java-tutorial

10) Collections: List, Set, Map

Goal

Use Java collections confidently and learn when to pick which.

List

• ordered
• allows duplicates

List<String> names = new ArrayList<>();
names.add("Asha");
names.add("Asha");

Set

• no duplicates
• order depends on implementation

Set<String> unique = new HashSet<>();
unique.add("Asha");
unique.add("Asha");
System.out.println(unique.size()); // 1

Map

• key/value

Map<String, Integer> counts = new HashMap<>();
counts.put("apple", 2);
counts.put("apple", 3); // overwrites

Big-O intuition (very rough)

• ArrayList random access: ~O(1)
• ArrayList insert/remove in middle: ~O(n)
• HashMap lookup by key: ~O(1) average

Exercises

1. Count word frequency using Map<String, Integer>.
2. Remove duplicates from a list using a Set.
3. Sort a list of custom objects (create Person(name, age) and sort by age).

Table of contents

1. Getting Started: Install, run, and your first program
2. Java Basics: types, variables, operators, formatting
3. Control Flow: if/switch/loops
4. Methods: parameters, return values, overloading
5. OOP: classes, objects, encapsulation
6. Inheritance & Polymorphism (and when not to use them)
7. Interfaces, abstract classes, and design basics
8. Exceptions and error handling
9. Strings, files, and I/O basics
10. Collections: List/Set/Map and Big-O intuition
11. Generics (the useful parts)
12. Lambdas & Streams
13. Dates and time (java.time)
14. Testing with JUnit 5 (basics)
15. Concurrency: threads, executors, futures
16. JVM basics: memory, GC, performance habits
17. Build tools: Maven essentials (recommended)
18. Next steps: projects to build