ArrayList & Collections Basics

Unlike a plain array, ArrayList can grow and shrink — it's the everyday go-to when you don't know the size up front.

import java.util.ArrayList;
import java.util.List;

List<String> names = new ArrayList<>();
names.add("Ada");
names.add("Grace");
System.out.println(names.get(0));   // Ada
System.out.println(names.size());    // 2

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

The <String> is a generic type parameter — it tells the compiler this list only ever holds Strings, so names.get(0) is already known to be a String with no cast needed (covered fully in Generics, in Level 2).

Common mistakes

• Using a raw array when the collection's size needs to change at runtime — reach for ArrayList instead.
• Calling .get(index) past the current .size() — throws IndexOutOfBoundsException.

Practice

Create an ArrayList<Integer>, add the numbers 1, 2, and 3, then print their sum using a for-each loop.

List<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(3);

int sum = 0;
for (int n : numbers) {
    sum += n;
}
System.out.println(sum); // 6