1. Introduction to Collections

1.1 What is the Collections Framework

A framework that provides classes and interfaces for storing and manipulating data.

1.2 Why Use Collections

Dynamic size
Built-in methods
Better performance

1.3 Key Components

Interfaces
Classes
Algorithms

1.4 Collection Hierarchy

Collection → List, Set
Map (separate hierarchy)

2. List Interface

2.1 Definition

An ordered collection that allows duplicate elements.

2.2 Common Classes

ArrayList
LinkedList
Vector

2.3 Example

import java.util.*;List<String> list = new ArrayList<>();
list.add("A");
list.add("B");
list.add("A");System.out.println(list);

2.4 Operations

add()
remove()
get()
set()

2.5 ArrayList vs LinkedList

ArrayList → faster access
LinkedList → faster insertion

3. Set Interface

3.1 Definition

A collection that does not allow duplicate elements.

3.2 Common Classes

HashSet
LinkedHashSet
TreeSet

3.3 Example

Set<Integer> set = new HashSet<>();
set.add(10);
set.add(20);
set.add(10);System.out.println(set);

3.4 Features

No duplicates
Unordered (HashSet)

4. Map Interface

4.1 Definition

Stores data in key-value pairs.

4.2 Common Classes

HashMap
LinkedHashMap
TreeMap

4.3 Example

Map<Integer, String> map = new HashMap<>();
map.put(1, "A");
map.put(2, "B");System.out.println(map.get(1));

4.4 Operations

put()
get()
remove()

5. Iterators

5.1 Definition

Used to traverse collections.

5.2 Methods

hasNext()
next()
remove()

5.3 Example

Iterator<String> it = list.iterator();while (it.hasNext()) {
    System.out.println(it.next());
}

5.4 ListIterator

Supports forward and backward traversal.

6. Comparable vs Comparator

6.1 Comparable

Used to define natural ordering.

class Student implements Comparable<Student> {
    int age;    public int compareTo(Student s) {
        return this.age - s.age;
    }
}

6.2 Comparator

Used to define custom sorting.

class SortByAge implements Comparator<Student> {
    public int compare(Student a, Student b) {
        return a.age - b.age;
    }
}

6.3 Differences

Comparable	Comparator
Inside class	Separate class
compareTo()	compare()

Conclusion

The Collections Framework is essential for handling data efficiently in Java. By understanding List, Set, Map, iterators, and sorting techniques, beginners can build scalable and efficient applications. Mastering collections is a key step toward becoming a proficient Java developer.

Collections Framework