AP CSA Interfaces and Abstract Classes 2

Question 1

Which keyword is used to declare an abstract class in Java?

Accepted Answer

abstract

Answer

interface

Answer

virtual

Answer

sealed

Question 2

Can an abstract class contain non-abstract (concrete) methods?

Accepted Answer

Yes, it can contain both abstract and concrete methods

Answer

No, all methods in an abstract class must be abstract

Answer

Yes, but only if it also implements an interface

Answer

No, concrete methods belong only in interfaces

Question 3

What is the result of trying to instantiate an abstract class directly?

Accepted Answer

A compile-time error

Answer

A NullPointerException at runtime

Answer

An object is created with null fields

Answer

The JVM creates a proxy object

Question 4

Which relationship does abstract class inheritance use in Java?

Accepted Answer

extends

Answer

implements

Answer

uses

Answer

includes

Question 5

Consider: `public abstract class Shape { public abstract double area(); }`. Which of the following correctly creates a subclass?

Accepted Answer

public class Circle extends Shape { public double area() { return 3.14 * r * r; } }

Answer

public class Circle extends Shape { }

Answer

public class Circle implements Shape { public double area() { return 3.14 * r * r; } }

Answer

public abstract class Circle extends Shape { }

Question 6

What distinguishes an abstract class from an interface in Java (in the context of AP CSA)?

Accepted Answer

Abstract classes can have constructors; interfaces cannot

Answer

Abstract classes support multiple inheritance; interfaces do not

Answer

Interfaces can have instance variables; abstract classes cannot

Answer

Abstract classes cannot have abstract methods

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?

Accepted Answer

Truck must also be declared abstract

Answer

Truck compiles fine because it provided at least one method

Answer

Truck automatically inherits a default `stop()` from Vehicle

Answer

A runtime error occurs when `stop()` is called