15. Java Abstract Classes

Abstract class can have

• Abstract methods

• Methods with code implementation

• Used to create objects

• Used to create subclasses

What is an abstract method?

• An abstract method is any method in an abstract class.

• An abstract method is a method which cannot be inherited.

• An abstract method is one without a body that is declared with the reserved word abstract.

• n abstract method is a method in the child class that overrids a parent method.

Can an abstract class define both abstract methods and non-abstract methods?

• No—it must have all one or the other.

• No—it must have all abstract methods.

• Yes—but the child classes do not inherit the abstract methods.

• Yes—the child classes inherit both.

Which of these class relies upon its subclasses for complete implementation of its methods?

• Object class

• abstract class

• ArrayList class

• None of the mentioned

Which of these keywords are used to define an abstract class?

• Abstract

• abstract

• abstract class

• abst

Which of the following is FALSE about abstract classes in Java

• If we derive an abstract class and do not implement all the abstract methods, then the derived class should also be marked as abstract using 'abstract' keyword

• Abstract classes can have constructors

• A class can be made abstract without any abstract method

• A class can inherit from multiple abstract classes.

Which of the following statement(s) with regard to an abstract class in JAVA is/are TRUE ? I. An abstract class is one that is not used to create objects. II. An abstract class is designed only to act as a base class to be inherited by other classes.

• Only I

• Only II

• Neither I nor II

• Both I & II

What is an abstract method?

• An abstract method is any method in an abstract class.

• An abstract method is a method which cannot be inherited.

• An abstract method is one without a body that is declared with the reserved word abstract.

• n abstract method is a method in the child class that overrids a parent method.

In order for the following code to be correct, what must be the type of the reference variable card?

_________ card;

card = new Valentine( "Joe", 14 ) ;

card.greeting();

card = new Holiday( "Bob" ) ;

card.greeting();

card = new Birthday( "Emily", 12 ) ;

card.greeting();

• Valentine

• Holiday

• Birthday

• Card

Which of the following classes fail to compile?

abstract class X { abstract void method(); }

class Y extends X { }

class Z extends Y {

void method() { System.out.println("Class Z"); } }

• X, Y, Z

• Y, Z

• Y

• Z