dissimilar or heterogeneous type of objects is called a Non-Generic Collection.

For example: List list = new ArrayList();

In the above collection, we can have any type of objects.

Difference Between Array and Collection

Comparison of Array and Collection in Java
Array	Collection
Array size is fixed.	Collection is re-sizable.
Array can hold homogeneous data type.	Collection can be homogeneous or heterogeneous.
Array can hold primitive and non-primitive data type.	Collection can hold non-primitive data type (Objects).
Array is faster than Collection.	Collection is slower than an array.
If you know the size of elements, then go for array.	If you don't know the size of elements, then go for collection.
In case of array, there are no ready-made methods available.	In case of collection, there are ready-made methods available.
In case of memory, array is not recommended.	In case of memory, collection is recommended.

Hierarchy of Collection Framework

The Java Collection Framework follows this interface hierarchy:

Java Collection Framework hierarchy diagram showing Iterable, Collection, List, Queue, Deque, and Set interfaces — Java Collection Framework — Interface Hierarchy

Iterable Interface

It is the root class of the Collection Framework.
This interface exists in the java.lang package.
It has an iterator() method which is used to iterate a collection.
This iterator() method returns an Iterator interface object which exists in the java.util package.

Iterator Interface

The Iterator interface provides methods to iterate over any Collection. We can get an iterator instance from a Collection using the iterator() method.
Iterators allow the caller to remove elements from the underlying collection during the iteration.
Iterators in collection classes implement the Iterator Design Pattern.

Iterator Methods

Iterator interface methods
Method	Description
`hasNext()`	Tests whether the next element is available in the collection or not during iteration.
`next()`	Returns an element from a collection during iteration.
`remove()`	Removes an element from the collection during iteration.

Collection Interface

Collection is a sub-interface of the Iterable interface, which has the following methods to perform different operations.

Collection interface methods
Method	Description
`size()`	Used to get the length of a collection.
`isEmpty()`	Tests whether the collection is empty or not.
`contains()`	Finds an element from a collection.
`toArray()`	Used to convert a collection into an array.
`add()`	Used to add an element into the collection.
`remove()`	Used to remove an element from the collection.
`clear()`	Used to remove all elements from the collection.
`addAll()`	Used to add one collection into another collection.
`removeAll()`	Used to remove a collection from another collection.
`retainAll()`	Used to remove all collection elements except those in the retained collection.
`containsAll()`	Finds a collection from another collection.

List Interface

List is a sub-interface of the Collection interface.
The List interface allows duplicate elements.
The List interface is an ordered collection (sometimes called a sequence).
The List interface is implemented by the ArrayList, LinkedList, and Vector classes.

List Methods

List interface methods
Method	Description
`get()`	Used to get an element based on index from the collection.
`set()`	Used to replace an element based on index in the collection.
`indexOf()`	Used to get the index based on the element.
`lastIndexOf()`	Used to get the index based on the last occurrence of the element.
`subList()`	Used to get a sublist based on a from-index and to-index.
`listIterator()`	Used to iterate elements in both directions — forward and backward.

Queue Interface

Queue is a sub-interface of the Collection interface.
The Queue interface holds elements in FIFO (First In First Out) order.
The Queue interface is implemented by the PriorityQueue class.

Queue Methods

Queue interface methods
Method	Description
`add()`	Used to add an element to the Queue.
`offer()`	Used to add an element to the Queue.
`remove()`	Used to get and remove the element at the top of the Queue.
`poll()`	Used to get and remove the element at the top of the Queue; returns null if the Queue is empty.
`element()`	Used to get the element at the top of the Queue.
`peek()`	Used to get the element at the top of the Queue; returns null if the Queue is empty.

Deque Interface

Deque is a sub-interface of the Queue interface.
In a Deque, we can insert and delete elements from both ends.
Methods are provided to insert, remove, and examine the element.
Each of these methods exists in two forms:
- One throws an exception if the operation fails.
- The other returns a special value (either null or false, depending on the operation).
The Deque interface is implemented by the ArrayDeque class.

Deque Methods

Deque interface methods
Method	Description
`addFirst()`	Used to add an element at the beginning.
`addLast()`	Used to add an element at the end.
`offerFirst()`	Used to add an element at the beginning and returns true or false.
`offerLast()`	Used to add an element at the end and returns true or false.
`removeFirst()`	Removes the first element; throws NoSuchElementException if the deque is empty.
`removeLast()`	Removes the last element; throws NoSuchElementException if the deque is empty.
`pollFirst()`	Removes the first element; returns null if the deque is empty.
`pollLast()`	Removes the last element; returns null if the deque is empty.
`getFirst()`	Gets the top element; throws NoSuchElementException if the deque is empty.
`getLast()`	Gets the last element; throws NoSuchElementException if the deque is empty.
`peekFirst()`	Gets the top element; returns null if the deque is empty.
`peekLast()`	Gets the last element; returns null if the deque is empty.
`removeFirstOccurrence()`	Removes the first occurrence of an element.
`removeLastOccurrence()`	Removes the last occurrence of an element.
`push()`	Adds an element at the top of the Deque.
`pop()`	Removes an element from the top.
`descendingIterator()`	Used to iterate in descending order.

Set Interface

Set is a sub-interface of the Collection interface.
The Set interface can never contain duplicate elements.
The Set interface is implemented by the HashSet and TreeSet classes.

Set Methods

Set interface methods
Method	Description
`size()`	Used to get the length of the collection.
`isEmpty()`	Tests whether the collection is empty or not.
`contains()`	Finds an element from a collection.
`toArray()`	Used to convert a collection into an array.
`add()`	Used to add an element into the collection.
`remove()`	Used to remove an element from the collection.
`clear()`	Used to remove all elements from the collection.
`addAll()`	Used to add one collection into another collection.
`removeAll()`	Used to remove a collection from another collection.
`retainAll()`	Used to remove all collection elements except those in the retained collection.
`containsAll()`	Finds a collection from another collection.

Collections Class

The Java Collections class consists exclusively of static methods that operate on or return collections. This class provides the following methods:

Collections utility class methods
Method	Description
`addAll()`	Used to add elements to a specific collection.
`binarySearch()`	Used to search the position of an element in a sorted list.
`copy()`	Used to copy all elements from a source list to a destination list.
`frequency()`	Returns the number of times an object appears in a collection.
`max()`	Gets the maximum element or value from a collection.
`min()`	Gets the minimum element or value from a collection.
`replaceAll()`	Replaces all occurrences of one specified value in a list with another.
`reverse()`	Reverses the order of list elements.
`rotate()`	Rotates the elements in the specified list by a given distance.
`shuffle()`	Randomly reorders the specified list elements.
`sort()`	Sorts a list of elements.
`swap()`	Swaps one element with another based on position in the list.
`synchronized...()`	Makes a collection or map thread-safe.
`unmodifiable...()`	Makes a collection or map immutable.

Frequently Asked Questions

What is the Collection Framework in Java?

The Java Collection Framework is a pre-defined architecture introduced in Java 1.2 that provides interfaces and classes to store and manipulate a group of objects. It offers sorting, searching, and other data-processing operations and lives in the java.util package.

What is the difference between Array and Collection in Java?

An array has a fixed size and can hold homogeneous primitive or non-primitive data, whereas a Collection is re-sizable and can hold homogeneous or heterogeneous objects. Collections provide ready-made methods and are recommended when the number of elements is unknown.

What is the difference between List and Set in Java?

A List allows duplicate elements and is ordered, whereas a Set does not allow duplicates. List is implemented by ArrayList, LinkedList, and Vector; Set is implemented by HashSet and TreeSet.

What is the difference between Queue and Deque in Java?

A Queue follows FIFO (First In, First Out) order, while a Deque (double-ended queue) allows insertion and removal at both ends. Queue is implemented by PriorityQueue; Deque is implemented by ArrayDeque.

What is the Collections class in Java?

The Collections class is a utility class in java.util that contains only static methods to operate on collections, such as sort(), reverse(), shuffle(), binarySearch(), max(), min(), and synchronized variants.

Conclusion

The Java Collection Framework is a powerful architecture that provides reusable data structures and algorithms out of the box. By understanding the hierarchy — Iterable → Collection → List, Queue, Deque, and Set — and the utility methods in the Collections class, you can write cleaner and more efficient Java code.

Related topics: Java ArrayList Tutorial, Java HashMap Tutorial, Java LinkedList Tutorial, Java TreeSet Tutorial, Java Generics, Java Iterator vs ListIterator

D.Tech Academic

Pages