Interfaces and Abstract Classes Flashcards

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

Answer: abstract

The `abstract` keyword is placed before the `class` keyword to declare a class as abstract, meaning it cannot be instantiated directly.

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

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.

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

Answer: A compile-time error

Attempting to instantiate an abstract class results in a compile-time error because abstract classes are incomplete by design.

Which relationship does abstract class inheritance use in Java?

Answer: extends

A subclass inherits from an abstract class using the `extends` keyword, the same as with concrete class inheritance.

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

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.

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

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.

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?

Answer: Truck must also be declared abstract

Since Truck does not implement all abstract methods inherited from Vehicle, Truck itself must be declared abstract.