Oop in kotlin

Object-Oriented Programming is a paradigm that provides many concepts, such as inheritance, data
binding, polymorphism, etc.
Simula is considered the first object-oriented programming language. The programming paradigm
where everything is represented as an object is known as a truly object-oriented programming
language.
Smalltalk is considered the first truly object-oriented programming language.
The popular object-oriented languages are Java, C#, PHP, Python, C++, etc.
The main aim of object-oriented programming is to implement real-world entities, for example,
object, classes, abstraction, inheritance, polymorphism, etc.

Variable , Constant, Method , Class

what is car?!!
what you understand when i say car?
What is the components of the car?
so car it is a general concept of some
topic.
This means car is the class.

object
An Object can be defined as an instance of a
class. An object contains an address and takes
up some space in memory. Objects can
communicate without knowing the details of
each other's data or code. The only necessary
thing is the type of message accepted and the
type of response returned by the objects.

class
Collection of objects is called class.
A class can also be defined as a
blueprint from which you can create an
individual object. Class doesn't
consume any space.

Object
Pen is an object.
Its name is
Reynolds; color is
white, known as
its state. It is used
to write, so writing
is its behavior.

Class
A class is a group of objects
which have common
properties. It is a template or
blueprint from which objects
are created. It is a logical
entity. It can't be physical.
A class in Java can contain:
● Fields
● Methods
● Constructors
● Blocks
● Nested class and
interface

OBJECT
3 Ways to initialize object
There are 3 ways to initialize object in Java.
1. By reference variable
2. By method
3. By constructor

Method Declaration
The method declaration provides information about method attributes, such as visibility,
return-type, name, and arguments. It has six components that are known as method header,
as we have shown in the following figure.

NAMING IN METHODS
While defining a method, remember that the method name must be a verb and start with a lowercase letter. If the method
name has more than two words, the first name must be a verb followed by adjective or noun. In the multi-word method name,
the first letter of each word must be in uppercase except the first word. For example:
Single-word method name: sum(), area()
Multi-word method name: areaOfCircle(), stringComparision()
It is also possible that a method has the same name as another method name in the same class, it is known as method
overloading.

Method Signature: Every method has a method signature. It is a part of the method declaration. It includes the method
name and parameter list.
Access Specifier: Access specifier or modifier is the access type of the method. It specifies the visibility of the method. Java
provides four types of access specifier:
● Public: The method is accessible by all classes when we use public specifier in our application.
● Private: When we use a private access specifier, the method is accessible only in the classes in which it is defined.
● Protected: When we use protected access specifier, the method is accessible within the same package or subclasses in
a different package.
● Default: When we do not use any access specifier in the method declaration, Java uses default access specifier by
default. It is visible only from the same package only.

Return Type: Return type is a data type that the method returns. It may have a primitive data type, object, collection, void,
etc. If the method does not return anything, we use void keyword.
Method Name: It is a unique name that is used to define the name of a method. It must be corresponding to the functionality
of the method. Suppose, if we are creating a method for subtraction of two numbers, the method name must be subtraction().
A method is invoked by its name.
Parameter List: It is the list of parameters separated by a comma and enclosed in the pair of parentheses. It contains the data
type and variable name. If the method has no parameter, left the parentheses blank.
Method Body: It is a part of the method declaration. It contains all the actions to be performed. It is enclosed within the pair of
curly braces.

PRE-Defined Method
In Java, predefined methods are the method that is already defined in the Java class libraries is known as predefined methods.
It is also known as the standard library method or built-in method. We can directly use these methods just by calling them
in the program at any point. Some pre-defined methods are length(), equals(), compareTo(), sqrt(), etc. When we call any
of the predefined methods in our program, a series of codes related to the corresponding method runs in the background that is
already stored in the library.
Each and every predefined method is defined inside a class. Such as print() method is defined in the java.io.PrintStream
class. It prints the statement that we write inside the method. For example, print("Java"), it prints Java on the console.

public class Demo
{
public static void main(String[] args)
{
// using the max() method of Math class
System.out.print("The maximum number is: "
+ Math.max(9,7));
}
}
PRE-Defined Method

STATIC METHOD
A method that has static keyword is known as static method. In other words, a method that belongs to a class rather than an
instance of a class is known as a static method. We can also create a static method by using the keyword static before the
method name.
The main advantage of a static method is that we can call it without creating an object. It can access static data members and
also change the value of it. It is used to create an instance method. It is invoked by using the class name. The best example of
a static method is the main() method.

Oop in kotlin

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