What is the Java Collections Framework?

The Java Collections Framework is a unified architecture for storing and manipulating groups of objects using interfaces like List, Set, and Map and implementing classes such as ArrayList, LinkedList, HashMap, and HashSet.

When should I use an ArrayList?

An ArrayList is ideal when you need fast random access and when insertions or deletions occur mostly at the end of the list. It is backed by a dynamic array structure.

When should I use a LinkedList?

LinkedList is preferred when an application requires frequent insertions or deletions in the middle of a list, as these operations are more efficient than with an ArrayList.

What is the purpose of a HashMap?

A HashMap stores data in key-value pairs and provides constant-time performance for get and put operations under average conditions. Keys must be unique, but values can be duplicated.

What is a HashSet used for?

A HashSet is used when you need to store unique elements with no duplicates. It is backed by a hash table and offers constant-time performance for add, remove, and contains operations.

Java Collections Framework – List, Set, Map, Iterator with Examples

The Java Collections Framework provides ready-made data structures such as List, Set, and Map. These classes help you store, search, sort, iterate, and manipulate data efficiently.

1. ArrayList

ArrayList is a resizable array. It allows duplicate values, maintains insertion order, and provides fast access by index.

import java.util.ArrayList;

public class ArrayListDemo {
    public static void main(String[] args) {
        ArrayList cars = new ArrayList<>();

        cars.add("Volvo");
        cars.add("BMW");
        cars.add("Tesla");

        System.out.println("Cars: " + cars);

        cars.remove(0); // Remove element at index 0

        System.out.println("After removal: " + cars);
    }
}

2. LinkedList

LinkedList is faster than ArrayList for adding/removing elements frequently. It supports operations like addFirst(), addLast(), removeFirst(), and getFirst().

import java.util.LinkedList;

public class LinkedListDemo {
    public static void main(String[] args) {
        LinkedList cars = new LinkedList<>();

        cars.add("Honda");
        cars.add("Toyota");
        cars.addFirst("Audi");
        cars.addLast("Ford");

        System.out.println("Cars: " + cars);

        cars.removeFirst();
        System.out.println("After removing first: " + cars);
    }
}

3. HashMap

HashMap stores data in key–value pairs. Keys must be unique. Values can be duplicated. HashMap does NOT maintain order.

import java.util.HashMap;

public class HashMapDemo {
    public static void main(String[] args) {
        HashMap capitalCities = new HashMap<>();

        capitalCities.put("India", "New Delhi");
        capitalCities.put("Japan", "Tokyo");
        capitalCities.put("USA", "Washington DC");

        System.out.println(capitalCities);

        System.out.println("Capital of Japan: " + capitalCities.get("Japan"));
    }
}

4. HashSet

HashSet stores only unique values. It does NOT maintain order and is extremely fast for search operations.

import java.util.HashSet;

public class HashSetDemo {
    public static void main(String[] args) {
        HashSet names = new HashSet<>();

        names.add("Raju");
        names.add("Ravi");
        names.add("Raju"); // Duplicate - ignored

        System.out.println(names);
    }
}

5. Iterator

Iterator is used to loop through any collection, especially when you need safe element removal during iteration.

import java.util.ArrayList;
import java.util.Iterator;

public class IteratorDemo {
    public static void main(String[] args) {
        ArrayList list = new ArrayList<>();

        list.add("A");
        list.add("B");
        list.add("C");

        Iterator it = list.iterator();

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

Java Collections