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Java Collection Framework Notes

1. Collection API

  • The Collection API in Java is a framework that provides architecture for storing and manipulating groups of objects.
  • It includes interfaces, classes, and methods that help in handling data efficiently.
  • Introduced in Java 2 (JDK 1.2), it replaced older data structures like Vector and Hashtable.

Key Features of Collection API

  • Supports different types of collections like List, Set, Queue, and Map.
  • Provides generic support to ensure type safety.
  • Offers built-in sorting, searching, and thread-safe operations.
  • Improves code reusability and performance.

2. Collection (Interface)

  • Collection<E> is the root interface of the Java Collection Framework.
  • It extends Iterable<E> and is further extended by List, Set, and Queue.

Hierarchy

Iterable<E>
   └── Collection<E>
          ├── List<E>
          ├── Set<E>
          ├── Queue<E>

Important Methods in Collection Interface

Method Description
add(E e) Adds an element to the collection.
remove(Object o) Removes an element from the collection.
size() Returns the number of elements in the collection.
isEmpty() Checks if the collection is empty.
contains(Object o) Checks if an element exists in the collection.
iterator() Returns an iterator to traverse elements.
toArray() Converts the collection into an array.
clear() Removes all elements from the collection.

3. Collections (Class)

  • Collections is a utility class in java.util package that provides static methods for working with collections.
  • It contains methods for sorting, searching, synchronization, and modifying collections.

Commonly Used Methods in Collections Class

Method Description
sort(List<T> list) Sorts the given list in ascending order.
reverse(List<T> list) Reverses the order of elements in a list.
shuffle(List<T> list) Randomly shuffles elements in a list.
binarySearch(List<T> list, T key) Searches for an element using binary search.
max(Collection<T> coll) Returns the maximum element.
min(Collection<T> coll) Returns the minimum element.
synchronizedList(List<T> list) Returns a thread-safe list.
unmodifiableList(List<T> list) Returns an unmodifiable (read-only) list.

Key Differences:

Feature Collection (Interface) Collections (Class)
Type Interface Utility Class
Belongs to java.util package java.util package
Purpose Defines core methods for working with collections. Provides static methods to manipulate collections.
Example List<String> list = new ArrayList<>(); Collections.sort(list);

Example Code

import java.util.*;

public class CollectionExample {
    public static void main(String[] args) {
        // Collection Interface usage with List
        Collection<Integer> numbers = new ArrayList<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);
        
        System.out.println("Collection: " + numbers);

        // Collections Class usage for sorting
        List<Integer> list = new ArrayList<>(numbers);
        Collections.sort(list);
        
        System.out.println("Sorted List: " + list);
    }
}
Output:
Collection: [10, 20, 30]
Sorted List: [10, 20, 30]

Summary

  • Collection API: The framework that provides collection classes & interfaces.
  • Collection (Interface): The root interface for working with collections.
  • Collections (Class): A utility class with static methods for collection manipulation.