Lecture 2

Question 1

What is a generalised class?

Answer 1

A super class that is general:
Car

Question 2

What is a specialised class?

Answer 2

Subclass of an abstract class:
(super) Car
(sub) ElectricCar and GasCar

Question 3

What is polymorphism?

Answer 3

When there are one or more classes or objects related to each other by inheritance (a task that performs a single action in different ways).
You make some specialisation of a generalisation.

Question 4

Give an example of polymorphism.

Answer 4

Say we have a super class Car that has the subclasses GasolineCar, ElectricCar etc. Let’s say we have another super class called CarGarage that has the method repair(Car). This means that this single action, repair(Car), will repair any subclass inherited from the super class Car.

Question 5

What does method overriding do?

Answer 5

It will change up the method implementation of the super class, but it will keep the method signature. When calling a super method, you say:
super.drive();
// alternative method implementation

Question 6

What is method overloading?

Answer 6

It provides additional method implementation, and will change the method signature. Methods of the same name need to have different parameters: drive(Direction direction) and drive(Location location)

Question 7

What visibility modifier are attributes usually assigned?

Answer 7

(-) Private (few cases where (+) public and (#) protected make sense)

Question 8

What visibility modifier are methods usually assigned?

Answer 8

(+) Public (internal use can either be private or protected)

Question 9

What does super do?

Answer 9

Super accesses ancestor

Question 10

What should you do, if you want to grant external access to private attributes?

Answer 10

Define getter and
setter methods.

Question 11

What is a getter method?

Answer 11

Provides read access of private or protected attributes.

Question 12

What is a setter method?

Answer 12

Provides write access of private or protected attributes.

Question 13

How do you make a getter method?

Answer 13

public int getAge() {
return age;
}

Question 14

When to use a setter?

Answer 14

Use a setter when you want to change a value (more than once)

Question 15

How do you make a setter method?

Answer 15

public void setAge(int age) {
this.age = age;
}

Question 16

What is enum?

Answer 16

Represent a fixed set of elements

Question 17

When should a constructor be used over setters?

Answer 17

When the attribute is not likely to change. (like breed/color)

Question 18

What is an abstract class?

Answer 18

A class that can’t be instantiated, because it doesn’t make logical sense. E.g. Pet has to always be either a Cat or a Dog, never just a Pet.

Question 19

What is an abstract method?

Answer 19

A method that doesn’t have an implementation yet, so the subclasses have to implement the method themselves

Question 20

How do you make a constructor?

Answer 20

public Animal(String breed, String color) {
this.breed = breed;
this.color = color;
}

Question 21

What is a concrete class?

Answer 21

The opposite of an abstract class. A “normal” class/default class

Question 22

Is it possible to have multiple constructors in one class?

Answer 22

Yes

Question 23

What is a static type?

Answer 23

Something that is known during development.
Pet somePet = new Dog(…);
PET IS STATIC

Question 24

What is a dynamic type?

Answer 24

Something that is known during execution.
Pet somePet = new Dog(…);
DOG IS DYNAMIC

Question 25

What are generics in collections?

Answer 25

Already made method to create List arrays, Integer arrays, etc.
E.g.: List<Pet> allPets = new ArrayList<Pet>();</Pet></Pet>

Question 26

What are the behaviours of generic collections?

Answer 26

If you insert a wrong element type to a list, it will get rejected.
You know all the elements in the list is of the same type.
If no generic argument is specified, Object is assumed, and all types are treated as Obejcts.

Question 27

What does “instanceof” do?

Answer 27

Checks whether object is an instance of the specified type.
if (pet instanceof Dog) {
//…
}

Question 28

What are lists good for?

Answer 28

Dynamically changing things, easy to access

Question 29

What are arrays good for?

Answer 29

Fixed number of values required, and when speed of access is more important than convenience of access

Question 30

What does classes in OOP do?

Answer 30

Combine coherent data and operations on that data

Question 31

What does abstraction mean?

Answer 31

Focusing on essentials and leaving out unnecessary details

Question 32

What does encapsulation mean?

Answer 32

Protects data and operations from unwanted external access.

Question 33

What does inheritance mean

Answer 33

Concept for code reuse as common data and operations can be shared
by multiple classes.

Question 34

What does “this” keyword mean?

Answer 34

Refers to the current instance of the class it is used in (“super()” if used in subclass)

Question 35

What does “super” keyword mean?

Answer 35

Used to call a previous method from the parent/super class, when a previous method has been overwritten.

Question 36

What is an interface?

Answer 36

Specify how a class’ public methods should look like (signature) but has no implementation. Defines a contract specifying a set of method signatures that classes must implement, enabling multiple inheritance of behavior.

Question 37

Difference between interface and abstract class?

Answer 37

Abstract class’ can have concrete methods or abstract methods that the subclasses must implement

Interface will contain methods that the class’ must implement themselves.

Question 38

What is the advantage of interfaces?

Answer 38

They have multiple inheritance. Makes sure you don’t forget to implement any methods in the implemented class.

Question 39

What does an interface look like?

Answer 39

// Interface
interface Animal {
public void animalSound(); // interface method (does not have a body)
}

// Pig “implements” the Animal interface
class Pig implements Animal {
public void animalSound() {
// The body of animalSound() is provided here
System.out.println(“The pig says: wee wee”);
}

Question 40

How does inheritance work for classes?

Answer 40

They have single inheritance, meaning a subclass can only have one parent

Question 41

What is a constructor?

Answer 41

A special method that is used to initialize objects (do not have a return type, only done once and are immutable). The constructor is called when the object is created.

All classes have constructors by default: if you do not create a class constructor yourself, Java creates one for you. However, then you are not able to set initial values for object attributes.

Question 42

Can software ever be done?

Answer 42

No, software is never “done”, change is a reality of life

Question 43

What is software development?

Answer 43

Software development is iterative and incremental