Difference between revisions of "Java/C3/Polymorphism/English"

• About Polymorphism in Java
• Run-time polymorphism
• Virtual Method Invocation
• Compile-time polymorphism

Software Requirements

• Ubuntu Linux Version 12.04
• JDK 1.7
• Eclipse 4.3.1

Prerequisites

• Java and Eclipse IDE.
• You must have knowledge of Subclassing & method overriding and overloading

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What is polymorphism?

The major advantages of Polymorphism are:

1. Reduction of complexity &

2. Code re-usability

Compile-time and Run-time polymorphism.

It is also called Static Binding.

Run-time polymorphism is essentially referred as method overriding.

It is also called Dynamic Binding.

I have already created a project named MyProject in the previous tutorial.

Manager class is the subclass.

Manager class contains an additional variable department.

We are calling the getDetails() method by Manager class object i.e. Manager.

Highlight department

Save and run the program.

So we can see department variable value in the output.

Therefore subclass method is invoked at runtime.

Therefore it is known as Runtime polymorphism or method overriding.

Save the file

Save the file

Uncomment the line manager.setName(“Nikkita Dinesh”);

Uncomment the value instance,

manager.setName(“Nikkita Dinesh”);

We uncommented this instance as we have uncommented the method setName() in Empolyee class

Employee emp1 = new Employee();

Type,

Employee emp1 = new Employee();

emp1.setName("Jayesh");

emp1.setEmail("jayesh@gmail.com");

Type,

emp1.setName("Jayesh");

emp1.setEmail("pqr@gmail.com");

System.out.println("Details of Employee class:" + "\n" + emp1.getDetails());

System.out.println("Details of Employee class:" + "\n" + emp1.getDetails());

Employee emp2 = new Manager();

Employee emp2 = new Manager();

We are able to do this because,

In Java, all objects are polymorphic since any object will pass the IS-A test for their own type and for the class Object.

• A Manager IS-A Employee
• A Manager IS-A Manager
• A Manager IS-A Object

Reference variables like emp1, emp2 and manager.

Here, we instantiated two Manager objects:

One which references Employee class.

Other references Manager class.

emp.setName("Ankita");

emp.setEmail("ankita@gmail.com");

Type,

emp2.setName("Ankita");

emp2.setEmail(“xyz@gmail.com”);

emp2.setDepartment(“IT”);

Highlight error,

The method setDepartment(String) is undefined for the type Employee

The method setDepartment(String) is undefined for the type Employee

emp.setDepartment("IT");

emp.setDepartment("IT");

System.out.println("Details of Manager class:" + "\n" + emp.getDetails());

System.out.println("Details of Manager class:" + "\n" + emp2.getDetails());

This is because we have not initialized department in Manager class using emp2 object .

public String department="IT";

So, go to Manager class and initialize the value for department.

Details of Manager class :

Name: Ankita

Email: ankita@gmail.com

Manager of:

Details of Manager class:

Name: Jayesh

Email: jayesh@gmail.com

Details of Manager class:

Name: Nikkita Dinesh

Email: abc@gmail.com

Manager of: Accounts

Employee object referring Employee class,

Manager object referring Employee class &

Manager object referring Manager class,

But when emp2 tried calling setDepartment, we got an error.

The reason for that is as follows:

So, it does not throw an error and validates the code.

At run time, however, the JVM invokes getDetails() in the Manager class.

As getDetails() of Manager class overrides getDetails() of Employee class.

So we get the output as per getDetails() of Manager class.

But the compiler does not see the setDepartment method in the Employee class.

Therefore, it raises an error in case of setDepartment call by emp2.

System.out.println("Details of Employee class:" + "\n" + emp1.getDetails());

The compiler references Employee class for getDetails() during emp1.getDetails().

At run time, the JVM invokes getDetails() in the Employee class.

So we get the output as per getDetails() of Employee class.

Thus JVM calls the appropriate method for the object that is referred to in each variable

This behavior is referred to as Virtual Method Invocation.

The methods are referred to as Virtual Methods.

Let us quickly know Compile time polymorphism in brief.

Compile time polymorphism is nothing but method overloading in java.

Class can have more than one methods.

The methods have same name but with different number of arguments.

Compiler is able to figure out the method call at compile-time.

That’s the reason it is known as compile time polymorphism.

Summary

• What is Polymorphism in Java?
• Run-time polymorphism
• Compile-time polymorphism
• Virtual Method Invocation

Assignment

Override methods for Vehicle and Bike class which we used in previous tutorials.

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