Week 5

Question 1

The process of acquiring the properties (methods and fields) of another

Answer 1

Inheritance

Question 2

Types of inheritance

Answer 2

Single
Multi-level
Hierarchical
Multiple

Question 3

In hierarchical inheritance, Class B and C both

Answer 3

Inherit from class A

Question 4

In multiple inheritance, Class C

Answer 4

inherits from both A and B

Question 5

The ______ keyword is used to enforce inheritance of the properties of a class

Answer 5

extends

Syntax: Class Subclass extends Superclass

Question 6

T/F - The vehicle superclass extends the car subclass

Answer 6

False

Question 7

What’s an advantage of inheritance?

Answer 7

Lets you reuse code instead of duplicating it.

Question 8

What does a subclass inherit from a sueprclass?

Answer 8

All members (methods, fields)

Question 9

T/F - Since a car is a special type of vehicle, we can use a car object in algorithms that manipulate vehicle objects

Answer 9

True

Question 10

Explain the substitution principle

Answer 10

You can always use a subclass object when a superclass object is expected.

IE. A method that processes vehicle objects can handle any kind of vehicle

Question 11

Is a quiz a subclass of a question?

Answer 11

No, quizzes and questions are not related in that way.

Question 12

You should use a single class for variation in…
Whereas you should use inheritance for variation in….

Answer 12

You should use a single class for variation in values
Whereas you should use inheritance for variation in behavior

Question 13

Class Manager and Class Employee, who would be the superclass? Who would be the subclass?

Answer 13

Manager would be a subclass of employee. You must first be an employee to be a manager.

Question 14

T/F - A subclass will inherit all methods, unless it overrides them

Answer 14

True

Question 15

What is important to specify when making a subclass?

Answer 15

Specify what makes it different from the superclass

Question 16

Do you re-declare instance variables in a subclass?

Answer 16

No, unless you want an instance variable that is not already part of the superclass

Question 17

Does a subclass inherit private members?

Answer 17

Subclasses do not.

Objects of a subclass contain the private members of the superclass. Despite not being able to access them, they are there.

Question 18

When inheriting methods, what do you do if the inherited behaviour is not required in the subclass?

Answer 18

Change the implementation of the methods

Question 19

T/F - When you provide a new implementation for an inherited method, you override the method

Answer 19

True

Question 20

ChoiceQuestion is a subclass of Question. What are 3 ways the ChoiceQuestion objects class would differ from the Question objects class?

Answer 20

1. There’s now a method for choosing/storing an answer
2. There’s objects to store the possible choices
3. There’s a display method to show the possible choices

Question 21

After making the 3 differences, the ChoiceQuestion class must _______ that these changes were made

Answer 21

specify

Question 22

T/F - Subclasses cannot access private instance variables, therefore they must use the public methods of the superclass

Answer 22

True

Question 23

Since subclasses cannot access private variables of the superclass, that means they were not inherited.

Answer 23

False. Private fields are inherited, you just can’t access them.

Question 24

T/F - An overriding method can extend or replace the functionality of the superclass method

Answer 24

True

Question 25

What is one possible solution when you need to access a private field of the superclass?

Answer 25

Use the reserved word “super” to call the required method
super.method(parameters if any)

Question 26

This reserved word forces the execution of the superclass method

Answer 26

super

Question 27

Are constructors considered members? What does that entail for inheritance? What is one caveat?

Answer 27

Constructors are not members. Therefore they’re not inherited by subclasses. The caveat is that constructors of the superclass can be invoked from the subclass.

Question 28

The super keyword is similar to the ____ keyword. How?

Answer 28

this. It differentiates the members of the superclass from the subclass, if they have the same names.

Question 29

Use the _____ keyword to differentiate between sub and super members

Answer 29

super

Question 30

If you wish to call the superclass’ getSalary() method from inside the subclass’ getSalary() method you must

Answer 30

type super.getSalary()

Question 31

Every class defined without the “extends” clause automatically extends what?

Answer 31

The class “object”. Sometimes referred to as “the cosmic superclass”

Question 32

Tell me about the class object

Answer 32

The “object” class is the direct or indirect superclass of every Java class.

Question 33

Name some methods that are defined in Java “object” class

Answer 33

toString - (String describing the object)
equals - (compares objects with each other)
hashCode (generates a numerical code)

Question 34

This method is called whenever you concatenate a string with an object

Answer 34

toString

Question 35

Why can the compiler invoke the toString method to concatenate?

Answer 35

It knows that every object has a toString method, since every class the object class

Question 36

When using toString on an object, what happens?

Answer 36

It prints the objects class name, and the hash code of the object

BankAccount momsSavings = new BankAccount(5000);
String s = momsSavings.toString();
// Sets s to something like “BankAccount@d24606bf”

Question 37

T/F - You can override the toString method to return something more to your liking

Answer 37

True

Question 38

How does equals differ from == ?

Answer 38

Equals checks whether two objects have the same content, == tests whether 2 references are identical.

Question 39

T/F - It’s not legal to store a superclass reference in a sublass variable

Check this one to ensure it’s accurate

Answer 39

False
IE.

ChoiceQuestion cq = new ChoiceQuestion()
Object obj = cq

Question 40

What operator can you use to detect the object type of unknown objects?

IE. Object obj

Answer 40

instanceof

Question 41

What is a ClassCast exception?

Answer 41

It’s a runtime error (important). When the application attempts to cast an object to another class, of which the original object is not an instance.

Question 42

How can do a safe cast using the instanceof operator?

Answer 42

if (obj instanceof Question)
{
Question q = (Question)obj;
}

Question 43

Why does System.out.println(System.out); produce java.io.PrintStream@7a84e4?

Answer 43

There’s no toString() method inside of the PrintStream class

Question 44

Will the following code compile?

Object obj = “Hello”;
System.out.println(obj.length());

Answer 44

The second line will not compile, as the class Object does not have the method “length”

Question 45

Object obj = “Who was the inventor of Java?”;
Question q = (Question) obj;
q.display()

Will it compile?

Answer 45

It will, but the 2nd line will throw a class cast exception. Question is not a superclass of String.

Question 46

Why do we not store all objects in variables of type object?

Answer 46

There are only a few method that can be invoked on variables of type object.

Question 47

If x is an object reference, what is the value of “x instanceof Object”

Answer 47

If x is null the value is false, true otherwise

Question 48

instanceof returns a ______ value

Answer 48

boolean

Question 49

Can every class be inherited?

Answer 49

No not every class. Any class declared final cannot be inherited.