AP CSA Interfaces and Abstract Classes 2 — Questions and Answers
Question 1: Which keyword is used to declare an abstract class in Java?
- interface
- abstract (Correct answer)
- virtual
- sealed
Correct answer: abstract
The `abstract` keyword is placed before the `class` keyword to declare a class as abstract, meaning it cannot be instantiated directly.
Question 2: Can an abstract class contain non-abstract (concrete) methods?
- No, all methods in an abstract class must be abstract
- Yes, it can contain both abstract and concrete methods (Correct answer)
- Yes, but only if it also implements an interface
- No, concrete methods belong only in interfaces
Correct answer: Yes, it can contain both abstract and concrete methods
An abstract class can have a mix of abstract methods (no body) and concrete methods (with a body), providing partial implementation.
Question 3: What is the result of trying to instantiate an abstract class directly?
- A NullPointerException at runtime
- A compile-time error (Correct answer)
- An object is created with null fields
- The JVM creates a proxy object
Correct answer: A compile-time error
Attempting to instantiate an abstract class results in a compile-time error because abstract classes are incomplete by design.
Question 4: Which relationship does abstract class inheritance use in Java?
- implements
- extends (Correct answer)
- uses
- includes
Correct answer: extends
A subclass inherits from an abstract class using the `extends` keyword, the same as with concrete class inheritance.
Question 5: Consider: `public abstract class Shape { public abstract double area(); }`. Which of the following correctly creates a subclass?
- public class Circle extends Shape { }
- public class Circle implements Shape { public double area() { return 3.14 * r * r; } }
- public class Circle extends Shape { public double area() { return 3.14 * r * r; } } (Correct answer)
- public abstract class Circle extends Shape { }
Correct answer: public class Circle extends Shape { public double area() { return 3.14 * r * r; } }
Circle must extend Shape (not implement) and must provide a concrete `area()` method to avoid being abstract itself.
Question 6: What distinguishes an abstract class from an interface in Java (in the context of AP CSA)?
- Abstract classes can have constructors; interfaces cannot (Correct answer)
- Abstract classes support multiple inheritance; interfaces do not
- Interfaces can have instance variables; abstract classes cannot
- Abstract classes cannot have abstract methods
Correct answer: Abstract classes can have constructors; interfaces cannot
Abstract classes can have constructors that subclasses can call via `super()`, whereas interfaces cannot have constructors because they cannot be instantiated.
Question 7: A class Truck extends an abstract class Vehicle that has abstract methods `start()` and `stop()`. Truck provides `start()` but NOT `stop()`. What must be true of Truck?
- Truck compiles fine because it provided at least one method
- Truck must also be declared abstract (Correct answer)
- Truck automatically inherits a default `stop()` from Vehicle
- A runtime error occurs when `stop()` is called
Correct answer: Truck must also be declared abstract
Since Truck does not implement all abstract methods inherited from Vehicle, Truck itself must be declared abstract.
AP CSA Interfaces and Abstract Classes 2
Which keyword is used to declare an abstract class in Java?