The ArrayList API

An ArrayList is a resizable list of objects. Unlike arrays, it grows and shrinks:

import java.util.ArrayList;

ArrayList<String> names = new ArrayList<String>();
names.add("An");
names.add("Binh");
names.add("Chi");

The <String> in angle brackets is the generic type: it declares what the list holds, and the compiler then enforces it — names.add(42) is a compile error. (The shorthand new ArrayList<>() is legal Java but the AP reference uses the explicit form; know both.)

The exam's method set:

names.size()               // 3  — method, not a field!
names.get(0)               // "An"
names.set(1, "Bao")        // replaces index 1; returns the old value
names.add("Duong")         // appends at the end
names.add(1, "Binh")       // INSERTS at 1, shifting the rest right
names.remove(0)            // removes index 0, shifting left; returns it
names.contains("Chi")      // true
names.indexOf("Chi")       // 1

Autoboxing: a list of numbers is ArrayList<Integer> (the wrapper class) — never ArrayList<int>. Java converts int ↔ Integer automatically when you add or get:

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(5);                    // int boxed to Integer
int first = nums.get(0);        // unboxed back to int