OOP

Question 1

What are the pillars of OOP?

Answer 1

1) Encapsulation - is distilling a concept down to a discrete set of data and behaviors.
2) Inheritance - is a form of code reuse. When a type extends another type, it incorporates all non-private super-class fields and methods as members.
3) Polymorphism - When one or more subclasses override a super-class method or implement an interface, their behavior is said to by polymorphic. All types have the same method signature, but the outcome differs between types.
4) Abstraction - is the principle of revealing only the essential characteristics of a type.

Question 2

What is the relationship between a class and object?

Answer 2

In Java, the definitions for object state and behavior are stored in a class. A class is a contract. It specifies the data that makes up each object instance. It defines the behaviors allowed.

In other words, a class is a template for an object. An object is an instance of a class.

Question 3

Access modifiers (public, protected, private) support which pillar?

Answer 3

Encapsulation

Question 4

What is inheritance? How does it work?

Answer 4

Inheritance is a code reuse strategy. Through inheritance, a child class gains access to a parent class’s fields and methods. It inherits a parent’s data and behavior. That means the code to create the data and behavior is only stored once, in the parent. When a child extends a parent, it reuses the parent’s code.

Question 5

Describe a syntax or keyword that supports another pillar.

Answer 5

A class encapsulates code that solves a problem.

Question 6

I’ve heard the phrase, “prefer composition over inheritance.” What does it mean? Is it right?

Answer 6

With composition, it’s easy to change behavior on the fly with Dependency Injection / Setters. Inheritance is more rigid as most languages do not allow you to derive from more than one type.

Question 7

How do the access modifiers (public, protected, private) interact with inheritance?

Question 8

How do I call a parent class’s constructor from a child class?

Answer 8

Through inheritcance.

Question 9

Tell me about the keyword: this. What does it mean? What does it do?

Answer 9

The this keyword accesses class members. A member is any field or method declared inside the class code block.

Question 10

What is the maximum number of classes you can extend (inherit)?

Answer 10

In Java, we can only extend a single class.

Question 11

What is the dot operator?

Answer 11

The (.) operator is also known as member operator it is used to access the member of a package or a class.

Question 12

Can one class both extend a parent and implement an interface?

Question 13

What is the maximum number of interfaces you can implement?

Answer 13

As many as needed.

Question 14

What is state? How does OOP manage it?

Question 15

What is a constructor?

Answer 15

A constructor is a method-like subroutine that ensures an object is created in a valid state. It ensures an object has the data it needs to operate.

Constructors can call other constructors by using the this keyword: this(argument, anotherAgrument);

Question 16

Explain the difference between the 3 access modifiers.

Answer 16

public - All classes can access the member.

protected - Only classes in the same package or a subclass can access the member.

private - The member is only visible from inside the class containing the definition. No outside classes may access it.

Question 17

What is the keyword for inheritance?

Answer 17

Extends

Question 18

What is an interface?

Answer 18

An interface enforces a contract for classes that implement it. It does not contain implementation. An interface specifies behavior without the need to create an explicit method.

Question 19

What is object instantiation?

Answer 19

It is the process of providing the data needed, if any, to code that constructs an object.

The new operator creates an object.

Question 20

Java, the definitions for object state and behavior are stored in what?

Answer 20

A class

Question 21

What is a class? Provide an example.

Answer 21

A class is a contract/blueprint. It specifies the data that makes up each object instance. It defines the behaviors allowed.

Ex: A blueprint is instructions for building a house, but it isn’t a house. It’s just specifications for a house. When a house is built using the blueprint, it’s an object produced from that blueprint.

Question 22

What data type is a class?

Answer 22

A class is a composite data type. A composite data type is a type made of other data types. For example, we could create a Pet data type by combining two strings: one for the pet’s name and one for its kind (dog, cat, ferret, etc).

The Pet class serves as a template for individual pets. It defines what’s possible. From the class, we can create instances of a pet.

Question 23

What is the difference between static and non-static methods?

Answer 23

Non-static methods have access to fields. Static methods don’t. One way to think about it is that static methods are just independent methods with no context.

Non-static methods can’t be executed independently. They’re part of an object. They have access to an object’s state.

Question 24

Static vs. Non-Static: What type of methods are executed using an instance of an object?

Answer 24

Non-static

Question 25

Static vs. Non-Static: What type of methods are executed using a class name?

Answer 25

Static methods

Question 26

What is polymorphism?

Answer 26

When two classes have identical method signatures but different behaviors, they exhibit polymorphism.

In plain English, it’s when an object of one type executes a method and an object of a different type executes the same method (or at least the same signature) and the behavior is different.

Question 27

What is composition?

Answer 27

Composition is a way to model relationships by including complex types as fields in other types.

Question 28

What is the keyword for interfaces?

Answer 28

Implements

Question 29

True or False?: Interfaces are Java data types.

Answer 29

True

Question 30

When should you use an interface?

Answer 30

Rule #1: Use an interface when there are two or more concrete implementations that solve one problem. For example, Visa, Discover, and American Express resolve payments in completely different ways. Their implementations are different. But their payment resolution contract is consistent (or should be). We can create a PaymentProvider interface that establishes the contract.

Rule #2: Use an interface when one concrete implementation solves a problem but there’s a possibility of different implementations. For example, we might create a game that’s initially limited to human players. If there’s a possibility of an AI player in the future, add a Player interface to establish the player contract, implement the interface in HumanPlayer, and leave the door open for other concrete Player implementations in the future.

Rule #3: Use an interface to control testing.