Collections Framework

50 min

The Java Collections Framework provides a comprehensive set of classes and interfaces for storing and manipulating groups of objects. It includes lists (ordered, allows duplicates), sets (no duplicates), maps (key-value pairs), and queues (FIFO/LIFO). The framework provides both interfaces (contracts) and implementations (concrete classes). Understanding the Collections Framework is essential for effective Java programming, as collections are used extensively in real-world applications.

Common interfaces include `List` (ordered collection, allows duplicates), `Set` (no duplicates), `Map` (key-value pairs), `Queue` (FIFO), and `Deque` (double-ended queue). Popular implementations include `ArrayList` (dynamic array, fast random access), `LinkedList` (doubly-linked list, fast insertion/deletion), `HashSet` (hash table, fast lookup), `TreeSet` (sorted set), `HashMap` (hash table, fast key-value access), and `TreeMap` (sorted map). Understanding interfaces and implementations helps you choose the right collection for each use case.

Generics enable type-safe collections that work with any data type. Instead of storing `Object` references and casting, generics allow you to specify the type: `List<String>`, `Map<Integer, String>`. This provides compile-time type checking, eliminates casting, and improves code readability. Generics use type parameters (like `<T>`) to represent types. Understanding generics helps you write type-safe, maintainable code.

The `Collections` utility class provides static methods for common collection operations: sorting (`sort()`), searching (`binarySearch()`), reversing (`reverse()`), shuffling (`shuffle()`), finding min/max, and more. These methods work with collections and provide convenient, optimized implementations. Understanding the Collections utility class helps you manipulate collections efficiently.

Performance characteristics vary between collection types. `ArrayList` provides O(1) random access but O(n) insertion/deletion in the middle. `LinkedList` provides O(1) insertion/deletion but O(n) random access. `HashSet`/`HashMap` provide O(1) average-case operations. `TreeSet`/`TreeMap` provide O(log n) operations but maintain sorted order. Understanding performance helps you choose appropriate collections for your use case.

Best practices include using generics for type safety, choosing the right collection type for your use case, using the Collections utility class for common operations, understanding performance characteristics, and using enhanced for loops for iteration. Collections should be used appropriately—don't use `ArrayList` when `HashSet` would be better. Understanding the Collections Framework enables you to work with data effectively in Java.

Key Concepts

Collections Framework provides classes for storing groups of objects.
Common interfaces: List, Set, Map, Queue.
Popular implementations: ArrayList, HashSet, HashMap.
Generics enable type-safe collections.
Collections utility class provides common operations.

Learning Objectives

Master

Working with List, Set, and Map collections
Using generics for type-safe collections
Choosing appropriate collection types for different use cases
Using Collections utility class for common operations

Develop

Data structure thinking and selection
Understanding collection performance characteristics
Designing efficient data handling solutions

Tips

Use generics for type-safe collections: List<String>, Map<Integer, String>.
Choose the right collection type based on your needs (List vs Set vs Map).
Use Collections utility class for sorting, searching, and other operations.
Understand performance characteristics to choose appropriate collections.

Common Pitfalls

Not using generics, requiring unsafe casting and losing type safety.
Using wrong collection type, causing poor performance or incorrect behavior.
Modifying collections during iteration, causing ConcurrentModificationException.
Not understanding collection performance, choosing inefficient types.

Summary

Collections Framework provides classes for storing groups of objects.
Generics enable type-safe collections with compile-time checking.
Different collection types serve different purposes (List, Set, Map).
Collections utility class provides common operations.
Understanding collections is essential for effective Java programming.

Exercise

Demonstrate the use of different collection types with generics.

import java.util.*;

public class CollectionsDemo {
    public static void main(String[] args) {
        // ArrayList example
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");
        System.out.println("Names: " + names);
        
        // HashSet example (no duplicates)
        Set<Integer> numbers = new HashSet<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(2); // Duplicate, won't be added
        numbers.add(3);
        System.out.println("Numbers: " + numbers);
        
        // HashMap example
        Map<String, Integer> ages = new HashMap<>();
        ages.put("Alice", 25);
        ages.put("Bob", 30);
        ages.put("Charlie", 35);
        
        System.out.println("Ages:");
        for (Map.Entry<String, Integer> entry : ages.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }
    }
}

Collections Framework