Lecture 7 - OOP Foundations

Question 1

What is a publicized service made available to others?

Answer 1

Interface

Question 2

TF: The implementation of an interface is usually visible to user of the interface

Answer 2

False. Implementation generally hidden

Question 3

At what time is a collection of classes organized into packages (hint: static point of view)?

Answer 3

At compile-time

Question 4

At what time is a program a collaborating community of objects that uses class features (data, method)?

Answer 4

At runtime

Question 5

What are Basic (primitive) types?

Answer 5

• int
• long
• double
• void
• boolean

Question 6

What are Reference types?

Answer 6

• Class
• Interface
• Array

Question 7

TF: A stack cannot hold the value of a primitive type

Answer 7

False. It can

Question 8

TF: A stack can hold the value NULL

Answer 8

True

Question 9

TF: A stack cannot hold a reference (pointer) to an object allocated in Heap

Answer 9

False. It can

Question 10

TF: A stack can hold Objects

Answer 10

False. It cannot

Question 11

TF: A heap can only hold objects that were created via “new”

Answer 11

True

Question 12

What are the two things defined by a class?

Answer 12

• A type
• An implementation

Question 13

What is defined by an interface?

Answer 13

• A type
  (without an implementation)
  Note: interface has no logic (logic defined by class implementing interface)

Question 14

What are the two types an expression can have?

Answer 14

• Compile-time (static) type
• Runtime (dynamic) type

Question 15

The compile-time type is also known as…

Answer 15

The declared type

Question 16

Animal a = new Cat();
What is the declared type of the provided code?

Answer 16

Animal

Question 17

Animal a = new Cat();
What is the run-time type of the provided code?

Answer 17

Cat

Question 18

Will the following code run?
Animal a = new Animal();
Cat c = (Cat) a;

Answer 18

No. Cannot cast downwards

Question 19

Will the following result in a compile-time error or runtime error?
Animal a = new Animal();
Cat c = (Cat) a;

Answer 19

Runtime error. The compiler thinks Animal “a” could be a Cat

Question 20

Will the following code run?
Cat c = new Cat();
Animal a = (Animal) c;

Answer 20

Yes. Casting upwards is allowed

Question 21

What is the purpose of Casting? (2)

Answer 21

• Inform compiler of (assumed) subtype of an object
  [ex: Animal a = new Cat()]
• So compiler can perform better type checking

Question 22

TF: Dynamic dispatching is when the runtime decides which implementation of a non-static method to call based on the calling object

Answer 22

True

Question 23

Which sleep method gets called for the following:
Animal a = new Cat();
a.sleep();

Answer 23

Since Cat is the actual type, the Cat’s sleep method gets called

Question 24

TF: Dynamic dispatching can occur on static methods

Answer 24

False. Only on non-static methods

Question 25

TF: Dynamic dispatching happens for constructors

Answer 25

False. Object constructor is decided at compile-time

Question 26

Given the following code,the compiler will check where for the existence of the method?
A a = new Cat();
a.m();

Answer 26

It will check the declared type (i.e. A) and all superclass of it for the method existence.