Deck 9: Polymorphism

Question 1

Consider a reference declared in the following manner.
Animal a;
This reference may only point to an object that created by instantiating the Animal class.

Answer 1

False
Explanation: This reference may point to an object of any type that is compatible with Animal. In particular, it may point to any object that is an instance of a class that is a subclass of Animal.

Question 2

The compareTo method of the Comparable interface returns a boolean value.

Answer 2

False
Explanation: The compareTo method returns an integer.

Question 3

Suppose that Horse is a subclass of Animal, and neither class is abstract. Which of the following is an invalid declaration and initialization?

A) Horse h = new Horse();
B) Horse h = new Animal();
C) Animal a = new Animal();
D) Animal a = new Horse();
E) all of the above are valid

Answer 3

B
Explanation: Since Horse is a subclass of Animal, choice b would require an explicit class.

Question 4

In Java, a(n) ___________________ is a collection of constants and abstract methods.

A) polymorphic reference
B) abstract class
C) implementation
D) interface
E) iterator

Question 5

In Java, polymorphic method binding occurs ____________________ .

A) at run time
B) at compile time
C) never
D) when a programmer writes the code
E) during the testing phase of software development

Question 6

Which of the following methods are included with any object that implements the Iterator interface?

A) next
B) hasNext
C) toString
D) all of the above
E) a and b

Question 7

The next method of the Iterator interface returns a reference to the next element in a collection and removes it.

Question 8

Let Animal be an interface. Then it is possible to create an object by instantiating the Animal interface.

Question 9

The Comparable interface contains which of the following methods?

A) isGreaterThan
B) isLessThan
C) equals
D) compareTo
E) all of the above

Question 10

Consider the following line of code. Comparable s = new String();
Which of the following statements is true about this line?

A) It will result in a compile-time error.
B) It will result in a run-time error.
C) It will create a String object pointed to by a Comparable reference.
D) Although it is perfectly valid Java, it should be avoided due to confusion.
E) none of the above are true

Question 11

Let Dog be a subclass of Animal, and suppose Animal has a method called speak() that is overridden in the Dog class. Consider the following code. Animal spot = new Dog();
Spot.speak();
Which of the following is true?

A) This code will result in a compile-time error.
B) This code will result in a run-time error.
C) The speak method defined in the Animal class will be called.
D) The speak method defined in the Dog class will be called.
E) The speak method will not be called at all.

Question 12

Let Object a be larger than Object b. What will the following method call return? a.compareTo(b)

A) it will return 0
B) it will return a number greater than 0
C) it will return a number less than 0
D) it will return true
E) it will return false

Question 13

In Java, polymorphic references can be created through the use of __________________ and ________________.

A) inheritance, interfaces
B) inheritance, abstract classes
C) interfaces, abstract classes
D) interfaces, iterators
E) none of the above

Question 14

Suppose Animal is an interface that specifies a single method - speak. Now suppose the Dog class implements the Animal interface. In addition to the speak method, the Dog class also has a method called wagTail. Now consider the following code. Animal a = new Dog();
A)wagTail();
Which of the following is true about this code?

A) It will result in a compile-time error.
B) It will result in a run-time error.
C) It will call the speak method defined in the Animal interface.
D) It will call the wagTail method defined in the Dog class.
E) none of the above are true.

Question 15

In order to create a class that implements an interface, the __________________ keyword is used.

A) extends
B) interfaces
C) implements
D) finalizes
E) abstracts

Question 16

Which GUI concepts use polymorphism to establish their relationship?

A) a listener and its associated component
B) a radio button and its default selection
C) a button and its label
D) a slider and its tick marks
E) none of the above

Question 17

A parameter to a method can be polymorphic.

Question 18

A polymorphic reference is one that can refer to _______________ type(s) of object(s).

A) exactly one
B) zero
C) multiple
D) abstract
E) static

Question 19

The commitment to execute certain code to carry out a method invocation is referred to as _________________.

A) execution
B) binding
C) polymorphism
D) inheritance
E) none of the above

Question 20

Late binding is _______________ than _______________ .

A) more efficient, compile-time binding
B) less efficient, compile-time binding
C) more efficient, run-time binding
D) less efficient, run-time binding
E)

Question 21

If a class implements an interface, it cannot extend another class.

Question 22

Suppose you are implementing the comparable interface in a class representing a Person, where the ordering is based on the age of the person. Write a compareTo method for this class. You may assume that there is an instance variable called age and a method called getAge.

Question 23

Consider the following inheritance hierarchy that is used in a video game.
Character
/ \
Friend Villain
/ \ / \
WiseMan ShopKeeper Dragon Skeleton
| |
FlyingDragon EliteSkeleton
Which of the following declarations and initializations will not cause a compiler error?
Character c = new FlyingDragon();
FlyingDragon f = new Character();
Dragon d = new Villain();
Villain v = new Skeleton();
Dragon d = new ShopKeeper();

Question 24

A single class may implement multiple interfaces.

Question 25

Describe the compareTo method the circumstances under which it returns different values.

Question 26

Are there any differences between extending a class and implementing an interface?

Question 27

What is polymorphism?

Question 28

Consider a class hierarchy that includes a class called Vehicle, with subclasses called Car and Airplane. The Vehicle class has a method called getMaxSpeed, which is overridden in the Car class. The getMaxSpeed of the Vehicle class returns 760 mph, while the getMaxSpeed method of the Car class is overridden to return 150 mph. What is the output of the following snippet of code? Explain your answer.
Vehicle v = new Car();
System.out.println(v.getMaxSpeed() + " mph");

Question 29

What is an interface?

Question 30

Write an interface for a CD player. It should have the standard operations (i.e. play, stop, etc) that usual CD players have.

Question 31

An interface name may be used as a reference type.

Question 32

How does inheritance relate to polymorphism in Java?

Question 33

Can references to interface types be polymorphic? Explain.

Question 34

An interface cannot declare any instance variables.

Question 35

Describe the Iterator interface.

Question 36

Give an example of a class that implements the Comparable interface, and explain how the implementation of the compareTo method determines its return value.

Question 37

Why can't an interface be instantiated?

Question 38

How do interfaces relate to multiple inheritance?

Question 39

Give an example of a class that implements the Iterator interface.