← Back to Week 3 Hub

ArrayList vs HashMap — Pick One

Two excellent collections, two different jobs. A quick decision guide, a comparison table, and 8 scenarios to test yourself.

ArrayList

An ordered sequence you access by position.

Keeps items in the order you add them
Access with get(0), get(1), …
Duplicates are fine
Natural fit for "a list of things"
Iterates in insertion order

HashMap

A fast key → value lookup table.

Find any value by its key in one step
Keys must be unique (second put overwrites)
No meaningful order (bucket order)
Natural fit for "given X, find Y"
Fast even with thousands of entries

Decision tree

Do you need to look up items by a unique identifier (like an id, SKU, username)?

YES

HashMap

Key = the identifier. get(key) returns the object.

NO

↓ next question

Does the order of items matter, or do you loop through all of them?

YES

ArrayList

Preserves insertion order, loops in sequence.

NO

ArrayList (default)

ArrayList is a fine default when lookup isn't needed.

Pick the right collection — 8 scenarios

Score: 0 / 0

Operation comparison

Need to…	ArrayList	HashMap
Add an item	list.add(x)	map.put(k, v)
Get by index	list.get(i)	not applicable
Get by key	loop + check	map.get(k) — fast
Check if item exists	list.contains(x)	map.containsKey(k)
Remove	list.remove(i)	map.remove(k)
Size / count	list.size()	map.size()
Loop through all	for (x : list)	for (Map.Entry e : map.entrySet())
Preserves order?	yes — insertion order	no — bucket order
Duplicates allowed?	yes	keys no, values yes

Tip: when in doubt, ask "do I need to find this by a specific key?" If yes → HashMap. If no → ArrayList.

← HashMap Operations & Iteration Waterfall vs Agile →