AP CSA Unit 3.1: Abstraction and Class Design Practice

January 5, 2026

Unit 3, Section 3.1

Day 1 Practice • January 7, 2026

🎯 Focus: When to Use Classes

Practice Question

A programmer is creating a student grade tracking system. Consider the following two approaches:

// Approach 1: Using separate variables
String student1Name = "Alice";
int student1Grade = 92;
String student1Email = "alice@school.edu";

String student2Name = "Bob";
int student2Grade = 87;
String student2Email = "bob@school.edu";

// Approach 2: Using a Student class
public class Student {
    private String name;
    private int grade;
    private String email;
    
    public Student(String n, int g, String e) {
        name = n;
        grade = g;
        email = e;
    }
}

Student student1 = new Student("Alice", 92, "alice@school.edu");
Student student2 = new Student("Bob", 87, "bob@school.edu");

The program needs to expand to track 100 students, and later add a new field for student ID. Which statement best describes why Approach 2 is better?

What This Tests: Section 3.1 introduces abstraction—the principle of grouping related data and behaviors into classes. This is the foundation of object-oriented programming and one of the most important concepts in AP CSA. This question tests your understanding of when and why to use classes instead of separate variables.

Key Concept: Abstraction and Grouping Related Data

Abstraction means hiding complex details and showing only the essential features. A class lets you group related data (like name, grade, email) into a single unit.

Why this matters:

Scalability: Adding student 100 with Approach 1 means creating 300 new variables (name, grade, email). With Approach 2, you create 1 new Student object.
Extensibility: Adding "student ID" with Approach 1 means adding 100 new variables. With Approach 2, you add 1 field to the class.
Organization: All student data stays together, making code easier to understand and maintain.

Detailed Explanation

Why B is correct: Approach 2 uses a class to group all related data about a student (name, grade, email) into a single entity. This makes the code much more scalable and maintainable.

Scaling to 100 students:

// Approach 1: Need 300 separate variables!
String student1Name, student2Name, ... student100Name;
int student1Grade, student2Grade, ... student100Grade;
String student1Email, student2Email, ... student100Email;

// Approach 2: Create objects in a loop or array
Student[] students = new Student[100];
students[0] = new Student("Alice", 92, "alice@school.edu");
students[1] = new Student("Bob", 87, "bob@school.edu");
// Much cleaner!

Adding a new field (studentID):

// Approach 1: Add 100 new variables
int student1ID, student2ID, ... student100ID;

// Approach 2: Add 1 field to class (automatic for all objects)
public class Student {
    private String name;
    private int grade;
    private String email;
    private int studentID;  // ONE change affects all students!
}

Common Mistakes

Mistake 1: Answer A (Less memory)

Classes don't necessarily use less memory. Both approaches store the same data (names, grades, emails). The advantage is organization and maintainability, not memory efficiency.

Mistake 2: Answer C (Faster execution)

Classes don't make programs run faster. Execution speed is nearly identical. The advantage is in code quality—easier to write, read, and maintain—not runtime performance.

Mistake 3: Answer D (Prevents access)

While the private keyword does control access (encapsulation), this is a separate concept. The main advantage here is abstraction—grouping related data. You could make fields public and still benefit from using a class.

Mistake 4: Answer E (Less code initially)

This is actually FALSE—Approach 2 requires MORE code initially (you have to write the class definition). But this upfront investment pays off massively when scaling or adding features. The question asks why Approach 2 is better when expanding to 100 students.

Real-World Analogy

Think of a filing system:

Approach 1 (Separate Variables): Like having loose papers scattered everywhere. Each student's name is in one pile, grades in another pile, emails in a third pile. To find all info about one student, you search through three different piles. To add a new category (phone number), you create a fourth pile for 100 papers.

Approach 2 (Class): Like having a folder for each student. All of Alice's info is in her folder. All of Bob's info is in his folder. To add a new category, you add one new page template to the folder design, and all existing folders automatically have space for it.

Practice Technique

When to Use a Class: The 3-Question Test

Ask yourself these questions:

Is there related data? (name + grade + email all describe one student)
Will I have multiple instances? (multiple students)
Might this data/behavior expand later? (adding student ID, GPA calculation, etc.)

If YES to 2+ questions → Use a class!

Additional Examples

Example 1: Bank Account System

// BAD: Separate variables
String account1Number, account2Number;
double account1Balance, account2Balance;

// GOOD: BankAccount class
public class BankAccount {
    private String accountNumber;
    private double balance;
    
    public void deposit(double amount) {
        balance += amount;
    }
}

Example 2: Game Character System

// BAD: For 10 characters, need 40 variables!
String char1Name, char2Name, ...
int char1Health, char2Health, ...
int char1Attack, char2Attack, ...
int char1Defense, char2Defense, ...

// GOOD: Character class
public class Character {
    private String name;
    private int health;
    private int attack;
    private int defense;
}

Character[] party = new Character[10];  // Clean!

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