Object Oriented Programming

Question 1

How to automate the creation of Getter and Setter?

Answer 1

IntelliJ –> Code –> Generate

Question 2

Constructors

Answer 2

for you to initialize the object that you are creating and do whatever else you wanna do at the time the object is being created.

• a special method that is used to initialize objects
• it is called when an object of a class is created
• it can be used to set initial values for object attributes
• it must have the same name as the class within which it is defined

Question 3

Constructor Chaining

Answer 3

• calling one constructor from another constructor
• it can be done using this() keyword for constructors in the same class
• you can also use super() keyword to call constructors from the base class

Question 4

Encapsulation

Answer 4

• refers to the bundling of data, along with the methods that operate on the data, into a single unit
• may also refer to a mechanism of restricting the direct access to some components of an object
• many programming languages use encapsulation frequently in the form of classes

Question 5

What are the benefits of encapsulation?

Answer 5

• Data hiding
• Increased Flexibility
• Reusability (Easy to reuse)
• Testing code is easy

Question 6

Reference variable

Answer 6

• reference type is used to point objects/values
• classes, interfaces, arrays, enumerators, and annotations are reference types in Java.
• reference variable can also store null values. By default, if no object is passed to a reference variable then it will store a null value
• You can access object members using a reference variable and dot syntax

Question 7

Class

Answer 7

xxx

Question 8

Object / Instance

Answer 8

xxx

Question 9

super keyword

Answer 9

• used to access/call the parent class members (variables and methods)
• can’t be used in static areas (the static block or a static method)
• Any attempt to do so will lead to compile-time errors
• is commonly used with method overriding

Question 10

this keyword

Answer 10

• used to access/call the current class members (variables and methods)
• this is required when we have a parameter with the same name as an instance variable (field)
• can’t be used in static areas (the static block or a static method)
• Any attempt to do so will lead to compile-time errors
• commonly used with constructors and setters

Question 11

Method Overriding

Answer 11

• if subclass (child class) has the same method as declared in the parent class
• in other words, if a subclass provides the specific implementation of the method that has been declared by one of its parent classes, it is known as method overriding.

Rules for Java Method Overriding

• the method must have the same name as in the parent class
• the method must have the same parameter as in the parent class
• there must be an IS-A relationship (inheritance)

Question 12

recursive call

Answer 12

the method would call itself forever until the memory is full

Question 13

What are the differences between a static method and an instance method?

Answer 13

Static Methods

• static methods are methods in Java that can be called without actually creating an object of that class
• it becomes part of the class (a class method: a method that belongs to a class rather than to an object)
• they are designed with the aim to be shared with all objects created within that class
• static methods can’t be overridden but can be overloaded

Instance Methods

• instance methods are methods that require an object of its class to be created before it can be called
• instance methods belong to the object of the class and not the class
• they can be overridden and overloaded

Question 14

How to pass objects as arguments?

Answer 14

code example

public double distance(Point obj) {
       int x = obj.getX();
       int y = obj.getY();
       return Math.sqrt((this.x - x) * (this.x - x) + (this.y - y) * (this.y - y));
   }

Question 15

Inheritance

Answer 15

• one class is allowed to inherit the features (fields and methods) of another class
• the keyword used for inheritance is extends
• the class whose features are inherited is known as superclass (base class or parent class)
• the class that inherits the other class is known as a subclass ( or a derived class, extended class, or child class)
• the subclass can add its own fields and methods in addition to the superclass fields and methods

Question 16

Composition

Answer 16

• a way to design or implement the “has-a” relationship
• the relationship is possible when one object contains another object and that object is fully dependent on it
• in composition, we use an instance variable that refers to another object
• to ensure the code reusability in the program

Question 17

Polymorphism

Answer 17

• same name, different behavior
• the ability of an object to take on many forms
• it allows us to perform a single action in different behavior in different situations
• the most common use of it in OOP occurs when a parent class reference is used to refer to a child class object

Question 18

What are the four basic concepts of OOP?

Answer 18

• Inheritance
• Abstraction
• Polymorphism
• Encapsulation

Question 19

getSimpleName()

Answer 19

• the method returns the simple name of this class in the form of String
• used to get the simple name of this class, as given in the source code

Question 20

static field vs. non-static field

Answer 20

static field

• a static field belongs to the class
• dont’ need an instance of that class to access its static field

non-static field

• non-static fields are located in the instances of the class.
• each instance of the class can have its own values for these fields
• you need to have an instance of the class from which you can access it