List, Set, and Map: Key Differences in Java Collections

When working with Java, understanding the differences between List, Set, and Map is essential.

List

Ordered collection, allows duplicates

Access by index

Common implementations: ArrayList, LinkedList

Real-world example: storing a user's transaction history

Use case: showing transactions in the exact order they occurred

List<Transaction> transactions = new ArrayList<>();
transactions.add(new Transaction("Deposit", 100));
transactions.add(new Transaction("Withdrawal", 50));
transactions.add(new Transaction("Deposit", 200)); // duplicates allowed

Set

Collection of unique elements (no duplicates)

Order depends on implementation (HashSet, LinkedHashSet, TreeSet)

Real-world example: each account number must be unique

Use case: preventing duplicate account entries in the system

Set<String> set = new HashSet<>();
set.add("apple");
set.add("banana");
set.add("apple"); // ignored

Map

Stores key-value pairs

Keys must be unique, values can repeat

Common implementations: HashMap, LinkedHashMap, TreeMap

Real-world example: account number as the key and balance as the value

Use case: quickly looking up the balance for a specific account

Map<String, Integer> map = new HashMap<>();
map.put("apple", 3);
map.put("banana", 5);
map.put("apple", 10); // overwrites previous

Think

List → array with duplicates

Set → bag of unique elements

Map → dictionary or phonebook

If you want to see how these structures show up in real systems, especially in finance, check out

Java Collections in Real Systems | Part 1: Set.