ArrayList

An ArrayList is a class from java.util that stores a sequence of object references and grows automatically as you add elements. Unlike arrays, its size changes over time.

ArrayList is a generic type: you specify the element type in angle brackets — ArrayList<String>, ArrayList<Integer>, ArrayList<Book>. Because the type must be an object, primitives use wrapper classes (Integer, Double) and benefit from autoboxing.

On the AP exam, you must know the standard ArrayList methods, the import line, and the common patterns for traversal, search, and removal — including the classic "removing while iterating forward" trap.

Import and create

import java.util.ArrayList;

ArrayList<String> names = new ArrayList<String>();

The angle brackets specify the element type. For primitives, use wrapper classes: ArrayList<Integer> rather than ArrayList<int>.

AP CSA ArrayList methods

• int size() — number of elements.
• boolean add(E obj) — append to the end; always returns true.
• void add(int index, E obj) — insert at index; shifts later elements right.
• E get(int index) — read the element at index.
• E set(int index, E obj) — replace the element at index; returns the old value.
• E remove(int index) — remove and return the element at index; shifts later elements left.

Examples

ArrayList<String> list = new ArrayList<String>();
list.add("apple");        // [apple]
list.add("banana");       // [apple, banana]
list.add(1, "blueberry"); // [apple, blueberry, banana]
list.set(0, "apricot");   // [apricot, blueberry, banana]
String x = list.get(2);   // "banana"
list.remove(0);           // [blueberry, banana]
int n = list.size();      // 2

Autoboxing and unboxing

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(7);              // autoboxing: int 7 -> Integer
int first = nums.get(0);  // unboxing: Integer -> int

Java converts between int and Integer automatically — but be aware that the list actually stores Integer objects.

Traversing an ArrayList

// indexed
for (int i = 0; i < list.size(); i++) {
    System.out.println(list.get(i));
}

// enhanced for
for (String s : list) {
    System.out.println(s);
}

Use list.size() as the bound (note the parentheses — it's a method, not a field).