What is the Java Collections Framework?

The Java Collections Framework provides architectures to store and manipulate groups of objects. It includes interfaces like List, Set, and Map.

1. List Interface (Ordered, Allows Duplicates)

import java.util.*;

public class ListExample {
    public static void main(String[] args) {
        // ArrayList - dynamic array
        List arrayList = new ArrayList<>();
        arrayList.add("Apple");
        arrayList.add("Banana");
        arrayList.add("Apple"); // Duplicate allowed
        System.out.println("ArrayList: " + arrayList);
        
        // Access by index - O(1)
        System.out.println("Element at index 1: " + arrayList.get(1));
        
        // LinkedList - doubly linked list
        List linkedList = new LinkedList<>();
        linkedList.add("First");
        linkedList.add("Second");
        linkedList.add(1, "Inserted");
        System.out.println("LinkedList: " + linkedList);
        
        // Iteration
        for (String item : arrayList) {
            System.out.println(item);
        }
    }
}

2. Set Interface (Unordered, No Duplicates)

public class SetExample {
    public static void main(String[] args) {
        // HashSet - hash table, no order
        Set hashSet = new HashSet<>();
        hashSet.add("Java");
        hashSet.add("Python");
        hashSet.add("Java"); // Duplicate - will not be added
        System.out.println("HashSet: " + hashSet);
        
        // TreeSet - sorted order
        Set treeSet = new TreeSet<>();
        treeSet.add("Banana");
        treeSet.add("Apple");
        treeSet.add("Cherry");
        System.out.println("TreeSet (sorted): " + treeSet);
        
        // LinkedHashSet - insertion order
        Set linkedHashSet = new LinkedHashSet<>();
        linkedHashSet.add("One");
        linkedHashSet.add("Two");
        linkedHashSet.add("Three");
        System.out.println("LinkedHashSet: " + linkedHashSet);
        
        // Check existence - O(1)
        System.out.println("Contains Java? " + hashSet.contains("Java"));
    }
}

3. Map Interface (Key-Value Pairs)

public class MapExample {
    public static void main(String[] args) {
        // HashMap - no order
        Map hashMap = new HashMap<>();
        hashMap.put("Apple", 10);
        hashMap.put("Banana", 20);
        hashMap.put("Orange", 15);
        
        // Iterate
        for (Map.Entry entry : hashMap.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }
        
        // Get value
        System.out.println("Value for Apple: " + hashMap.get("Apple"));
        
        // TreeMap - sorted by keys
        Map treeMap = new TreeMap<>();
        treeMap.put("Banana", 20);
        treeMap.put("Apple", 10);
        treeMap.put("Cherry", 30);
        System.out.println("TreeMap (sorted): " + treeMap);
        
        // Check if key exists
        System.out.println("Contains key Banana? " + hashMap.containsKey("Banana"));
    }
}

Collection Hierarchy

/*
Iterable (Interface)
    └── Collection (Interface)
        ├── List (Interface)
        │   ├── ArrayList
        │   ├── LinkedList
        │   └── Vector
        └── Set (Interface)
            ├── HashSet
            ├── LinkedHashSet
            └── TreeSet

Map (Interface)
    ├── HashMap
    ├── LinkedHashMap
    ├── TreeMap
    └── Hashtable
*/

Practical Example - Student Management

class Student {
    int id;
    String name;
    
    Student(int id, String name) {
        this.id = id;
        this.name = name;
    }
    
    @Override
    public String toString() {
        return id + ": " + name;
    }
}

public class StudentManagement {
    public static void main(String[] args) {
        // List - store all students
        List students = new ArrayList<>();
        students.add(new Student(1, "Alice"));
        students.add(new Student(2, "Bob"));
        students.add(new Student(3, "Charlie"));
        
        // Set - unique course names
        Set courses = new HashSet<>();
        courses.add("Java");
        courses.add("Python");
        courses.add("Java"); // Won't add duplicate
        
        // Map - student id to student object
        Map studentMap = new HashMap<>();
        for (Student s : students) {
            studentMap.put(s.id, s);
        }
        
        // Retrieve by id
        Student found = studentMap.get(2);
        System.out.println("Found student: " + found);
    }
}

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