Java Collections Framework

The Java Collections Framework provides a set of classes and interfaces that implement commonly used data structures, allowing developers to store, retrieve, and manipulate data in a variety of ways. It provides a unified architecture for working with different types of collections such as lists, sets, queues, and maps.

Key Interfaces of Java Collections Framework

1. Collection Interface

The Collection interface is the root interface of the collection hierarchy. It represents a group of objects, known as elements.

• Methods:
  • add(E e): Adds an element to the collection.
  • remove(Object o): Removes an element from the collection.
  • clear(): Removes all elements from the collection.
  • size(): Returns the number of elements in the collection.
  • isEmpty(): Checks if the collection is empty.

2. List Interface

The List interface extends the Collection interface and represents an ordered collection that may contain duplicate elements.

• Implementing Classes: ArrayList, LinkedList, Vector, Stack

• Methods:

  • add(int index, E element): Adds an element at the specified index.
  • get(int index): Retrieves the element at the specified index.
  • set(int index, E element): Replaces the element at the specified index.
  • remove(int index): Removes the element at the specified index.

3. Set Interface

The Set interface extends Collection and represents a collection that does not allow duplicate elements.

• Implementing Classes: HashSet, LinkedHashSet, TreeSet

• Methods:

  • add(E e): Adds an element to the set (only if not already present).
  • remove(Object o): Removes the element from the set.
  • contains(Object o): Checks if the element exists in the set.
  • size(): Returns the number of elements in the set.

4. Queue Interface

The Queue interface represents a collection designed for holding elements before processing. It is typically used to model a queue of tasks, where elements are processed in FIFO (First In First Out) order.

• Implementing Classes: LinkedList, PriorityQueue

• Methods:

  • add(E e): Inserts the specified element into the queue.
  • peek(): Retrieves, but does not remove, the head of the queue.
  • poll(): Retrieves and removes the head of the queue.
  • offer(E e): Adds an element to the queue (may fail or return false if the queue is full).

5. Map Interface

The Map interface represents a collection of key-value pairs. It maps keys to values, where each key is associated with exactly one value.

• Implementing Classes: HashMap, LinkedHashMap, TreeMap

• Methods:

  • put(K key, V value): Adds a key-value pair to the map.
  • get(Object key): Retrieves the value associated with the key.
  • containsKey(Object key): Checks if a key exists in the map.
  • remove(Object key): Removes the key-value pair for the given key.

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Common Implementing Classes

1. ArrayList

An ArrayList is a resizable array implementation of the List interface. It allows dynamic resizing and provides fast random access to elements.

• Characteristics:
  • Allows duplicate elements.
  • Elements are ordered.
  • Provides O(1) time complexity for accessing elements by index.

List<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");