Lesson 3.1: Abstraction and Program Design

Unit 3 · Lesson 3.1 · Conceptual

Lesson 3.1: Abstraction and Program Design

🕑 25–30 min· 8 Practice Questions· 4 Mastery Questions· Output Predictor + Bug Hunt

What You'll Learn

Define abstraction and explain why it is central to class design
Distinguish instance variables from class (static) variables
Apply the three-question method to design a class from a specification
Explain why instance variables should be private

Key Vocabulary

Term	Definition
abstraction	Reducing complexity by focusing on the main idea; hiding irrelevant details (EK 3.1.A.1)
data abstraction	Separates what data represents from how it is stored; gives data a name without exposing implementation (EK 3.1.A.2)
attribute	Data defined in a class outside any method; represents a property of objects (EK 3.1.A.3)
instance variable	An attribute whose value is unique to each instance of the class (EK 3.1.A.3)
class variable	An attribute shared by ALL instances; declared with the `static` keyword (EK 3.1.A.3)
behavior	An action a class can perform; implemented as a method
UML diagram	Unified Modeling Language diagram showing a class's attributes and behaviors

CED EK 3.1.A.1–3.1.A.3

Abstraction reduces complexity by hiding implementation details and exposing only what is necessary. An instance variable's value is unique per object; a class variable (static) is shared by all objects.

What Is Abstraction?

A BankAccount class abstracts a real account. You include balance and methods like deposit(). You ignore the bank's server architecture and network protocols.

Identifying Attributes and Behaviors

Scenario: model a Student in a grade-book program.

Category	Examples
Attributes (data)	`name` (String), `grade` (int), `gpa` (double)
Behaviors (methods)	`getGpa()`, `setGrade(int g)`, `toString()`
Irrelevant details	home address, locker number, lunch preference

Instance Variables vs Class Variables

Instance vs Class Variable

public class Student {
    private String name;           // instance variable -- unique per object
    private int grade;
    private static int total = 0;  // class variable -- shared by ALL objects

    public Student(String n, int g) {
        name = n; grade = g;
        total++;  // increments the ONE shared copy
    }
}

After creating two Students, each has its own name, but total is 2 for both.

Designing a Class: Three Questions

Question	Guides You To
What is it?	The class name (a noun)
What does it have?	Instance variables (attributes)
What can it do?	Methods (behaviors)

AP Trap: Attributes vs Behaviors

Attributes are nouns (data stored in the object). Behaviors are verbs (actions the object performs). getBalance() is a behavior, NOT an attribute.

AP Trap: Static vs Instance

Every object needs its OWN copy → instance variable. All objects share ONE copy → class variable (static). A counter of objects created is almost always static.

AP Trap: Private Instance Variables

Instance variables should be private unless the specification says otherwise. Public instance variables break encapsulation and appear as distractors on the AP exam.

Real-World Connection: A GPS app models roads as objects with attributes (speedLimit, laneCount) and behaviors (getCongestion()).

Summary

Abstraction hides irrelevant details and focuses on essential properties.
An instance variable belongs to a specific object; a class variable (static) is shared by all objects.
Design a class by identifying its name, attributes, and behaviors.
Instance variables representing attributes should be declared private.

Tier 2 · AP Practice

Practice Questions

MCQ 1

Which BEST describes abstraction in OOP?

A Ensuring all instance variables are public

B Reducing complexity by focusing on essential properties and hiding irrelevant details

C Requiring every class to extend another class

D Forcing all methods to return void

B is correct. EK 3.1.A.1.

MCQ 2

For a TrafficLight class, which is BEST classified as an attribute (instance variable)?

A changeToRed()

B currentColor

C getCycleTime()

D flashWarning()

B is correct. currentColor stores state. A, C, D are actions (behaviors).

MCQ 3

private static int totalCreated; is inside Counter. Which is TRUE?

A Each Counter object has its own copy of totalCreated

B totalCreated cannot be accessed by any Counter method

C All Counter objects share a single copy of totalCreated

D totalCreated resets to zero when a new Counter is created

C is correct. static = one shared copy for all instances.

MCQ 4

Which class design BEST applies abstraction for a Book class?

⚠ Predict the answer before reading the options.

A Attributes: title, author, isbn; Behaviors: getTitle(), isCheckedOut()

B Attributes: title, author, isbn; Behaviors: none

C Attributes: none; Behaviors: getTitle(), getAuthor()

D Attributes: printerModel, networkSpeed; Behaviors: scanPage()

A is correct. A well-designed class has relevant attributes AND behaviors.

MCQ 5

The following class violates standard OOP practice. Why?

public class Thermometer {
    public double temperature;
    public double humidity;
}

A Constructors should be private, not public

B The class is missing a static counter variable

C Instance variables should be private to enforce encapsulation

D A class with two instance variables cannot be properly abstracted

C is correct. Public instance variables allow direct external modification, breaking encapsulation.

MCQ 6

Which is the MOST appropriate use of a static variable?

A Storing the name of each individual Employee

B Counting the total number of Employee objects created

C Storing the salary of each Employee

D Tracking the department of each individual Employee

B is correct. A total count is shared by all objects -- perfect for static. A, C, D vary per individual object.

MCQ 7

Which statements about a Vehicle class are TRUE?
I. make and model are appropriate instance variables.
II. accelerate() and brake() are appropriate behaviors.
III. roadSurfaceComposition is an appropriate Vehicle instance variable.

A I only

B I and II only

C II and III only

D I, II, and III

B is correct. I and II are valid. III is false: road surface belongs to a Road class, not Vehicle.

MCQ 8

After Dog d1 = new Dog("Rex",3); and Dog d2 = new Dog("Spot",5);, where dogCount is static and incremented per constructor call, which describes the state?

A d1.name="Rex", dogCount=1

B d1.name=d2.name="Rex", dogCount=2

C d1.name="Rex", d2.name="Spot", dogCount=2

D d1.name="Rex", d2.name="Spot", dogCount=1

C is correct. Each object has its own name. dogCount is static, incremented twice = 2.

PRACTICE WITH A GAME — CHOOSE ONE:

Output Predictor: Static vs Instance

Predict the printed output.

Question 1 of 3 · Score: 0

Tier 3 · AP Mastery

Mastery: Abstraction and Program Design

MCQ 1

Which bank account design demonstrates the BEST abstraction?

A Attributes: networkLatency, serverPort; Behaviors: getBalance()

B Attributes: balance, accountNumber; Behaviors: none

C Attributes: none; Behaviors: deposit(double), withdraw(double)

D Attributes: balance, accountNumber; Behaviors: deposit(double), withdraw(double)

D is correct. Complete abstraction needs both relevant attributes and behaviors.

MCQ 2

The following class is NOT correctly designed. Which change would BEST fix it?

public class Inventory {
    public String itemName;
    public int quantity;
}

A Change static int to a non-static instance variable

B Change public itemName and public quantity to private

C Remove all static variables from the class

D Change the class to private class Inventory

B is correct. itemName and quantity should be private (EK 3.3.A.5).

MCQ 3

After lib1.addCheckout(); lib1.addCheckout(); lib2.addCheckout(); where addCheckout() increments the static variable totalCheckouts, what is the value of totalCheckouts?

A 1

B 3

C 2

D 0

B is correct. totalCheckouts is static (shared). Each call increments it. 3 calls = 3.

MCQ 4

Which CORRECTLY distinguishes instance variable from class variable?

A Instance: shared by all; Class: unique per object

B Instance: declared with static; Class: no static keyword

C Instance: unique per object, no static keyword; Class: shared by all, declared with static

D Instance: only primitive types; Class: only reference types

C is correct. EK 3.1.A.3. A and B reverse the definitions. D is false.

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