⚠️ Did you know? Starting with JDK 21, LinkedHashSet implements the new SequencedCollection interface — allowing insertion at the start, end, and even reversing the collection!

This brings new power — and sometimes unexpected behavior — to a class that has long been predictable.

🧠 What Changed?

LinkedHashSet is now:

✅ Ordered (like before)

✅ No duplicates (like before)

✅ Now supports:

  • addFirst(E)
  • addLast(E)
  • reversed()
  • getFirst()
  • getLast()

🧪 Example: A New Way to Add 

Set<String> set = new LinkedHashSet<>();
set.add("A");
set.add("B");

((SequencedSet<String>) set).addFirst("Z");

System.out.println(set); // [Z, A, B]

Wait, what?! 😲 LinkedHashSet at the front?

🔄 Reversing Collections 

SequencedSet<String> reversed = ((SequencedSet<String>) set).reversed();
System.out.println(reversed); // [B, A, Z]

The reversed view is live, not a copy! Changes in one reflect in the other.

🤯 Why This Can Surprise You

Let’s say you’ve always assumed LinkedHashSet maintains insertion order and doesn’t allow element repositioning...

Suddenly, calling addFirst() breaks that assumption.

myMethod(Set<String> values) {
    values.add("X");
}

If someone passes a LinkedHashSet with addFirst() logic inside, the method might change order unexpectedly. 🚨

🧰 Best Practices

  • 🔍 Be aware of runtime type when accepting Set interfaces
  • 📖 Check whether your code expects pure insertion order — now it may not be guaranteed!
  • 🧪 When migrating to JDK 21+, review any code depending on LinkedHashSet ordering

🧵 TL;DR

Feature Old LinkedHashSet New LinkedHashSet (JDK 21+)
Maintains order ✅ Yes ✅ Yes
No duplicates ✅ Yes ✅ Yes
addFirst() ❌ No ✅ Yes
reversed() ❌ No ✅ Yes

👋 Final Thoughts

Java’s evolution with SequencedCollection adds great flexibility — but with power comes surprises. If you're using JDK 21 or newer, check your assumptions about the collections you're using!

🔗 Let me know if you'd like a deep dive into SequencedMap or custom implementations of SequencedCollection next!