Mastering Java Collections: Beyond ArrayList and HashMap

As a software developer, throughout my journey of building projects, fixing bugs, and preparing for placements, I learned something simple yet powerful: choosing the right Java Collection makes your code cleaner, faster, and easier to scale.

Most of us start with just ArrayList and HashMap. I did too. But once I started working on real use-cases, I realized there’s a whole ecosystem of smarter data structures in Java.

In this blog, I’ll break down 7 of the most useful Java Collections beyond the basics — with real examples and exactly when to use them.

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1. LinkedList

What it is

A doubly linked list — each element points to both the previous and the next. It implements both List and Deque.

Why not ArrayList?

ArrayList is backed by an array. So when you insert or delete from the middle, every element after it needs to shift. LinkedList, on the other hand, just updates some pointers. It’s faster for frequent add/remove operations.

Code Example

LinkedList<String> tasks = new LinkedList<>();
tasks.add("Wake up");
tasks.addFirst("Brush teeth");
tasks.addLast("Eat breakfast");
System.out.println(tasks); // [Brush teeth, Wake up, Eat breakfast]

Where it's helpful

Undo lists, dynamic queues, and cases where order frequently changes like browsing history.

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2. TreeMap / TreeSet

What it is

TreeMap and TreeSet are based on a Red-Black tree, which keeps elements sorted. TreeMap is a map that sorts entries by key, either naturally or with a comparator, making it ideal for storing key-value pairs in order. TreeSet is a set that uses TreeMap internally, maintaining unique elements in ascending order, which is perfect for keeping items sorted and with no duplicates.

Why not HashMap?

HashMap is fast but doesn’t care about order. TreeMap gives you automatic sorting.

Code Example

TreeMap<Integer, String> scores = new TreeMap<>();
scores.put(90, "Sham");
scores.put(100, "Sundar");
System.out.println(scores); // {90=Sham, 100=Sundar}

Where it's helpful

Any feature where sorting is key — like ranked lists or auto-ordered maps.

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3. LinkedHashMap

What it is

It combines the features of a hash map and a linked list, allowing it to keep track of the order in which entries are inserted or accessed.

Why not HashMap?

If you want predictable iteration (like recently used data first), HashMap won’t help.

• Tracking recent user activity we use LinkedHashMap

Code Example

LinkedHashMap<Integer, String> map = new LinkedHashMap<>();
map.put(1, "Login");
map.put(2, "Search");
map.put(3, "Logout");
System.out.println(map); // {1=Login, 2=Search, 3=Logout}

Where it's helpful

We use LinkedHashMap when maintaining order is important.

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4. PriorityQueue

What it is

A queue based on priority — internally a min-heap. Elements are served based on their natural order.

Use Case

• Scheduling tasks based on priority

• Algorithms like Dijkstra’s

Code Example

PriorityQueue<Integer> pq = new PriorityQueue<>();
pq.add(50);
pq.add(10);
pq.add(30);
System.out.println(pq.poll()); // 10

Where it's helpful

Any task queue or scheduling system, or when working with graph algorithms.

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5. Deque (ArrayDeque)

What it is

A double-ended queue — supports adding/removing from both ends. It's faster than Stack and LinkedList.

Use Case

• Undo/Redo stacks

• Solving sliding window problems

Code Example

Deque<String> actions = new ArrayDeque<>();
actions.push("Open");
actions.push("Edit");
System.out.println(actions.pop()); // Edit

Where it's helpful

Text editors, navigational history, problems involving moving windows or intervals.

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6. EnumSet / EnumMap

What it is

Collections designed specifically for enums. Internally optimized using bitwise operations.

Why not use HashSet or HashMap with enums?

These are much faster and memory-efficient when the keys are enums.

Use Case

• Managing roles or states

• Defining fixed permissions or days

Code Example

enum Role { ADMIN, STUDENT, TEACHER }
EnumSet<Role> allowed = EnumSet.of(Role.STUDENT, Role.TEACHER);
System.out.println(allowed); // [STUDENT, TEACHER]

Where it's helpful

Role-based access, state machines, or any feature using enum values.

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7. ConcurrentHashMap

What it is

A thread-safe version of HashMap. Allows multiple threads to read and write without corrupting the structure.

Why not HashMap?

In multi-threaded code, HashMap can fail or even crash the program. ConcurrentHashMap handles concurrency efficiently.

Use Case

• Analytics counters

• Shared state in server applications

Code Example

ConcurrentHashMap<String, Integer> counter = new ConcurrentHashMap<>();
counter.put("pageViews", 1);
counter.compute("pageViews", (k, v) -> v + 1);

Where it's helpful

Web apps, server logs, user counters, or any data that is modified by multiple threads.

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Real Project Where I Used These

I applied many of these collections in a backend project for managing event registrations: EventRegistration-Management (GitHub)

Some highlights:

• Used TreeMap to sort participant scores in real-time

• Used ConcurrentHashMap to safely track live registration counts from different threads

• Used LinkedHashMap to build a recent activity tracker

• Used PriorityQueue to assign event slots based on priority rules

This was the first time I realized how different collections make different parts of your system cleaner and more efficient.

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What’s Next?

I’m currently exploring Java Streams — especially how map(), filter(), reduce(), and collect() can simplify and speed up your code. If you’ve ever written nested for-loops and wondered “Is there a better way?”, Streams are worth learning.

That’s what I’ll be breaking down in my next blog.

see you There! Cheers!!!