Java 21 introduced a significant enhancement to the collection framework: SequencedCollection. This new interface brings order to the world of collections, providing standardized ways to interact with elements based on their sequence.

What are Sequenced Collections?

Imagine a list where the order of elements matters. That's the essence of a SequencedCollection. It extends the existing Collection interface, offering additional functionalities specific to ordered collections.

Key Features:

  • Accessing first and last elements: Methods like getFirst() and getLast() grant direct access to the first and last elements in the collection, respectively.
  • Adding and removing elements at ends: Efficiently manipulate the beginning and end of the sequence with methods like addFirst(), addLast(), removeFirst(), and removeLast().
  • Reversed view: The reversed() method provides a view of the collection in reverse order. Any changes made to the original collection are reflected in the reversed view.

Benefits:

  • Simplified code: SequencedCollection provides clear and concise methods for working with ordered collections, making code easier to read and maintain.
  • Improved readability: The intent of operations becomes more evident when using methods like addFirst() and removeLast(), leading to better understanding of code.

Example Usage:

Consider a Deque (double-ended queue) implemented using ArrayDeque:

import java.util.ArrayDeque;
import java.util.Deque;

public class SequencedCollectionExample {
    public static void main(String ... args) {
        Deque<String> tasks = new ArrayDeque<>();

        // Add tasks (FIFO order)
        tasks.addLast("Buy groceries");
        tasks.addLast("Finish homework");
        tasks.addLast("Call mom");

        // Access and process elements
        System.out.println("First task: " + tasks.getFirst());

        // Process elements in reverse order
        Deque<String> reversedTasks = tasks.reversed();
        for (String task : reversedTasks) {
            System.out.println("Reversed: " + task);
        }
    }
}

This example demonstrates how SequencedCollection allows for efficient access and manipulation of elements based on their order, both forward and backward.

Implementation Classes:

While SequencedCollection is an interface, existing collection classes automatically become SequencedCollection by virtue of inheriting from Collection. Here's a brief overview:

  • Lists: ArrayList, LinkedList, and Vector
  • Sets: Not directly applicable, but LinkedHashSet maintains order within sets.
  • Queues: ArrayDeque and LinkedList
  • Maps: Not directly applicable, but LinkedHashMap and TreeMap (based on key order) maintain order for key-value pairs.

Remember, specific functionalities and behaviors might vary within these classes. Refer to the official Java documentation for detailed information.

Conclusion:

SequencedCollection is a valuable addition to the Java collection framework, offering a structured and efficient way to work with ordered collections. By understanding its features and functionalities, you can write more readable, maintainable, and expressive code when dealing with ordered data structures in Java 21 and beyond.