Java SequencedCollection for Ordered Data Structures: SequencedList, SequencedSet, SequencedDeque

🚀 Introduction: Why SequencedCollection in Java 21?

Before Java 21, working with ordered data structures required different methods for accessing the first and last elements.

For example:

• List → list.get(0), list.get(list.size() - 1)
• Deque → deque.getFirst(), deque.getLast()
• Set (e.g., LinkedHashSet) → No direct way to get the first/last element

💡 Java 21 introduces SequencedCollection, a new interface that provides consistent methods for handling ordered data structures.

📌 In this article, you’ll learn:
✅ What SequencedCollection is and why it matters
✅ How it improves List, Set, and Deque
✅ Complete examples demonstrating each SequencedCollection type

🔍 What is SequencedCollection in Java 21?

SequencedCollection is a new interface in Java 21 that provides a unified way to manage ordered data in collections.

📌 Key Features of SequencedCollection

✅ Access first & last elements easily (getFirst(), getLast())
✅ Reverse collections (reversed())
✅ Efficient insertion/removal at both ends (addFirst(), addLast(), removeFirst(), removeLast())

Classes Implementing SequencedCollection:
1️⃣ SequencedList (For ordered List implementations)
2️⃣ SequencedSet (For ordered Set implementations)
3️⃣ SequencedDeque (For Deque structures like ArrayDeque and LinkedList)

🚀 1️⃣ Using SequencedList (Ordered Lists with First & Last Access)

Before Java 21, retrieving the first and last elements in a List was verbose.

✔ Traditional Way (Before Java 21)

List<String> names = new ArrayList<>(List.of("Raj", "Amit", "Neha"));

// Getting first and last elements
String first = names.get(0);
String last = names.get(names.size() - 1);

System.out.println("First: " + first + ", Last: " + last);

📌 Problems:
❌ Hardcoded indices (0 and size() - 1)
❌ Prone to IndexOutOfBoundsException

✅ Java 21 SequencedList (Better Way)

import java.util.*;

public class SequencedListExample {
    public static void main(String[] args) {
        SequencedList<String> names = new ArrayList<>(List.of("Raj", "Amit", "Neha"));

        System.out.println("First: " + names.getFirst());  // ✅ Direct access
        System.out.println("Last: " + names.getLast());

        names.addFirst("Vikram");  // ✅ Adds at the beginning
        names.addLast("Priya");    // ✅ Adds at the end

        System.out.println("Updated List: " + names);
        System.out.println("Reversed List: " + names.reversed()); // ✅ Reverse list
    }
}

📌 Advantages:
✅ Cleaner syntax (getFirst(), getLast())
✅ Easy insertion/removal at both ends (addFirst(), addLast())
✅ Reversing a list is now a single call (reversed())

🚀 2️⃣ Using SequencedSet (Ordered Sets with First & Last Elements)

Before Java 21, Set collections did not allow direct access to the first and last elements.

✔ Traditional Way (Before Java 21)

Set<String> cities = new LinkedHashSet<>(Set.of("Mumbai", "Delhi", "Bangalore"));
String first = cities.iterator().next();  // Gets first element (Inefficient)
System.out.println("First city: " + first);

📌 Problems:
❌ No built-in way to get the last element
❌ Must iterate manually

✅ Java 21 SequencedSet (Better Way)

import java.util.*;

public class SequencedSetExample {
    public static void main(String[] args) {
        SequencedSet<String> cities = new LinkedHashSet<>(Set.of("Mumbai", "Delhi", "Bangalore"));

        System.out.println("First City: " + cities.getFirst());  // ✅ Direct access
        System.out.println("Last City: " + cities.getLast());

        cities.addFirst("Kolkata");  // ✅ Adds at the beginning
        cities.addLast("Chennai");   // ✅ Adds at the end

        System.out.println("Updated Set: " + cities);
        System.out.println("Reversed Set: " + cities.reversed()); // ✅ Reverse order
    }
}

📌 Advantages:
✅ First and last element access without iteration
✅ Maintains order while allowing first/last insertions
✅ Reversing a set is now simple

🚀 3️⃣ Using SequencedDeque (Efficient Double-Ended Queues)

Before Java 21, Deque (double-ended queue) had separate methods:

• addFirst(), addLast()
• getFirst(), getLast()

💡 Now, SequencedDeque unifies the API with SequencedCollection methods.

✅ Java 21 SequencedDeque Example

import java.util.*;

public class SequencedDequeExample {
    public static void main(String[] args) {
        SequencedDeque<Integer> numbers = new ArrayDeque<>(List.of(10, 20, 30));

        System.out.println("First: " + numbers.getFirst());  // ✅ Unified API
        System.out.println("Last: " + numbers.getLast());

        numbers.addFirst(5);   // ✅ Adds at the beginning
        numbers.addLast(40);   // ✅ Adds at the end
        numbers.removeFirst(); // ✅ Removes first element
        numbers.removeLast();  // ✅ Removes last element

        System.out.println("Updated Deque: " + numbers);
        System.out.println("Reversed Deque: " + numbers.reversed()); // ✅ Reverse order
    }
}

📌 Advantages:
✅ Unified API across collections
✅ Efficient insertions and removals from both ends
✅ Easy to reverse order

🔥 Key Takeaways

✅ Java 21’s SequencedCollection provides a cleaner API for ordered data structures.
✅ SequencedList simplifies list operations (getFirst(), getLast(), reversed()).
✅ SequencedSet allows direct access to the first & last elements without iteration.
✅ SequencedDeque unifies the API for efficient queue operations.

By adopting SequencedCollection, your Java code will be more readable, efficient, and easy to maintain! 🚀