Unit 3 — Class Creation

Unit 3 is where you learn to design classes and write methods like AP expects: instance variables, constructors, method headers, return values, scope, and the this keyword. Study topics 3.1–3.9 first, then take the Unit 3 quiz: 15 MCQ + 1 FRQ.

Topic pages

Go in order (3.1 → 3.9).

Abstraction and Program Design3.1 Impact of Program Design3.2 Anatomy of a Class3.3 Constructors3.4 Methods: How to Write Them3.5 Methods: Passing and Returning References of an Object3.6 Class Variables and Methods3.7 Scope and Access3.8 this Keyword3.9

How to use the quiz

Answer MCQs first. Don’t check as you go.
Click Check Answers to see score + explanations.
Try the FRQ, then reveal the sample solution.

Unit 3 Quiz — Multiple Choice (15)

Choose the best answer. (One correct option each.)

1) Which best describes abstraction in AP CSA?

Topic 3.1

A. Rewriting code so it runs faster B. Using only primitive types C. Managing complexity by hiding details and exposing essential features D. Printing variable values to debug a program

2) Which design choice most improves code reuse?

Topic 3.2

A. Copying and pasting similar code into many places B. Writing a method to perform a repeated task C. Using longer variable names D. Avoiding parameters in methods

3) Which line is a valid instance variable declaration inside a class?

Topic 3.3

A. private int score; B. public static void main(String[] args) C. return score; D. if (score > 0)

4) Which is true about constructors?

Topic 3.4

A. Constructors must have a return type B. Constructors can be called without creating an object C. A constructor name can be different from the class name D. Constructors initialize new objects and have the same name as the class

5) What is returned by this method call?

Topic 3.5

public int addOne(int x) {
  return x + 1;
}

A. The method prints x + 1 B. The method returns x + 1 to the caller C. The method changes the value of the caller’s variable automatically D. The method must return a double

Answer: B. return sends a value back to whoever called the method.

6) Which is true about parameters?

Topic 3.5

A. Parameters receive values passed into a method call B. Parameters can only be primitives, not objects C. Parameters must be declared static D. Parameters are the same thing as instance variables

7) Which statement about object references is correct?

Topic 3.6

A. Assigning one object reference to another creates a new object B. Two references can never point to the same object C. Two variables can reference the same object in memory D. References store the entire object’s data inside the variable

8) Which is a class (static) method call?

Topic 3.7

A. name.length() B. list.add(3) C. student.getScore() D. Math.abs(-5)

Answer: D. Math.abs is called on the class name (static method).

9) What is the scope of a local variable declared inside a method?

Topic 3.8

A. The entire program B. From its declaration to the end of the block it’s declared in C. All methods in the class D. Only inside constructors

10) Which use of `this` is most common in constructors?

Topic 3.9

A. Distinguishing instance variables from parameters with the same name B. Calling a static method C. Creating a new object D. Ending a loop early

Answer: A. this.x = x; assigns the parameter into the instance variable.

11) In a well-designed class, why are instance variables often `private`?

Topic 3.3

A. So they can be accessed from any class B. So constructors do not run C. To preserve encapsulation and control access through methods D. Because public variables cannot store integers

12) Which method signature correctly returns a `String` and takes an `int` parameter?

Topic 3.5

A. public void make(int x) B. public int make(String s) C. public String make() D. public String make(int x)

Answer: D. Return type is String, parameter type is int.

13) What happens when you assign one object reference variable to another?

Topic 3.6

A. The object is copied into a new memory location B. Both variables refer to the same object C. The object is automatically deleted D. The program will not compile

14) Which is true about static (class) variables?

Topic 3.7

A. There is one shared copy for the class, not one per object B. Each object gets its own separate copy C. They can only store Strings D. They are declared inside methods

15) Which statement about access is correct?

Topic 3.8

A. Private methods can be called from any class B. Public fields are required for encapsulation C. Public methods provide controlled access to private data D. Access modifiers do not affect what can be accessed

Unit 3 Quiz — FRQ (1)

This FRQ focuses on designing a class with instance variables, a constructor, and methods (Unit 3 skills).

FRQ 1) `Counter` class

Write a class Counter that represents a counter that starts at an initial value. The class must include:

An instance variable that stores the current count.
A constructor public Counter(int start) that sets the initial count to start.
A method public void increment() that increases the count by 1.
A method public int getValue() that returns the current count.

Write your solution here:

Sample solution

public class Counter {
  private int count;

  public Counter(int start) {
    count = start;
  }

  public void increment() {
    count++;
  }

  public int getValue() {
    return count;
  }
}

Uses a private instance variable, constructor initialization, and simple instance methods.

Unit 3 — Class Creation

Topic pages

How to use the quiz

Unit 3 Quiz — Multiple Choice (15)

1) Which best describes abstraction in AP CSA?

2) Which design choice most improves code reuse?

3) Which line is a valid instance variable declaration inside a class?

4) Which is true about constructors?

5) What is returned by this method call?

6) Which is true about parameters?

7) Which statement about object references is correct?

8) Which is a class (static) method call?

9) What is the scope of a local variable declared inside a method?

10) Which use of this is most common in constructors?

11) In a well-designed class, why are instance variables often private?

12) Which method signature correctly returns a String and takes an int parameter?

13) What happens when you assign one object reference variable to another?

14) Which is true about static (class) variables?

15) Which statement about access is correct?

Unit 3 Quiz — FRQ (1)

FRQ 1) Counter class

Sample solution

Done?

10) Which use of `this` is most common in constructors?

11) In a well-designed class, why are instance variables often `private`?

12) Which method signature correctly returns a `String` and takes an `int` parameter?

FRQ 1) `Counter` class