Topic 4.5 — ArrayList Creation and Access

Goal: create ArrayList objects, understand how they store references, and access/update elements using get and set.

The big idea

An ArrayList is a resizable list. Unlike arrays, it can grow and shrink. You still use zero-based indexing, but you access elements with methods like get and set.

If you’re thinking “array, but flexible,” you’re on the right track.

ArrayList vs Array (quick compare)

Feature	Array	ArrayList
Size changes?	No (fixed)	Yes (resizable)
Length/size	`arr.length`	`list.size()`
Access	`arr[i]`	`list.get(i)`
Update	`arr[i] = x`	`list.set(i, x)`

Create an ArrayList

You usually use generics: ArrayList<Type>. In AP CSA, you’ll often see ArrayList<Integer> and ArrayList<String>.

import java.util.ArrayList;

ArrayList<Integer> nums = new ArrayList<Integer>();
ArrayList<String> words = new ArrayList<String>();

Important: primitives vs wrapper classes

ArrayLists store objects, not primitives.

Primitive	Wrapper (object)
`int`	`Integer`
`double`	`Double`
`boolean`	`Boolean`

So you write ArrayList<Integer>, not ArrayList<int>.

Size and indexes

First index is 0.
Last index is list.size() - 1.
size() is a method (parentheses required).

int n = nums.size();

Out of bounds (still a thing)

If you use an invalid index, you’ll get an exception (similar idea to arrays).

ArrayList<String> list = new ArrayList<String>();
list.get(0); // ❌ list is empty

Access and update: `get` and `set`

ArrayList<String> names = new ArrayList<String>();
names.add("Ada");
names.add("Linus");

// Access:
String first = names.get(0);   // "Ada"

// Update (replaces existing value):
names.set(1, "Grace");         // now ["Ada", "Grace"]

set(i, value) overwrites what’s already at index i.

ArrayList stores references

For object types, the list stores references to objects. If two variables refer to the same object, changes to that object are visible through both references.

// Example idea (details depend on class):
// Two references can point to the same object.

What you should be able to do on AP

Create an ArrayList with a type parameter
Use size() correctly
Access values with get
Replace values with set
Recognize index bounds: 0 to size()-1

Quick self-check

Why do we use Integer instead of int in an ArrayList?
What’s the last valid index if list.size() is 7?
What does set(i, x) do?
How is list.get(i) different from list.set(i, x)?
Why is list.length incorrect?

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