Java is an Object Oriented Programming language i.e. it supports four major pillars of OOPS.

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1 Java Polymorphism Java is an Object Oriented Programming language i.e. it supports four major pillars of OOPS. 1. Abstraction 2. Encapsulation 3. Inheritance 4. Polymorphism Polymorphism : In simple terms polymorphism means 'ability to take more than one forms'. This feature of Java allows us to perform single action using different approaches. We have a class 'Animal' and it has method 'makesound()' as this is a generic class we cannot provide implementation like : Roar, Neigh, Bark etc. We had to give a generic message. <p> public class Animal{</p> <div>...</div> <div> public void makesound(){</div> <div> System.out.println("Animal is making a sound"); </div> <div> }</div> <div>}</div> Now let's say we have two classes Lion and Dog, which extend Animal class, here we can provide implementation for 'makesound()' method in both the classes(method Overriding).

2 <p>public class Lion extends Animal{</p> <div>...</div> <div> public void makesound(){</div> <div> System.out.println(" Roar ");</div> <div> }</div> <div>}</div> <div> </div> <div>public class Dog extends Animal{</div> <div>...</div> <div> public void makesound(){</div> <div> System.out.println(" Bark ");</div> <div> }</div> <div>}</div> Here we can see that 'makesound()' method is same but it's implementation is different in each class, so we are performing single action using different approaches.

3 Types of Polymorphism : There are two types of polymorphism in Java. 1. Compile Time Polymorphism (Static Polymorphism) : e.g. Method Overloading 2. Runtime Polymorphism (Dynamic Polymorphism) : e.g. Method Overriding 1. Compile Time Polymorphism : As its title suggests Static/Compile Time polymorphism gets resolved at compile time. Method Overloading : In method overloading we can have more than one methods with same name but different signature i.e. different parameter list (data type of parameter, number of parameters, order of parameters) in same class. Let's say we have to perform same kind of operation with different ways i.e. for different inputs, sometimes it becomes really difficult to find meaningful names for each method so here method overloading becomes useful. a. Different types of parameters : Consider we have ' Calculate' class in which we want to implement two methods. In first method we want to add 2 integers and in second one we want to add 2 double, so here we can have same method names and just change type of parameters for each method.

4 <p> class Calculation{</p> <div> // adding two int values </div> <div><span style="white-space:pre"> </span>public int add(int a, int b){</div> <div> </div> <div> int sum = a+b;</div> <div> return sum;</div> <div> }</div> <div> </div> <div> // adding two double values.</div> <div> public double add(double a, double b){</div> <div> </div> <div> double sum = a+b;</div> <div> return sum;</div> <div> }</div> <div> </div> <div>}</div> <div> </div> <div>class DemoOverloading {</div> <div> public static void main (String[] args) {</div> <div> </div> <div> <span style="whitespace:pre"> </span>calculation obj = new Calculation();</div> <div> </div> <div> int sum1 = obj.add(4,2);</div> <div> System.out.println("Addition of the two integers is :" + sum1);</div> <div> double sum2 = obj.add(3.4,1.4);</div> <div> System.out.println("Addition of the two integers is :" + sum2);</div> <div> </div> <div> }</div> <div>}</div>

5 b. Number of Parameters : Consider we have ' Calculate' class in which we want to implement two methods. In first method we want to add 2 integers and in second one we want to add 3 integers, so here we can have same method names and just change number of parameters for each method.

6 <p> class Calculation{</p> <div> </div> <div> // adding two integer values.</div> <div> public int add(int a, int b){</div> <div> </div> <div> int sum = a+b;</div> <div> return sum;</div> <div> }</div> <div> </div> <div> // adding three integer values.</div> <div> public int add(int a, int b, int c){</div> <div> </div> <div> int sum = a+b+c;</div> <div> return sum;</div> <div> }</div> <div> </div> <div>}</div> <div> </div> <div>class DemoOverloading {</div> <div> public static void main (String[] args) {</div> <div> </div> <div> <span style="white-space:pre"> </span>calculation obj = new Calculation();</div> <div> </div> <div> int sum1 = obj.add(4,2);</div> <div> System.out.println("Addition of the two integers is :" + sum1);</div> <div> int sum2 = obj.add(5,1,4);</div> <div> System.out.println("Addition of the three integers is :" + sum2);</div> <div> </div> <div> }</div> <div>}</div>

7 c. Order of parameter : We can also overload methods by changing order of parameters. Take a look at following code. Here we in class 'Test' we are printing Name and ID passed from Object 'obj' in method 'register'. For method overloading we have just changed order of parameters, take a look at the parameters. But still it will print the desired output.

8 <p> class Test{<span style="white-space:pre"> </span></p> <div><span style="white-space:pre"> </span>public void register(string name, int id){</div> <div> </div> <div> System.out.println("Name:"+ name +" "+"Id:"+ id);</div> <div> }</div> <div> //overloaded method with change in</div> <div> //parameter order</div> <div> public void register(int id, String name){</div> <div> </div> <div> System.out.println("Name:"+ name +" "+"Id :"+ id);</div> <div> }</div> <div> </div> <div>}</div> <div> </div> <div>class DemoOverloading {</div> <div><span style="white-space:pre"> </span> public static void main (String[] args) {</div> <div> </div> <div><span style="white-space:pre"> </span> Test obj = new Test();</div> <div><span style="white-space:pre"> </span> </div> <div><span style="whitespace:pre"> </span> obj.register("john", 101);</div> <div><span style="white-space:pre"> </span> obj.register("sarah", 102);</div> <div><span style="whitespace:pre"> </span> </div> <div><span style="white-space:pre"> </span> }</div> <div>}</div> <div> </div>

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10 2. Runtime Polymorphism : In runtime polymorphism call to an overridden method is resolved at runtime. When superclass reference calls a method which is overridden, it is the JVM who decides which method to call (either superclass or subclass) based on the type of object that was referred at the time of call. JVM uses concept of Upcasting to resolve calls made to overridden methods at run time. Upcasting : Process of storing reference of subclass object inside superclass reference variable is called as upcasting. Consider we have superclass as 'Person' and its subclass as 'Employee'. Person person = new Employee; //upcasting Process of converting reference variable of subclass into reference variable of superclass is called as upcasting. Employee employee = new Employee //ref var of subclass Person person = employee //upcasting Therefore, if a 'Person' contains a method that is overridden by a 'Employee', then when different types of objects are referred to through a 'Person' class's reference variable, different versions of the method are executed. In java runtime polymorphism can be achieved through Method Overriding. Method Overriding : The process of reimplementing the methods of superclass inside subclass is known as Method Overriding. In simple words subclass can implement same method of superclass having same name. Rules for Method Overriding : 1. Name of the methods must be same as that of superclass method. 2. Number of parameters and data types of parameters must be same. 3. Access modifiers of subclass method should be same or wider than superclass access modifiers. 4. Exception list in subclass method should be same or it should be subset of exception list specified in superclass method. Example of Method Overriding

11 <p> //Demo Method Overriding (Java)</p> <div> </div> <div>class Person</div> <div>{</div> <div> void show() { System.out.println("Person's show()"); }</div> <div>}</div> <div> </div> <div>// Inherited class</div> <div>class Employee extends Person</div> <div>{</div> <div> // This method overrides show() of Person</div> <div> void show() { System.out.println("Employee's show()"); }</div> <div>}</div> <div> </div> <div>// Driver class</div> <div>class DemoOverriding</div> <div>{</div> <div> public static void main(string[] args)</div> <div> {</div> <div> // If a Person type reference refers</div> <div> // to a Person object, then Person's</div> <div> // show is called</div> <div> <span style="white-space:pre"> </span>person obj1 = new Person();</div> <div> obj1.show();</div> <div> </div> <div> // If a Person type reference refers</div> <div> // to a Employee object Employee's show()</div> <div> // is called. This is called RUN TIME</div> <div> // POLYMORPHISM.</div> <div> Person obj2 = new Employee();</div> <div> obj2.show();</div> <div> }</div> <div>}</div>

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