1Z0-811 Inheritance and Polymorphism 3

Question 1

Which statement is true about abstract classes in Java?

Accepted Answer

Abstract classes can have both abstract and concrete methods

Answer

Abstract classes can be instantiated directly with new

Answer

Abstract classes cannot contain any concrete methods

Answer

Abstract classes must implement all methods of any interface they implement

Question 2

What does the 'instanceof' operator return in Java?

Accepted Answer

A boolean indicating if an object is an instance of a given type

Answer

The class name of the object as a String

Answer

A new instance of the specified class

Answer

The memory address of the object

Question 3

What exception is thrown when an invalid cast is attempted at runtime?

Accepted Answer

ClassCastException

Answer

IllegalArgumentException

Answer

NullPointerException

Answer

ClassNotFoundException

Question 4

What is a covariant return type in method overriding?

Accepted Answer

The overriding method may return a subtype of the parent method's return type

Answer

The overriding method must return the exact same type as the parent method

Answer

The overriding method may return a supertype of the parent method's return type

Answer

The overriding method must declare void as its return type

Question 5

Which of the following is a valid polymorphic variable assignment, where Dog extends Animal?

Accepted Answer

Animal a = new Dog();

Answer

Dog d = new Animal();

Answer

Object o = new int();

Answer

Dog d = (Dog) new Object();

Question 6

What happens if a subclass constructor does not explicitly call super()?

Accepted Answer

The compiler automatically inserts a call to the no-arg super() constructor as the first statement

Answer

A compilation error always occurs

Answer

The parent class constructor is never invoked

Answer

A RuntimeException is thrown when the object is created

Question 7

A concrete class that extends an abstract class must:

Accepted Answer

Implement all abstract methods or itself be declared abstract

Answer

Override all methods inherited from java.lang.Object

Answer

Be declared final to prevent further subclassing

Answer

Declare at least one public constructor