Learn · OOP & Java

Java Collections

Java hands you ready-made containers so you never rebuild a list or a lookup table by hand. Knowing which to pick is half of writing clean code — and a constant interview question.

Before we start

📋 What you’ll learn

What a collection is and the four you must know
List vs Set vs Map vs Queue — and when to reach for each
Why a Set can’t hold duplicates (the trick underneath)
How these build directly on Arrays & Hashing

✅ After this you’ll be able to

Pick the right collection for a problem, on purpose
Explain why a Set removes duplicates automatically
Use List, Set, Map and Queue in real Java code

Why you’re learning it: collections appear in every Java program and every interview (“List vs Set?”). They’re the practical face of your Arrays & Hashing lesson. ⏱️ ~25 min.

The idea — the right container for the job

You wouldn’t carry soup in a basket or books in a bucket. Data is the same: each collection is a container shaped for a job. Pick by asking two questions — do I need order? and do I find things by position, by uniqueness, or by a key? Here are the four, drawn out:

📋 List — an ordered, numbered shelf

Keeps order, allows duplicates, reach any item by its index. (Like the array you already know — but it grows.)

Note the two As — a List is happy with duplicates.

🚫 Set — a guest list (each name once)

Add the same thing twice and the second is silently ignored. No order, but instant “is this in here?”.

ABC ← add “A” again → A

🗺️ Map — labelled drawers (key → value)

Store a value under a key, fetch it instantly by that key. A dictionary; your phone contacts.

"d@x.com"→User #123

"a@x.com"→User #124

🎟️ Queue — a line at the counter (FIFO)

First in, first served. Add at the back, remove from the front.

front → job 1→job 2→job 3 ← back

Choose the right one

Need	Use	(Java class)	Reach for it when…
List	List	`ArrayList`	a sequence — a playlist, rows from a query
Set	Set	`HashSet`	no duplicates — unique visitors, “seen before?”
Map	Map	`HashMap`	look up by name/id — user by email
Queue	Queue	`LinkedList / ArrayDeque`	process in order — job/task queues

The key insight

A Set has no duplicates because underneath it’s a Map using the items themselves as keys — and keys are unique by definition. A HashMap gives O(1) lookups for the exact reason from your hashing lesson (the hash points straight to the slot). So collections aren’t new magic — they’re the data structures you already learned, gift-wrapped.