AP CSA Unit 4 | Classes & Objects

Unit 4: Classes & Objects

Classes define blueprints. Objects are individual instances with their own data. AP class-design FRQs commonly ask for constructors, fields, mutators, accessors, and boundary logic.

classobjectconstructorprivategettersettertoString

AP habit: trace values by hand before trusting your eyes. Most AP mistakes are loop bounds, type conversion, and reference mistakes.

Core Examples

Basic Class

public class Book {
    private String title;
    private int pagesRead;

    public Book(String t) {
        title = t;
        pagesRead = 0;
    }
}

Private instance variables protect object state.

Accessor and Mutator

public int getPagesRead() {
    return pagesRead;
}

public void read(int pages) {
    pagesRead += pages;
}

Methods control how data changes.

Reference Variables

Book a = new Book("Hobbit");
Book b = a;
b.read(10);

Both variables can refer to the same object.

Try It in Java

Run and modify the examples below. Add print statements, change values, and test edge cases.

Code Tracing Drop-Down Practice

If b = a and b changes the object, what does a see?

The same change.

Why use private fields?

Encapsulation prevents uncontrolled data changes.

What does a constructor do?

It initializes a new object.

Debugging Practice

Bug: accessing a private field from another class.

Use a public getter or method.

Bug: constructor has a return type.

Constructors have no return type.

AP-Style Multiple Choice

Which line creates an object?

Book b = new Book("Hobbit");

What method returns a readable summary?

toString()

What is stored in an object variable?

A reference.

FRQ Practice

Write a Stopwatch class with private seconds and running fields. Include a constructor, start(), stop(), tick(), getSeconds(), and toString().

Scoring focus

Look for the correct method signature, correct data use, clear control flow, and boundary-case handling.

Physics / Real-World Mini Project

Create a Particle class with mass, position, and velocity. Include methods to update position and calculate kinetic energy.