Collection-in-Java

Collection

A Collection in Java is a framework that provides an architecture to store and manipulate a group of objects efficiently. It provides various interfaces and classes to work with different types of data structures like Lists, Sets, and Maps.

• Collection is like a container that holds multiple objects.
• It provides dynamic memory allocation, unlike arrays which have a fixed size.
• It allows easy addition, deletion, searching, and sorting of elements.

List Interface in Java

The List interface in Java is part of the Collections Framework and is used to store ordered and indexed elements, allowing duplicates.

Key Features of List

• Ordered Collection → Elements are stored in the order they are inserted.
• Allows Duplicates → Can have repeated elements.
• Index-based Access → Elements can be accessed using an index (list.get(index)).
• Supports Null Values → Can store null values.
• Dynamic Resizing → Unlike arrays, lists grow dynamically.

Basic ArrayList Operation

add(E e)	                Adds an element to the list

add(int index, E e)	        Adds element at a specific index

get(int index)	                Returns element at the given index

remove(Object o)	        Removes first occurrence of given element

remove(int index)	        Removes element at the specified index

set(int index, E e)	        Replaces the element at the given index

size()	                        Returns the number of elements in the list

contains(Object o)	        Checks if the list contains the element

LinkedList

- LinkedList is a doubly linked list implementation in Java.
- It allows fast insertions and deletions compared to ArrayList.
- It implements List and Deque interfaces.

Basic LinkedList Operation

add(E e)	            Adds an element at the end of the list

add(int index, E e)	    Inserts an element at the specified position

addFirst(E e)	            Adds an element at the beginning of the list

addLast(E e)	            Adds an element at the end of the list (same as add(E e))

get(int index)	            Returns the element at the given index

getFirst()	            Retrieves the first element

getLast()	            Retrieves the last element

remove(int index)	    Removes the element at the given index

remove(Object o)	    Removes the first occurrence of the specified element

removeFirst()	            Removes and returns the first element

removeLast()	            Removes and returns the last element

set(int index, E e)	    Replaces the element at the given index

size()	                    Returns the number of elements in the list

contains(Object o)	    Checks if the list contains the element

clear()	                    Removes all elements from the list

indexOf(Object o)	    Returns the index of the first occurrence of the specified element

lastIndexOf(Object o)	    Returns the index of the last occurrence of the specified element

peekFirst()	            Retrieves, but does not remove, the first element

peekLast()	            Retrieves, but does not remove, the last element

pollFirst()                 Retrieves and removes the first element, or returns null if empty

pollLast()	            Retrieves and removes the last element, or returns null if empty

Set

The Set interface in Java represents a collection of unique elements, meaning it does not allow duplicates. It is commonly used when you need to store a group of distinct elements.

Key Features of Set

• No duplicates allowed
• Unordered (HashSet) or sorted (TreeSet) storage
• Faster lookups compared to lists

HashSet in Java

HashSet is the most commonly used Set implementation in Java. It is fast because it uses a hash table for storing elements.

Basic HashSet Operation

- add(E e)	Adds an element to the set

- remove(Object o)	Removes an element from the set

- contains(Object o)	Checks if the set contains the element

- size()	Returns the number of elements

- isEmpty()	Checks if the set is empty

- clear()	Removes all elements

- iterator()	Returns an iterator for traversal