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The Java Collections Framework is a powerful library that provides data structures and algorithms to store, retrieve, and manipulate data efficiently. Whether you're building a simple application or a large-scale system, mastering Java collections is essential.

Core Interfaces in Java Collections

The framework is built around several key interfaces:

1. Collection (root interface for List, Set, Queue)
2. List – Ordered, allows duplicates
3. Set – Unordered, no duplicates
4. Queue – FIFO structure
5. Map – Key-value pair mapping (not a Collection)

1. List Interface

A List maintains the order of insertion and allows duplicate elements.

Common Implementations:

• ArrayList – Fast random access, slower insert/delete in the middle
• LinkedList – Slower access, faster insertion/deletion

Example:

java

List<String> fruits = new ArrayList<>();
fruits.add("Apple");
fruits.add("Banana");
fruits.add("Apple"); // allows duplicate

2. Set Interface

A Set stores unique elements only. Duplicate elements are not allowed.

Common Implementations:

• HashSet – No ordering, fast lookup
• LinkedHashSet – Maintains insertion order
• TreeSet – Sorted order, uses a Red-Black tree

Example:

java

Set<String> countries = new HashSet<>();
countries.add("India");
countries.add("Canada");
countries.add("India"); // ignored

3. Map Interface

A Map stores key-value pairs. Keys must be unique.

Common Implementations:

• HashMap – No order, fast lookup
• LinkedHashMap – Maintains insertion order
• TreeMap – Sorted by keys
• ConcurrentHashMap – Thread-safe for concurrent access

Example:

java

Map<String, Integer> scores = new HashMap<>();
scores.put("Alice", 90);
scores.put("Bob", 85);

4. Queue Interface

A Queue follows the First-In-First-Out (FIFO) principle.

Common Implementations:

• LinkedList – Also implements Queue
• PriorityQueue – Elements ordered by priority

Example:

java

Queue<String> queue = new LinkedList<>();
queue.add("Task1");
queue.add("Task2");
queue.poll(); // removes "Task1"

Useful Utility Classes

• Collections: Contains static methods like sort(), reverse(), shuffle(), synchronizedList().
• Arrays: Provides utility methods for arrays like asList() and sort().

Example:

java

Collections.sort(fruits);
Collections.reverse(fruits);

Best Practices

• Prefer interfaces (List, Set, Map) over concrete classes.
• Choose the right data structure for performance (e.g., ArrayList vs LinkedList).
• Use Collections.unmodifiableList() for read-only access.
• Be aware of synchronization needs in multithreaded environments.

The Java Collections Framework is a backbone of Java programming. Understanding the behavior and use cases of each type helps you write efficient, clean, and scalable code.