* Hybrid inheritance
This is the combination of multiple and multilevel inheritances and in java, multiple inheritances are not supported as it leads to ambiguity and this type of inheritance can only be achieved through interfaces.

Consider that class a is the parental or base class of class b and class c and in turn, class b and class c are parental or a base class of class d. Class b and class c are derived classes from class a and class d is derived class from class b and class c.

The following program creates a superclass called add and a subclass called sub, using extend keyword to create a subclass add.

// a simple example of inheritance
// create a superclass
class Add {
int my;
int by;
void setmyby (int xy, int hy) {
my=xy;
by=hy;
}
}
// create a sub class
class b extends add {
int total;
void sum() {
public Static void main(String args[]) {
b sub0b = new b();
sub0b.Setmyby(10,12);
sub0b.Sum();
System.out.println("total = " + sub0b.Total);
}
}

What is Polymorphism ?
Polymorphism refers to many forms, or it is a process that performs a single action in different ways. It occurs when we have many classes related to each other by inheritance. Polymorphism is of two different types, i.e., compile-time polymorphism and runtime polymorphism. One of the examples of Compile time polymorphism is that when we overload a static method in java. Run time polymorphism also called a dynamic method dispatch is a method in which a call to an overridden method is resolved at run time rather than compile time. In this method, the overridden method is always called through the reference variable. By using method overloading and method overriding, we can perform polymorphism. Generally, the concept of polymorphism is often expressed as one interface, and multiple methods. This reduces complexity by allowing the same interface to be used as a general class of action.

public class Bird {
..
Public void sound() {
System.out.println("birds sounds");
}
}
public class pigeon extends Bird {
..
@Override
public void sound() {
System.out.println("cooing");
}
}
public class sparrow extends Bird () {
..
@override
Public void sound() {
System.out.println("chip");
}
}

In the above example, we can see common action sound () but there are different ways to do the same action. This is one of the examples which shows polymorphism.

Polymorphism in java can be classified into two types:

1. Static / Compile-Time Polymorphism
2. Dynamic / Runtime Polymorphism

What is Compiler - Time Polymorphism in Java ?
Compile-Time polymorphism in java is also known as Static Polymorphism. to resolved at compile-time which is achieved through the Method Overloading.

What is Runtime Polymorphism in Java ?
Runtime polymorphism in java is also known as Dynamic Binding which is used to call an overridden method that is resolved dynamically at runtime rather than at compile time.

What is Encapsulation ?
Encapsulation is one of the concepts in OOPs concepts; it is the process that binds together the data and code into a single unit and keeps both from being safe from outside interference and misuse. In this process, the data is hidden from other classes and can be accessed only through the current class’s methods. Hence, it is also known as data hiding. Encapsulation acts as a protective wrapper that prevents the code and data from being accessed by outsiders. These are controlled through a well-defined interface.

Encapsulation is achieved by declaring the variables as private and providing public setter and getter methods to modify and view the variable values. In encapsulation, the fields of a class are made read-only or write-only. This method also improves reusability. Encapsulated code is also easy to test for unit testing.

class animal {
// private field
private int age'
// getter method
Public int gerage() {
return age;
}
// setter method
public void setAge (int age) {
this.Age = age;
}
}
class Main {
public static void main(String args[]);
// create an object of person
Animal a1= new Animal();
// change age using setter
A1.setAge(12);
//access age using getter
System.out.println("animal age is " + a1.getage());
}
}

In this example, we declared a private field called age that cannot be accessed outside of the class.

To access age, we used public methods. These methods are called getter and setter methods. Making age private allows us to restrict unauthorized access from outside the class. Hence this is called data hiding.

Coupling in Java
Coupling refers to the relationship between two classes. It indicates the knowledge one object or class has of another. That means that if one class changes its properties or behaviour, it will affect the dependent changes in the other class. Therefore, these changes will depend upon the level of interdependence the two classes have between them. There are two types of coupling, namely tight coupling, and loose coupling.

* Tight coupling: If one class is strongly interrelated to another class, it is said to have a tight coupling with that class.

public class College{
public void status() {
System.out.println("College is open today");
}
}
public class Student{
College obj = new College();
public void goToCollege() {
obj.status();
}
}

In the above code example, the student class is dependent on the college class. That is, any change in the college class requires student classes to change. Here, therefore, student class and college class are tightly coupled with each other.

* Loose coupling: If one class is weakly interrelated to another class, it is said to have loose coupling with that class. Loose coupling is preferred over tight coupling. A class can achieve this with the help of interfaces, as shown below.

public interface College {
void status();
}
class CollegeStatus1 implements College{
public void status() {
System.out.println("College is open monday to friday");
}
}
class CollegStatus2 implements College{
public void status() {
System.out.println("College is open in saturday");
}
}
public class Student{
College obj = new CollegeStatus1();
public void goToCollege() {
obj.status();
}
}

In the above code example, CollegeStatus1 and CollegeStatus2 are loosely coupled. Here, student class is not directly or tightly coupled with a CollegeStatus1 or CollegeStatus2 class. By applying a dependency injection mechanism, the loose coupling implementation is achieved to allow a student to go to college with any class which has implemented a college interface. In addition, it means we can use CollegeStatus2 whenever the college is open on Saturday.

Cohesion In Java
Java Cohesion measures how the methods and the attributes of a class are meaningfully and strongly related to each other and how focused they are on performing a single well-defined task for the system. This is used to indicate the degree to which a class has a single, well-focused responsibility. More cohesive classes are good to keep them for code reusability. Low cohesive classes are difficult to maintain as they have a less logical relationship between their methods and properties. It is always better to have highly cohesive classes to keep them well focused for a single work.

* Low Cohesion: In the following code, we have a class called Book. But it is less cohesive because it comprises less focussed and independent attributes and methods to the class. This class should contain information related to the Book. Therefore, the person’s name and age method are making this classless cohesive.

class Book {
int price = 299; // related attribute
String name = "Sam"; // unrelated attribute
// related methods to Book class
public String author(String name) {
return name;
}
public String title(String subject) {
return subject;
}
public int id(int number) {
return number;
}
// unrelated methods to Book class
public int age(int age) {
return age;
}
}

*High Cohesion: When the class has a single well-defined purpose or task, it is said to be highly cohesive. So, in the above example code, if we remove the information related to the person, then the class becomes highly cohesive, as shown below.

class Book {
int price = 299; // related attribute
// related methods to Book class
public String author(String name) {
return name;
}
public String title(String subject) {
return subject;
}
public int id(int number) {
return number;
}
}

Association in Java
Association is a relation between two separate classes that establishes with the help of their Objects. It specifies the relationship between two or more Objects. Association can be one-to-one, one-to-many, many-to-one, and many-to-many. Let us understand this with real-world examples, suppose the relationship between the bus and the passengers. A bus can have only one driver(one-to-one). Many passengers can associate with the single bus(many-to-one). A single passenger can associate with many different buses(one-to-many). Also, many passengers can associate with the many different buses(many-to-many). One object is associated with another object to use the functionality and services provided by another object.