AP CSA Unit 1: Using Objects and Methods - Complete Study Guide

AP Computer Science A – Unit 1: Using Objects and Methods (2025)

Unit 1 introduces the building blocks of Java programs: variables, data types, expressions, input/output, the Math class, APIs, objects, and method calls. Everything else in AP CSA builds on these ideas, so this is the unit where you set yourself up for a 4 or 5.

📘 1.1 Unit Overview & Exam Context

In the 2025 AP Computer Science A Course and Exam Description (CED), Unit 1: Using Objects and Methods accounts for about 15–25% of the multiple-choice exam. Even though it's the first unit, it introduces concepts you’ll use all year:

Variables, data types, and expressions
Assignment and simple input/output
Casting and the range of primitive types
Compound assignment operators (like +=)
Documentation with comments
APIs and libraries (like the Java standard library)
Method signatures and method calls (including class/static methods)
The Math class and basic numerical methods
Objects, instantiation, and calling instance methods
String objects and common string operations

Why Unit 1 matters: If you can confidently create variables, call methods, and reason about objects and Strings, you’ll be in excellent shape for later topics like iteration, arrays, ArrayLists, and FRQs.

🧠 1.2 Algorithms, Programs, and Compilers

An algorithm is a step-by-step process for solving a problem. A program is how we express that algorithm in a programming language (like Java).

Compiler vs. Source Code

Java is a compiled language. You write source code in .java files. Then, the compiler translates this code into bytecode that runs on the Java Virtual Machine (JVM).

Plain English Algorithm: 1. Ask the user for their age. 2. Add 1. 3. Print "Next year you will be " + (age + 1). ↓ translated into Java code ↓ int age = input.nextInt(); int next = age + 1; System.out.println("Next year you will be " + next);

On the AP exam, you don’t need to know all compiler internals, but you must understand that Java source code must be translated before the computer can execute it.

🔤 1.3 Variables, Data Types, and Expressions

A variable is a named storage location in memory. Every variable has:

A type (what kind of data it holds)
A name
A value

Primitive Data Types (AP Required)

Type	Description	Example
`int`	Integer value	`int count = 10;`
`double`	Decimal number	`double price = 4.99;`
`boolean`	true or false	`boolean done = false;`
`char`	Single character	`char grade = 'A';`

Expressions & Operators

An expression combines variables, values, and operators to produce a result:

int x = 5;
int y = 2;
int sum = x + y;        // 7
int product = x * y;    // 10
double avg = (x + y) / 2.0;  // 3.5

AP Tip: Know the difference between integer division and floating-point division. 5 / 2 is 2, but 5 / 2.0 is 2.5.

📥 1.4 Assignment, Input, Casting & Compound Operators

Assignment Statements

The = symbol in Java means “assign”, not “equals in math”.

int a = 10;
a = a + 5;   // now a is 15

Simple Input (Scanner)

AP CSA expects you to understand the idea of input, often with Scanner:

Scanner input = new Scanner(System.in);
int age = input.nextInt();

Casting & Range

Casting converts between compatible types:

double d = 5.8;
int n = (int) d;  // n is 5 (fraction is discarded)

Casting from double to int truncates (drops) the decimal part — it does not round.

Compound Assignment

These operators update and assign in one step:

x += 3;   // x = x + 3
x -= 2;   // x = x - 2
x *= 4;   // x = x * 4
x /= 5;   // x = x / 5

📄 1.5 Comments, APIs & Method Signatures

Comments & Documentation

Comments help explain your code to humans (they are ignored by the compiler):

// This is a single-line comment

/*
   This is a
   multi-line comment
*/

Writing clear comments is part of good programming style and can be especially helpful on FRQs.

APIs & Libraries

An API (Application Programming Interface) tells you:

What methods are available
What parameters they take
What types they return

For example, the String API shows methods like length(), substring(), indexOf(), equals(), and compareTo().

Method Signatures

A method signature includes the method name and parameter types:

public static int max(int a, int b)

Return type: int
Method name: max
Parameters: two ints (a, b)

The AP exam expects you to read unfamiliar method signatures and call them correctly.

🧮 1.6 Class (Static) Methods & the Math Class

A class method (also called a static method) belongs to the class itself, not to a particular object. You call these using ClassName.methodName(...).

Math Class Basics

double root = Math.sqrt(25);   // 5.0
double pow  = Math.pow(2, 3);  // 8.0
double abs  = Math.abs(-4.5);  // 4.5

Math.random() returns a double in the range [0.0, 1.0) (0 inclusive, 1 exclusive).

Random Integer in a Range

// 0 to n-1
int r1 = (int)(Math.random() * n);

// 1 to n
int r2 = (int)(Math.random() * n) + 1;

// a to b inclusive
int r3 = (int)(Math.random() * (b - a + 1)) + a;

AP Tip: Always test your range formulas with the smallest and largest possible values from Math.random(): 0.0 and just under 1.0.

🧱 1.7 Objects, Instantiation & References

Java is an object-oriented language. Many things you work with are objects: String, Scanner, Random, and eventually your own classes.

Creating Objects (Instantiation)

Scanner input = new Scanner(System.in);
Random rand = new Random();
String name = new String("APCSA"); // usually just: String name = "APCSA";

The new keyword creates (instantiates) an object in memory and returns a reference to it. That reference is stored in the variable.

Think of this mental model: Scanner input = new Scanner(System.in); +-----------+ +------------------------+ | input | ---> | Scanner object in heap | +-----------+ +------------------------+ The variable holds a reference (arrow), not the entire object.

This idea (variables storing references to objects) is crucial later when you pass objects to methods or assign one object variable to another.

📞 1.8 Calling Instance Methods

An instance method is a method you call on a specific object instance:

String word = "hello";
int len = word.length();          // instance method
String upper = word.toUpperCase();

General pattern:

objectVariable.methodName(parameters);

More Examples

Scanner input = new Scanner(System.in);
int value = input.nextInt();   // calls nextInt() on that Scanner

Random rand = new Random();
int r = rand.nextInt(10);      // 0–9

On the AP exam, you'll be asked to interpret method calls, determine parameter types, and predict return values.

🧵 1.9 Strings & String Manipulation

String is one of the most important classes in AP CSA. It represents a sequence of characters like "hello" or "AP CSA".

Key Ideas About Strings

Strings are objects, not primitive types.
Strings are immutable – they cannot be changed after creation.
Many AP questions involve substring, indexOf, equals, and compareTo.

Common String Methods

String s = "computer";

// length
int n = s.length();          // 8

// substring
String sub1 = s.substring(0, 3); // "com"
String sub2 = s.substring(3);    // "puter"

// indexOf
int idx1 = s.indexOf("pu");  // 3
int idx2 = s.indexOf("x");   // -1 (not found)

// equality
boolean b1 = s.equals("computer");  // true
boolean b2 = s.equals("Computer");  // false (case-sensitive)

Never use == to compare two Strings. Use equals() instead, because == compares references, not contents.

String compareTo

"apple".compareTo("banana");   // negative (apple comes before banana)
"zoo".compareTo("ant");        // positive (zoo comes after ant)
"java".compareTo("java");      // 0 (equal)

You rarely care about the exact number—just whether it is negative, zero, or positive.

🧩 1.10 Putting It All Together – Practice Examples

Example 1 – Using Math and Strings

int a = 7;
int b = 3;
double avg = (a + b) / 2.0;
String msg = "Average: " + avg;
System.out.println(msg);

Output: Average: 5.0

Example 2 – Simple Input and Output

Scanner input = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = input.nextLine();

System.out.println("Hello " + name + "!");

Example 3 – String + Random

Random rand = new Random();
String[] prizes = {"Sticker", "Pen", "Notebook"};
int index = rand.nextInt(prizes.length);
System.out.println("You won: " + prizes[index]);

These “combine everything” examples are similar to early AP CSA MCQs and helpful warm-ups for FRQs.

⚠️ 1.11 Common Unit 1 Mistakes

Using == instead of equals() for Strings
Forgetting integer vs. double division rules
Assuming Math.random() includes 1.0
Forgetting that String methods don’t modify the original string (immutability)
Misreading method signatures (wrong parameter types or order)
Confusing variables that hold values vs. variables that hold object references

When practicing, always ask yourself: “Is this a primitive or an object? Does this method change something, or does it return a new value?”

📝 Unit 1 Quiz – Using Objects and Methods (10 Questions)

Question 1

What is printed by the following code?

int a = 5;
int b = 2;
double result = a / b;
System.out.println(result);

A. 2.0
B. 2.5
C. 3.0
D. 3

Answer: A
a / b is integer division (5/2 = 2), then stored as 2.0 in a double.

Question 2

Which of the following correctly creates a Scanner object to read from the keyboard?

A. Scanner input = new Scanner();
B. Scanner input = new Scanner(System.in);
C. Scanner = new Scanner(System.in);
D. new Scanner input = Scanner(System.in);

Question 3

What best describes the purpose of an API in Java?

A. It automatically fixes syntax errors.
B. It describes available classes, methods, and how to use them.
C. It converts Java code to machine code.
D. It creates random numbers for testing.

Question 4

What is the value of x after this code runs?

int x = 10;
x += 3;
x -= 4;

A. 7
B. 9
C. 13
D. 14

Question 5

Which expression correctly generates a random integer from 1 to 6 inclusive?

A. (int)(Math.random() * 6)
B. (int)(Math.random() * 6) + 1
C. (int)(Math.random() * 5) + 1
D. (int)(Math.random() * 7) + 1

Answer: B
Pattern: 1 to n → (int)(Math.random() * n) + 1. Here n = 6.

Question 6

What is printed?

String s = "APCSA";
System.out.println(s.substring(1, 4));

A. "APC"
B. "PCS"
C. "CSA"
D. "PCSA"

Question 7

Which of the following compares two strings for equal text?

A. str1 == str2
B. str1.equals(str2)
C. str1 = str2
D. str1.compareTo(str2) == 1

Answer: B
equals() checks if the characters in the two strings match exactly.

Question 8

Which of the following is a valid method signature?

A. public int method(String x, int y)
B. public method(String x, int y)
C. public static void(int x)
D. int method(x, y)

Question 9