Lecture 9

Question 1

What is inheritance?

Answer 1

Inheritance allows us to create classes that are subclasses of other classes.

Question 2

What are three different ways of describing: “A inherits B”

Answer 2

A is a subclass of B, A is derived from B, A is the child of B.

Question 3

What are two different ways of describing: “B is inherited by A”

Answer 3

B is a superclass of A, B is the parent of A.

Question 4

What two things are true about an object of class A, if class A inherits class B?

Answer 4

Every object, that is an instance of the subclass has all the properties from the superclass
On such an object, you can call all methods of the superclass.

Question 5

What is the: “Liskov substitution principle”?

Answer 5

If A is a superclass of B, then everywhere in the program where an object of Class A is requested, an object of class B is also accepted.

Question 6

How do you define in Java that A is a subclass of B?

Answer 6

In the signature of A add “extends B”

Question 7

What can access something that is ‘public’?

Answer 7

everything

Question 8

What can access something that is ‘private’?

Answer 8

Only things inside the class

Question 9

What can access something that has no access type specified?

Answer 9

The class and the package

Question 10

What can access something that is ‘protected’?

Answer 10

The class, the package, and subclasses.

Question 11

How can you call the constructor of a parent?

Answer 11

super()

Question 12

What is the concept of Polymorphism

Answer 12

Everywhere where a class X is allowed, a subclass Y of x is also allowed, but NOT the other way around.

Question 13

When can you get a ‘ClassCastException’?

Answer 13

When you try to cast something to a class, which it can’t be cast to.

Question 14

What is method overriding?

Answer 14

Creating a method with the same signature as an already existing method in the inheritance-architecture but in a different class of the inheritance-architecture.

Question 15

What is method overloading?

Answer 15

Creating a method with the same name as another method inside the class (or inheritance-architecture), but with a different parameter list (different amount, types or order).

Question 16

What is dynamic binding?

Answer 16

Dynamic binding means that an object can be an instance of multiple classes (of a certain inheritance-architecture), but that it is unknown to the compiler (a.k.a. depending on circumstances) of what class this object is an instance.

Question 17

If there is dynamic binding, who “decides” what class an object is an instance of?

Answer 17

the JVM

Question 18

What is the superclass of all classes in java?

Answer 18

Object

Question 19

What does the final keyword do if added to an attribute?

Answer 19

It turns the attribute into a constant

Question 20

What does the final keyword do if added to a class?

Answer 20

It prevents you from making subclasses

Question 21

What does the final keyword do if added to a method?

Answer 21

It prevents you from redefining the method in subclasses

Question 22

What do you use an abstract class for?

Answer 22

To “combine” classes, by creating a class for a category to which they all belong, even though this category itself can’t “exist” in the real world.

Question 23

What does the abstract keyword do if added to a class?

Answer 23

It prevents you from creating instances of this class

Question 24

What does the abstract keyword do if added to a method?

Answer 24

It forces you to implement this method, when creating a subclass.

Question 25

When can you use the abstract keyword on methods? How do you define these methods?

Answer 25

You can only use it on methods in abstract classes. You don’t define them at all. abstract exampleMethod(). NO {}

Question 26

What keyword do you add to a class signature to signify that it uses an interface?

Answer 26

implements InterfaceNameHere

Question 27

What are the two differences between an interface and an abstract class?

Answer 27

an interface cannot have attributes, an abstract class can.
an interface cannot have implementations for methods, an abstract class can.

Question 28

Why would we use an interface and not an abstract class?

Answer 28

A class can only inherit from one other class, (so if it already inherits something else, it can’t inherit an extra abstract class), but they can implement as many interfaces as you’d like.

Question 29

What keyword is “invisibly” added to all methods in an interface?

Answer 29

abstract, (YOU DON”T ADD THIS YOURSELF!!!)