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Visual Review
AP Computer Science A Unit 1 Visual ReviewA topic-by-topic visual walkthrough of Using Objects and Methods — variables, data types, expressions, casting, the Math and String classes, and object creation.
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TOPIC 1.1
Algorithms, Programming & Compilers
Key definitions
ALGORITHM: a finite, ordered set of
steps that solves a problem.
PROGRAM: an algorithm written in a
language a computer can execute.
Java is a compiled language
A COMPILER translates your source
code into bytecode the computer runs.
A SYNTAX error stops compilation —
the program won't run until it's fixed.
// every Java program lives inside a class
public class HelloWorld {
public static void main (String[] args) {
System.out.println("Hello, World!" ); } }
An algorithm becomes a program ; a compiler turns Java source into runnable code.
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.2
Variables and Data Types
A variable is a named memory location that holds a value of one type
Java is STATICALLY TYPED: you declare a type, and the variable can only ever hold that type of value.
int score = 100; double gpa = 3.75; boolean passed = true;
int
Whole numbers.
No decimal point.
42, -7, 0
Stores 32-bit integers.
double
Decimal (real) numbers.
Has a fractional part.
3.14, -0.5, 2.0
Stores 64-bit floating point.
boolean
A truth value.
Only two options.
true, false
Used for conditions/logic.
The three primitive types tested on the exam: int, double, boolean .
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.3
Expressions and Output
Arithmetic operators
+ − * / %
% is the REMAINDER (mod) operator.
Precedence: * / % before + −.
Use ( ) to force order.
17 % 5 → 2 13 / 4 → 3
Printing output
System.out.print(x);
prints with NO new line after
System.out.println(x);
prints, then moves to a new line
+ joins ("concatenates") strings.
// string concatenation with + mixes text and values
System.out.println("Total: " + count ); // Total: 7
Mind operator precedence; println adds a new line, print doesn't .
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.4
Assignment Statements and Input
= assigns right-to-left
The = operator EVALUATES the right side, then stores that value in the variable on the left.
int x = 5; x = x + 3; // x is now 8 — the old x (5) is read first
Reading input with Scanner
Scanner in =
new Scanner(System.in);
int n = in.nextInt();
nextInt(), nextDouble(), nextLine()
Declare vs. assign
DECLARE once (with a type):
int count;
ASSIGN as often as you like:
count = 10; count = 11;
= evaluates the right side first ; a Scanner reads user input by type.The Review Hub · AP Computer Science A Unit 1
TOPIC 1.5
Casting and Range of Variables
Integer division truncates
int / int gives an int — the fraction is
dropped (NOT rounded).
7 / 2 → 3
7.0 / 2 → 3.5
(double) 7 / 2 → 3.5
Casting between types
(double) x
int → double (widening, always safe)
(int) 4.9 → 4
double → int TRUNCATES toward zero
Cast binds tightly — cast before dividing.
Range & overflow
An int holds about ±2.1 billion (−2³¹ to 2³¹−1). Exceeding it OVERFLOWS and wraps to a wrong value.
int/int truncates ; cast to double to keep the fraction, cast to int to drop it.The Review Hub · AP Computer Science A Unit 1
TOPIC 1.6
Compound Assignment Operators
Shorthand for "update a variable using itself"
Each compound operator performs the arithmetic, then reassigns the result back into the variable.
x += 3; is exactly x = x + 3;
x += 5 x = x + 5
x -= 5 x = x - 5
x *= 5 x = x * 5
x /= 5 x = x / 5
x %= 5 x = x % 5
Increment & decrement
x++; // add 1
x--; // subtract 1
Very common as loop counters.
x++ is shorthand for x = x + 1.
+=, -=, *=, /=, %= update a variable in place; ++ / -- change it by one.The Review Hub · AP Computer Science A Unit 1
TOPIC 1.7
API and Libraries
What an API is
An Application Program Interface (API)
documents the classes and methods a
library provides, and how to call them.
A LIBRARY = reusable prewritten code.
Why it matters on the exam
You don't memorize every method —
you READ the API to find the right one
and its parameters & return type.
The AP Java Quick Reference is provided.
// reading a method signature from the API tells you how to call it
int length() // takes nothing, returns an int String substring(int from, int to) // two int params An API documents a library's methods — read it to learn parameters and return types.
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.8
Documentation with Comments
// single-line comment — to the end of the line
/* block comment
spanning multiple lines */
/** Javadoc comment — documents a method */
// the compiler ignores all comments
Why comment?
Explain intent for human readers.
Comments never affect execution.
Preconditions & postconditions
PRECONDITION: what must be true before.
POSTCONDITION: what's true after it runs.
Comments (//, /* */, /** */ ) are for humans — the compiler ignores them.
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.9
Method Signatures
int max (int a, int b)return type
name
parameter list (types + names)
Signature = name + parameters
The signature is what you need to CALL
a method: its name and the ordered
list of parameter types.
Arguments must match the parameter types.
Parameters vs. arguments
PARAMETER: the placeholder in the
method definition (int a).
ARGUMENT: the actual value you pass
in when calling — max(3, 8).
A method signature = name + parameter types ; arguments must match in order and type.
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.10
Calling Class (Static) Methods
Static methods belong to the CLASS, not an object
Call them on the class name — no object needed. Format: ClassName.methodName(arguments)
int big = Math.max(3, 8); double r = Math.sqrt(16);
// the receiver is the CLASS name
Math .abs (-5) → 5 Integer .parseInt ("42" ) → 42 A method that returns a value can be
used anywhere that value type is valid.
Void vs. returning methods
A void method does an action but gives
back no value (e.g. println).
A non-void method RETURNS a value you
can store or use in an expression.
Match the return type to how you use it.
Call static methods on the class name: ClassName.method(args) .
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.11
The Math Class
// the four Math methods on the AP Java Quick Reference
Math .abs (x) absolute value Math .pow (b, e) b to the power e (returns double) Math .sqrt (x) square root (returns double) Math .random () double in [0.0, 1.0) Random int in a range — a classic exam formula
(int)(Math.random() * (max - min + 1)) + min
e.g. a fair die (1–6): (int)(Math.random() * 6) + 1. The (int) cast truncates to a whole number.
Know abs, pow, sqrt, random — and the random-int-in-range formula.
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.12
Objects: Instances of Classes
Class vs. object
A CLASS is a blueprint/template that
defines state (data) and behavior
(methods).
An OBJECT is one specific INSTANCE
built from that blueprint.
Many objects can come from one class.
Analogy: cookie cutter
The CLASS is the cookie cutter.
Each OBJECT is a cookie it stamps out.
Cookies share a shape but can differ
in their own attribute values.
String is a class; "hi" is an object.
Reference types
Objects are REFERENCE types: the variable stores a reference (address), not the object itself.
A class is the blueprint; an object is one instance built from it.
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.13
Object Creation (Instantiation)
Robot r = new Robot (5 , 10 );type
new keyword
constructor call + arguments
The new keyword
new allocates memory for the object
and runs its CONSTRUCTOR to set up
the initial state.
It returns a reference to the new object.
null & aliasing
A reference not yet pointing at an object
is null. Using it → NullPointerException.
Two variables can ALIAS the same object,
so a change through one shows in both.
new ClassName(args) allocates the object and runs its constructor.The Review Hub · AP Computer Science A Unit 1
TOPIC 1.14
Calling Instance Methods
Instance methods are called ON an object
Format: objectReference.methodName(arguments). The object is the RECEIVER of the call.
String s = "hello"; int n = s.length(); // n = 5
// receiver . method ( args )
account .deposit (50 );account .getBalance ();Different objects keep their own state,
so the same call can give different results.
Instance vs. static — the difference
STATIC: called on the class
Math.sqrt(9)
INSTANCE: called on an object
myStr.length()
Instance methods can use the object's fields.
Instance methods use object.method(args) — the object is the receiver.
The Review Hub · AP Computer Science A Unit 1
TOPIC 1.15
String Manipulation
String s = "COMPUTER"; // indices 0..7, length 8
s.length() → 8 s.substring(0, 4) → "COMP" (0 up to but NOT 4) s.indexOf("P") → 3 (-1 if not found) s.equals("computer") → false (case-sensitive) Zero-indexed & immutable
First character is index 0; last is
length() − 1. Strings never change.
Compare with .equals, not ==
== compares references; .equals
compares the actual characters.
Strings are zero-indexed and immutable ; compare with .equals, and substring excludes the end.
The Review Hub · AP Computer Science A Unit 1
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How to use the visual review
Spend 30 seconds per slide before clicking next. Look at the code, then ask yourself: "Could I predict this output or write this method call from memory?"
Use the fullscreen button () on desktop for the best experience. Use arrow keys to navigate. Tap "Show all slides" to jump around.
This is great for review the night before the exam — fast, visual, and covers every idea you need to recognize in Unit 1.