AP Computer Science A Unit 4 Essentials: Key Terms & Core Concepts

Key Concept 1

Arrays and ArrayList store collections you traverse with loops

An array is a fixed-size, indexed collection (indices 0 to length−1, size given by the field arr.length); an ArrayList is a resizable list of objects with methods add, get, set, remove, and size(). You process either with traversals — a standard for loop for index access, or the enhanced for-each loop to read each element — implementing standard algorithms like finding a min/max, summing, counting, and filtering. Because ArrayList stores objects, it uses wrapper classes (Integer, Double) with autoboxing.

Array ArrayList Traversals

Key Concept 2

2D arrays are grids traversed with nested loops

A 2D array is an array of arrays — a rectangular grid accessed as matrix[row][col]. The number of rows is matrix.length and the number of columns is matrix[0].length. You traverse it with nested loops (row-major order: outer loop over rows, inner over columns) to implement algorithms across the whole grid. You can also read data into collections from text files using a Scanner.

2D Array Nested Loops Text Files

Key Concept 3

Searching, sorting, and recursion are the standard algorithms

Sequential (linear) search checks each element in turn and works on unsorted data; binary search halves a sorted array each step and is far faster. The standard sorts are selection sort (repeatedly swap in the smallest remaining element) and insertion sort (insert each element into a growing sorted region). Recursion — a method that calls itself with a base case and a recursive case — expresses algorithms like recursive binary search and merge sort.

Searching Sorting Recursion

Array

A fixed-size, indexed collection of same-type elements.

Arrays

Index

Position in an array, from 0 to length−1.

Arrays

arr.length

The size of an array (a field, no parentheses).

Arrays

ArrayIndexOutOfBoundsException

Run-time error from an invalid index.

Arrays

Traversal

Visiting each element of a collection.

Traversals

Enhanced for (for-each)

for (T x : coll) — reads elements, can’t assign/remove.

Traversals

ArrayList

A resizable list of objects (java.util).

ArrayList

add / get / set / remove / size

The core ArrayList methods.

ArrayList

Wrapper class

Integer/Double, wrapping a primitive as an object.

ArrayList

Autoboxing / unboxing

Automatic primitive↔wrapper conversion.

ArrayList

2D array

An array of arrays; a grid matrix[row][col].

2D Arrays

Row-major traversal

Nested loops: outer rows, inner columns.

2D Arrays

Text file / Scanner

Reading data from a file with a Scanner.

Files

Sequential (linear) search

Check each element until found (unsorted).

Searching

Binary search

Halve a sorted array each step.

Searching

Selection sort

Repeatedly swap in the smallest remaining element.

Sorting

Insertion sort

Insert each element into a sorted region.

Sorting

Recursion

A method that calls itself.

Recursion

Base case

The stopping condition of a recursion.

Recursion

Merge sort

A recursive divide-and-merge sort.

Recursion