AP Computer Science A Reference Sheet

AP Computer Science A Reference Sheet

Complete Java Quick Reference for the 2025-2026 AP CSA Exam

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String Methods

Method Returns Example
length() int "Hello".length() → 5
substring(start, end) String "Hello".substring(1, 4) → "ell"
substring(start) String "Hello".substring(2) → "llo"
indexOf(str) int "Hello".indexOf("l") → 2
equals(str) boolean "Hi".equals("hi") → false
compareTo(str) int "A".compareTo("B") → negative
charAt(index) char "Hello".charAt(0) → 'H'
💡 Remember: String indices start at 0. substring(start, end) includes start but excludes end.

Math Methods

Method Returns Example
Math.abs(x) int/double Math.abs(-5) → 5
Math.pow(base, exp) double Math.pow(2, 3) → 8.0
Math.sqrt(x) double Math.sqrt(16) → 4.0
Math.random() double Returns 0.0 ≤ x < 1.0
💡 Random Range Formula: (int)(Math.random() * range) + min
Example: Random 1-6 (dice roll): (int)(Math.random() * 6) + 1

ArrayList Methods

Method Returns Description
size() int Number of elements
get(index) E Element at index
set(index, value) E Replace element, returns old value
add(value) boolean Add to end
add(index, value) void Insert at index, shifts others right
remove(index) E Remove at index, shifts others left

Arrays vs ArrayList

Operation Array ArrayList
Declare int[] arr = new int[5]; ArrayList list = new ArrayList<>();
Size arr.length list.size()
Access arr[i] list.get(i)
Modify arr[i] = value; list.set(i, value);
Add element N/A (fixed size) list.add(value);

2D Array Basics

Operation Code
Declare int[][] grid = new int[rows][cols];
Number of rows grid.length
Number of columns grid[0].length
Access element grid[row][col]
Get entire row grid[row] (returns 1D array)

2D Array Traversal

for (int row = 0; row < grid.length; row++) { for (int col = 0; col < grid[0].length; col++) { // process grid[row][col] } }

Operators

Type Operators
Arithmetic + - * / % (mod = remainder)
Comparison == != < > <= >=
Logical && (and) || (or) ! (not)
Assignment = += -= *= /= %=
Increment/Decrement x++ ++x x-- --x

Boolean Operators (Truth Table)

A B A && B A || B !A
true true true true false
true false false true false
false true false true true
false false false false true
💡 Memory Trick: AND (&&) requires BOTH true. OR (||) requires at least ONE true.

compareTo() Return Values

Expression Returns Meaning
a.compareTo(b) negative (e.g., -1) a comes BEFORE b alphabetically
a.compareTo(b) 0 a EQUALS b
a.compareTo(b) positive (e.g., 1) a comes AFTER b alphabetically
💡 Example: "Apple".compareTo("Banana") → negative (A comes before B)

Wrapper Classes (Autoboxing)

Primitive Wrapper Why You Need It
int Integer ArrayList — can't use primitives in generics
double Double Autoboxing: Integer x = 5; (auto-converts)
boolean Boolean Unboxing: int y = x; (auto-converts back)

Loop Syntax

Type Syntax
for loop for (int i = 0; i < n; i++) { }
while loop while (condition) { }
for-each loop for (Type item : collection) { }
💡 For-each limitation: Cannot modify the collection or access the index. Use a regular for loop if you need either.

Selection Statements

Type Syntax
if if (condition) { }
if-else if (condition) { } else { }
if-else if if (c1) { } else if (c2) { } else { }

Class Structure (FRQ 2)

public class ClassName { private int instanceVar; // instance variable public ClassName(int param) { // constructor instanceVar = param; } public int getInstanceVar() { // accessor (getter) return instanceVar; } public void setInstanceVar(int v) { // mutator (setter) instanceVar = v; } }

Common Traps (Watch Out!)

Trap Problem Fix
= vs == if (x = 5) assigns, not compares Use == for comparison
Integer division 5/2 = 2, not 2.5 Cast: (double)5/2
String == Compares references, not content Use .equals()
Off-by-one < vs <=, wrong start index Trace with examples
Null pointer Calling method on null Check != null first
; after if/while if (x > 0); { } — empty body! Remove the semicolon
ArrayList forward delete Skips elements when removing Loop backward

Essential Code Patterns

Accumulator (Sum)

int total = 0; for (int i = 0; i < arr.length; i++) { total += arr[i]; } return total;

Find Maximum

int max = arr[0]; for (int i = 1; i < arr.length; i++) { if (arr[i] > max) { max = arr[i]; } } return max;

ArrayList Removal (Loop Backward!)

for (int i = list.size() - 1; i >= 0; i--) { if (list.get(i).equals(target)) { list.remove(i); } }

Linear Search

for (int i = 0; i < arr.length; i++) { if (arr[i] == target) { return i; } } return -1; // not found

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