List and Set

15.1.1 What is a Collection?

In Java, a collection is a container for storing multiple elements together under one name.
Instead of creating ten variables like int a, b, c, d..., you put them in a List, Set, or Map.

Collections are part of the Java Collections Framework (JCF) — a powerful set of classes and interfaces to handle groups of data efficiently.

Why use collections?

• You can add, remove, search, and loop through elements easily.

• They help you manage dynamic data: the number of items can grow or shrink at runtime.

• They come with powerful methods for sorting, filtering, and transforming data.

15.1.2 List Basics

A List is an ordered collection.

• It allows duplicates.

• Elements have an index: first element is index 0.

• Most common implementation: ArrayList.

Example uses: a list of names, scores, tasks in order.

Mini Lab: Using ArrayList

java
import java.util.ArrayList;

public class ListMiniLab {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();

        names.add("Alice");
        names.add("Bob");
        names.add("Charlie");

        System.out.println("First name: " + names.get(0));
        System.out.println("Total names: " + names.size());

        for (String name : names) {
            System.out.println(name);
        }
    }
}

Key points:

• add() adds an element.

• get(index) reads an element.

• .size() returns how many elements.

• You can loop with for-each.

15.1.3 Set Basics

A Set is an unordered collection that does not allow duplicates.
The most common implementation is HashSet.

Sets are perfect for:

• Keeping unique items.

• Checking if something exists quickly.

Mini Lab: Using HashSet

java
import java.util.HashSet;

public class SetMiniLab {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();

        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Apple"); // Duplicate - ignored

        System.out.println("Total fruits: " + fruits.size());

        for (String fruit : fruits) {
            System.out.println(fruit);
        }
    }
}

Key points:

• Adding a duplicate does nothing.

• No guarantee of order.

• Fast checks: contains() tells you if an item exists.