Java · Lesson 8 of 9

Collections & generics

List, Set, Map, generics and a first look at streams.

The Collections Framework provides ready-made data structures:

List (ArrayList) — ordered, allows duplicates, index access.
Set (HashSet, TreeSet) — unique elements.
Map (HashMap, TreeMap) — key → value lookups.
Queue/Deque (ArrayDeque) — FIFO/LIFO processing.

Generics (List<String>) tell the compiler what type a collection holds, catching mistakes at compile time.

Streams (Java 8+) process collections declaratively: filter, map, sorted, collect.

Example program

Run with JDK 17+: java Main.java

import java.util.*;
import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        List<String> skills = new ArrayList<>(List.of("Java", "SQL", "Spring", "SQL", "Git"));
        Set<String> unique = new TreeSet<>(skills);
        System.out.println(unique);

        Map<String, Integer> openings = new HashMap<>();
        openings.put("Kochi", 12);
        openings.put("Thiruvananthapuram", 9);
        openings.merge("Kochi", 3, Integer::sum);      // add 3 more
        System.out.println("Kochi openings: " + openings.get("Kochi"));
        System.out.println(openings.getOrDefault("Kannur", 0));

        List<String> longSkills = skills.stream()
            .distinct()
            .filter(s -> s.length() > 3)
            .map(String::toUpperCase)
            .collect(Collectors.toList());
        System.out.println(longSkills);
    }
}

Output

[Git, Java, SQL, Spring]
Kochi openings: 15
0
[JAVA, SPRING]

Save the code as Main.java and run java Main.java (JDK 11+ runs single files directly). Names and figures are fictional.

Key points

List = ordered; Set = unique; Map = key → value.
Generics give compile-time type safety.
Streams express filter/map/collect pipelines.

Quick check

Which collection rejects duplicate elements?

ArrayList LinkedList HashSet ArrayDeque

Practice task

Count how many times each word appears in a List<String> using a HashMap and merge().

Interview corner

How does HashMap work internally?

It hashes the key to pick a bucket in an array; collisions are stored in a linked list that becomes a balanced tree when large (Java 8+). get/put are O(1) on average.

ArrayList vs LinkedList?

ArrayList gives fast random access and is usually faster overall. LinkedList allows cheap inserts/removals at known positions but slow index access.