AP CS A · Guide

AP Computer Science A Unit 4: Data Collections

Unit 4 is the single heaviest unit on the exam — and it's the basis for two of the four free-response questions. If you master one unit, make it this one.

Updated for the 2025–26 (digital, 4-unit) exam

Data Collections accounts for roughly 30–40% of the multiple-choice questions — more than any other unit — and it directly drives the ArrayList and 2D array free-response questions. Together with Unit 2 (Selection & Iteration), it makes up the largest share of the exam.

What's in Unit 4

1D arrays — fixed-size collections of one type
ArrayList — a resizable list of objects
2D arrays — grids of rows and columns
Traversals — visiting elements with loops
Standard algorithms — searching and sorting that operate on these structures

Arrays — the foundation

An array holds multiple values of the same type (primitives or object references).
Its length is fixed at creation and cannot change.
Created with new, elements start at defaults: int → 0, double → 0.0, boolean → false, objects → null.
An initializer list creates and fills in one step: int[] nums = {1, 2, 3};
Valid indices run from 0 to length - 1. Going outside that throws an ArrayIndexOutOfBoundsException.
Traverse with a standard for loop (when you need the index) or an enhanced for-each loop (when you just need the values).

ArrayList — when the size can change

Unlike an array, an ArrayList grows and shrinks as you add and remove.
Core methods: add, get, set, remove, size (and add at a specific index).
It stores objects, so use wrapper classes — Integer, Double — for primitive values.

The classic trap: removing elements while looping forward shifts every later index down by one, so you skip elements. Loop backward, or don't increment the index on a removal.

2D arrays

A 2D array is a grid; you access an element with arr[row][col].
Row-major traversal: an outer loop over the rows, an inner loop over the columns.
arr.length is the number of rows; arr[r].length is the number of columns in row r.

Standard algorithms to recognize

Linear search — check each element in order until you find the target; works on any data.
Binary search — repeatedly check the middle of a sorted array and eliminate half each time.
Selection, insertion, and merge sort — know how each one orders data. Merge sort is the most efficient of the three.

Mistakes that cost the most points

Off-by-one errors and reading past the end of an array.
Using == instead of .equals() to compare objects such as Strings.
Forgetting that an array's length is fixed (you can't add to an array — that's what ArrayList is for).
Modifying an ArrayList while iterating forward over it.

Practice the Unit 4 free-response style

Write a method on an array or ArrayList and get it graded against a rubric — instant feedback on your loop bounds, edge cases, and return value. No account needed.

Grade my FRQ — free Create a free account

How this maps to the exam

Free-response question 3 asks you to work with an ArrayList, and question 4 asks you to traverse and manipulate a 2D array. Both reward correct loop bounds and clean traversal — exactly the Unit 4 skills above. Practicing these two question types is the most direct way to turn Unit 4 study into points.

The 2025–26 redesign consolidated the course into four units and removed inheritance, polymorphism, extends, and super from the required content, while adding File and Scanner for reading text files. Exam weighting and content can change — always confirm current details on the official College Board AP Central page. Silver is an independent study tool and is not affiliated with or endorsed by the College Board.