📚 Collections

Introduction

Java Collections Framework (JCF) provides a set of interfaces and classes to store and manipulate groups of objects efficiently. Collections help manage dynamic data structures like lists, sets, and maps.

Why Use Collections?

Dynamic sizing (unlike arrays)
Built-in sorting and searching
Better performance and memory management

Core Interfaces

1. List (Ordered Collection, Allows Duplicates)

ArrayList: Fast random access, slower insert/delete
LinkedList: Fast insert/delete, slower access

Example:

import java.util.*;
public class ListExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("Apple");
        list.add("Banana");
        list.add("Apple"); // Allows duplicates
        System.out.println(list);
    }
}

2. Set (Unique Elements, No Duplicates)

HashSet: Unordered, fast operations
TreeSet: Sorted order
LinkedHashSet: Maintains insertion order

Example:

import java.util.*;
public class SetExample {
    public static void main(String[] args) {
        Set<String> set = new HashSet<>();
        set.add("Apple");
        set.add("Banana");
        set.add("Apple"); // Ignored
        System.out.println(set);
    }
}

3. Map (Key-Value Pairs)

HashMap: Fast access, unordered
TreeMap: Sorted by key
LinkedHashMap: Maintains insertion order

Example:

import java.util.*;
public class MapExample {
    public static void main(String[] args) {
        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Apple");
        map.put(2, "Banana");
        map.put(1, "Grapes"); // Overwrites previous value
        System.out.println(map);
    }
}

Utility Classes

Collections: Sort, reverse, shuffle, etc.
Arrays: Convert arrays to lists

Example:

import java.util.*;
public class UtilityExample {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(5, 3, 8, 1);
        Collections.sort(numbers);
        System.out.println(numbers); // [1, 3, 5, 8]
    }
}

Conclusion

Java Collections provide a powerful way to manage data structures efficiently. Choose the right collection type based on your use case for optimal performance.

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