Understanding Java Collections Framework

What is the Collections Framework?

Java's Collections Framework provides ready-made data structures — lists, sets, maps, queues — so you don't reinvent them.

The Main Interfaces

Type	Allows duplicates?	Ordered?
List (ArrayList)	Yes	Yes (index)
Set (HashSet)	No	No
Map (HashMap)	Keys: No	No
Queue (LinkedList)	Yes	FIFO

List Example

List<String> names = new ArrayList<>();
names.add("Sara");
names.add("Ali");
names.get(0); // "Sara"

Map Example

Map<String, Integer> ages = new HashMap<>();
ages.put("Sara", 25);
ages.get("Sara"); // 25

Set Example

Set<Integer> unique = new HashSet<>();
unique.add(1); unique.add(1); // only stored once

FAQs

ArrayList vs LinkedList?

ArrayList is faster for random access; LinkedList for frequent insert/delete at ends. More in our Java guides.

HashMap vs TreeMap?

HashMap is unordered and fast; TreeMap keeps keys sorted.