Polymorphism in Java

Polymorphism in Java

The process of representing one form in multiple forms is known as Polymorphism.

Polymorphism is derived from 2 greek words: poly and morphs. The word "poly" means many and "morphs" means forms. So polymorphism means many forms.

Polymorphism is not a programming concept but it is one of the principal of OOPs. For many objects oriented programming language polymorphism principle is common but whose implementations are varying from one objects oriented programming language to another object oriented programming language.

Real life example of polymorphism

Suppose if you are in class room that time you behave like a student, when you are in market at that time you behave like a customer, when you at your home at that time you behave like a son or daughter, Here one person present in different-different behaviors.

How to achieve Polymorphism in Java ?

In java programming the Polymorphism principal is implemented with method overriding concept of java.

Polymorphism principal is divided into two sub principal they are:

• Static or Compile time polymorphism
• Dynamic or Runtime polymorphism

Note: Java programming does not support static polymorphism because of its limitations and java always supports dynamic polymorphism.

Let us consider the following diagram

Here original form or original method always resides in base class and multiple forms represents overridden method which resides in derived classes.

In the above diagram the sum method which is present in BC class is called original form and the sum() method which are present in DC1 and DC2 are called overridden form hence Sum() method is originally available in only one form and it is further implemented in multiple forms. Hence Sum() method is one of the polymorphism method.

Example of runtime polymorphism.

In below example we create two class Person an Employee, Employee class extends Person class feature and override walk() method. We are calling the walk() method by the reference variable of Parent class. Since it refers to the subclass object and subclass method overrides the Parent class method, subclass method is invoked at runtime. Here method invocation is determined by the JVM not compiler, So it is known as runtime polymorphism.

Example of Polymorphism in Java

class Person
{
void walk()
{
System.out.println("Can Run....");
}
}
class Employee extends Person
{
void walk()
{
System.out.println("Running Fast...");
}
public static void main(String arg[])
{
Person p=new Employee(); //upcasting
p.walk();
}
}

Output

Running fast...

Dynamic Binding

Dynamic binding always says create an object of base class but do not create the object of derived classes. Dynamic binding principal is always used for executing polymorphic applications.

The process of binding appropriate versions (overridden method) of derived classes which are inherited from base class with base class object is known as dynamic binding.

Advantages of dynamic binding along with polymorphism with method overriding are.

• Less memory space
• Less execution time
• More performance

Static polymorphism

The process of binding the overloaded method within object at compile time is known as Static polymorphism due to static polymorphism utilization of resources (main memory space) is poor because for each and every overloaded method a memory space is created at compile time when it binds with an object. In C++ environment the above problem can be solve by using dynamic polymorphism by implementing with virtual and pure virtual function so most of the C++ developer in real worlds follows only dynamic polymorphism.

Dynamic polymorphism

In dynamic polymorphism method of the program binds with an object at runtime the advantage of dynamic polymorphism is allocating the memory space for the method (either for overloaded method or for override method) at run time.

Conclusion

The advantage of dynamic polymorphism is effective utilization of the resources, so java always use dynamic polymorphism. Java does not support static polymorphism because of its limitation.

Encapsulation

Encapsulation in Java

Encapsulation is a process of wrapping of data and methods in a single unit is called encapsulation. Encapsulation is achieved in java language by class concept.

Combining of state and behavior in a single container is known as encapsulation. In java language encapsulation can be achieve using class keyword, state represents declaration of variables on attributes and behavior represents operations in terms of method.

Advantage of Encapsulation

The main advantage of using of encapsulation is to secure the data from other methods, when we make a data private then these data only use within the class, but these data not accessible outside the class.

Real life example of Encapsulation

The common example of encapsulation is capsule. In capsule all medicine are encapsulated in side capsule.

Benefits of encapsulation

• Provides abstraction between an object and its clients.
• Protects an object from unwanted access by clients.
• Example: A bank application forbids (restrict) a client to change an Account's balance.

Let's see the Example of Encapsulation in java

Example

class Employee
{  
private String name;  
   
public String getName()
{  
return name;  
}  
public void setName(String name){  
this.name=name;
}  
}  

class Demo
{  
public static void main(String[] args)
{  
Employee e=new Employee();  
e.setName("Harry");  
System.out.println(e.getName());  
}  
}  

Output

Harry

Abstraction In Java

Abstraction in Java

Abstraction is the concept of exposing only the required essential characteristics and behavior with respect to a context.

Hiding of data is known as data abstraction. In object oriented programming language this is implemented automatically while writing the code in the form of class and object.

Real Life Example of Abstraction

Abstraction shows only important things to the user and hides the internal details for example when we ride a bike, we only know about how to ride bike but can not know about how it work ? and also we do not know internal functionality of bike.

Note: Data abstraction can be used to provide security for the data from the unauthorized methods.

Note: In java language data abstraction can be achieve using class.

Example of Abstraction

class Customer
{
int account_no;
float balance_Amt;
String name;
int age;
String address;
void balance_inquiry()
{
/* to perform balance inquiry only account number
is required that means remaining properties 
are hidden for balance inquiry method */
}
void fund_Transfer()
{
/* To transfer the fund account number and 
balance is required and remaining properties 
are hidden for fund transfer method */
}

How to achieve Abstraction ?

There are two ways to achieve abstraction in java

• Abstract class (0 to 100%)
• Interface (Achieve 100% abstraction)

Read more about Interface and Abstract class in previous section.

Difference between Encapsulation and Abstraction

Encapsulation is not provides fully security because we can access private member of the class using reflation API, but in case of Abstraction we can't access static, abstract data member of class.

Interface

Interface in Java

Interface is similar to class which is collection of public static final variables (constants) and abstract methods.

The interface is a mechanism to achieve fully abstraction in java. There can be only abstract methods in the interface. It is used to achieve fully abstraction and multiple inheritance in Java.

Why we use Interface ?

• It is used to achieve fully abstraction.
• By using Interface, you can achieve multiple inheritance in java.
• It can be used to achieve loose coupling.

properties of Interface

• It is implicitly abstract. So we no need to use the abstract keyword when declaring an interface.
• Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
• Methods in an interface are implicitly public.
• All the data members of interface are implicitly public static final.

How interface is similar to class ?

Whenever we compile any Interface program it generate .class file. That means the bytecode of an interface appears in a .class file.

How interface is different from class ?

• You can not instantiate an interface.
• It does not contain any constructors.
• All methods in an interface are abstract.
• Interface can not contain instance fields. Interface only contains public static final variables.
• Interface is can not extended by a class; it is implemented by a class.
• Interface can extend multiple interfaces. It means interface support multiple inheritance

Behavior of compiler with Interface program

In the above image when we compile any interface program, by default compiler added public static final before any variable and public abstract before any method. Because Interface is design for fulfill universal requirements and to achieve fully abstraction.

Declaring Interfaces:

The interface keyword is used to declare an interface.

Example

interface Person
{  
 datatype variablename=value;
 //Any number of final, static fields
 returntype methodname(list of parameters or no parameters)
 //Any number of abstract method declarations
}  

Explanations

In the above syntax Interface is a keyword interface name can be user defined name the default signature of variable is public static final and for method is public abstract. JVM will be added implicitly public static final before data members and public abstract before method.

Example

public static final datatype variable name=value; ----> for data member
public abstract returntype methodname(parameters)---> for method

Implementing Interfaces:

A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

Example

interface Person
{  
void run();
}  
class Employee implements Person
{
public void run()
{
System.out.println("Run fast");
} 
}

When we use abstract and when Interface

If we do not know about any things about implementation just we have requirement specification then we should be go for Interface

If we are talking about implementation but not completely (partially implemented) then we should be go for abstract

Rules for implementation interface

• A class can implement more than one interface at a time.
• A class can extend only one class, but implement many interfaces.
• An interface can extend another interface, similarly to the way that a class can extend another class.

Relationship between class and Interface

• Any class can extends another class

• Any Interface can extends another Interface.

• Any class can Implements another Interface

• Any Interface can not extend or Implements any class.

Difference between Abstract class and Interface

	Abstract class	Interface
1	It is collection of abstract method and concrete methods.	It is collection of abstract method.
2	There properties can be reused commonly in a specific application.	There properties commonly usable in any application of java environment.
3	It does not support multiple inheritance.	It support multiple inheritance.
4	Abstract class is preceded by abstract keyword.	It is preceded by Interface keyword.
5	Which may contain either variable or constants.	Which should contains only constants.
6	The default access specifier of abstract class methods are default.	There default access specifier of interface method are public.
7	These class properties can be reused in other class using extend keyword.	These properties can be reused in any other class using implements keyword.
8	Inside abstract class we can take constructor.	Inside interface we can not take any constructor.
9	For the abstract class there is no restriction like initialization of variable at the time of variable declaration.	For the interface it should be compulsory to initialization of variable at the time of variable declaration.
10	There are no any restriction for abstract class variable.	For the interface variable can not declare variable as private, protected, transient, volatile.
11	There are no any restriction for abstract class method modifier that means we can use any modifiers.	For the interface method can not declare method as strictfp, protected, static, native, private, final, synchronized.

Example of Interface

interface Person
{  
void run();  // abstract method
}  
class A implements Person
{  
public void run()
{
System.out.println("Run fast");
}   
public static void main(String args[])
 {  
 A obj = new A();  
 obj.run();  
 }  
}  

Output

Run fast

Multiple Inheritance using interface

Example

interface Developer
{  
void disp();
}  
interface Manager
{
void show();
} 

class Employee implements Developer, Manager
{
public void disp()
{
System.out.println("Hello Good Morning");  
}
public void show()
{
System.out.println("How are you ?");  
}
public static void main(String args[])
{
Employee obj=new Employee();
obj.disp();
obj.show();
}  
} 

Output

Hello Good Morning
How are you ?

Marker or tagged interface

An interface that have no member is known as marker or tagged interface. For example: Serializable, Cloneable, Remote etc. They are used to provide some essential information to the JVM so that JVM may perform some useful operation.

Example

//Way of writing Serializable interface  
public interface Serializable
{  
}  

Why interface have no constructor ?

Because, constructor are used for eliminate the default values by user defined values, but in case of interface all the data members are public static final that means all are constant so no need to eliminate these values.

Other reason because constructor is like a method and it is concrete method and interface does not have concrete method it have only abstract methods that's why interface have no constructor.

Overriding

Method Overriding in Java

Whenever same method name is existing in both base class and derived class with same types of parameters or same order of parameters is known as method Overriding. Here we will discuss about Overriding in Java.

Note: Without Inheritance method overriding is not possible.

Advantage of Java Method Overriding

• Method Overriding is used to provide specific implementation of a method that is already provided by its super class.
• Method Overriding is used for Runtime Polymorphism

Rules for Method Overriding

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

Understanding the problem without method overriding

Lets understand the problem that we may face in the program if we do not use method overriding.

Example Method Overriding in Java

class Walking
{  
void walk()
{
System.out.println("Man walking fastly");
}  
}  
class OverridingDemo
{  
public static void main(String args[])
{  
Man obj = new Man();  
obj.walk();  
}
}

Output

Man walking

Problem is that I have to provide a specific implementation of walk() method in subclass that is why we use method overriding.

Example of method overriding in Java

In this example, we have defined the walk method in the subclass as defined in the parent class but it has some specific implementation. The name and parameter of the method is same and there is IS-A relationship between the classes, so there is method overriding.

Example

class Walking
{  
void walk()
{
System.out.println("Man walking fastly");
}  
}  
class Man extends walking
{  
void walk()
{
System.out.println("Man walking slowly");
}
}

class OverridingDemo
{  
public static void main(String args[])
{  
Man obj = new Man();  
obj.walk();  
}
}

Output

Man walking slowly

Note: Whenever we are calling overridden method using derived class object reference the highest priority is given to current class (derived class). We can see in the above example high priority is derived class.

Note: super. (super dot) can be used to call base class overridden method in the derived class.

Accessing properties of base class with respect to derived class object

class A
{
int x;
void f1()
{
x=10;
System.out.println(x);
}
void f4()
{
System.out.println("this is f4()");
System.out.println("-----------------");
}
};
class B extends A
{
int y;
void f1()
{
int y=20;
System.out.println(y);
System.out.println("this is f1()");
System.out.println("------------------");
}
};
class C extends A
{
int z;
void f1()
{
z=10;
System.out.println(z);
System.out.println("this is f1()");
}
};
class Overide
{
public static void main(String[] args)
{
A a1=new B();
a1.f1();
a1.f4();
A c1=new C();
c1.f1();
c1.f4();
}
}

Example of Implement overriding concept

class Person
{
String name;
void sleep(String name)
{
this.name=name;
System.out.println(this.name +"is sleeping+8hr/day");
}
void walk()
{
System.out.println("this is walk()");
System.out.println("-----------------");
}
};
class Student extends Person
{
void writExams()
{
System.out.println("only student write the exam");
}
void sleep(String name)
{
super.name=name;
System.out.println(super.name +"is sleeping 6hr/day");
System.out.println("------------------");
}
};
class Developer extends Person
{
public void designProj()
{
System.out.println("Design the project");
}
void sleep(String name)
{
super.name=name;
System.out.println(super.name +"is sleeping 4hr/day");
System.out.println("------------------");
}
};
class OverideDemo
{
public static void main(String[] args)
{
Student s1=new Student();
s1.writExams();
s1.sleep("student");
s1.walk();
Developer d1=new Developer();
d1.designProj();
d1.sleep("developer");
}
}

Difference between Overloading and Overriding

	Overloading	Overriding
1	Whenever same method or Constructor is existing multiple times within a class either with different number of parameter or with different type of parameter or with different order of parameter is known as Overloading.	Whenever same method name is existing multiple time in both base and derived class with same number of parameter or same type of parameter or same order of parameters is known as Overriding.
2	Arguments of method must be different at least arguments.	Argument of method must be same including order.
3	Method signature must be different.	Method signature must be same.
4	Private, static and final methods can be overloaded.	Private, static and final methods can not be override.
5	Access modifiers point of view no restriction.	Access modifiers point of view not reduced scope of Access modifiers but increased.
6	Also known as compile time polymorphism or static polymorphism or early binding.	Also known as run time polymorphism or dynamic polymorphism or late binding.
7	Overloading can be exhibited both are method and constructor level.	Overriding can be exhibited only at method label.
8	The scope of overloading is within the class.	The scope of Overriding is base class and derived class.
9	Overloading can be done at both static and non-static methods.	Overriding can be done only at non-static method.
10	For overloading methods return type may or may not be same.	For overriding method return type should be same.

Note: In overloading we have to check only methods names (must be same) and arguments types (must be different) except these the remaining like return type access modifiers etc. are not required to check 
But in overriding every things check like method names arguments types return types access modifiers etc.