Polymorphism

Polymorphism means one method or object behaving differently in different situations.

It helps in:

Writing flexible code
Managing large applications easily

In simple terms:
One form, many behaviors

Types of Polymorphism

Compile-Time Polymorphism (Static Polymorphism)
Runtime Polymorphism (Dynamic Polymorphism)

1. Compile-Time Polymorphism

Achieved using Method Overloading

Method Overloading

Method overloading means:

Same method name
Different parameters

Rules:

Method name must be same
Parameters must be different
Return type can be same or different

java

class Soft {
    public static void main(String[] args) {
        A obj = new A();
        obj.m1();
        obj.m1(10);
        obj.m1(10, "Soft");
    }
}

class A {
    void m1() {
        System.out.println("m1()");
    }

    void m1(int a) {
        System.out.println("m1(int)");
    }

    void m1(int a, String s) {
        System.out.println("m1(int, String)");
    }
}

Execution depends on method parameters at compile time

2. Runtime Polymorphism

Achieved using:

Dynamic Method Dispatch
Method Overriding

Dynamic Method Dispatch

It means:

A parent class reference can hold child class object

java

class A {}

class B extends A {}

class Soft {
    public static void main(String[] args) {
        A obj = new B(); // valid
    }
}

Important Cases

A obj = new B(); → Valid
B obj = new A(); → Compile-time error
Casting wrong object → Runtime error

Method Overriding

Method overriding means:

A child class provides a new implementation of a parent class method

Rules:

Method name must be same
Parameters must be same
Return type must be same
Access modifier can be same or more open

java

class A {
    void show() {
        System.out.println("A class method");
    }
}

class B extends A {
    void show() {
        System.out.println("B class method");
    }
}

class Soft {
    public static void main(String[] args) {
        A obj = new B();
        obj.show(); // calls B method
    }
}

Key Point:

Method call is decided at runtime

java

class University {
    void generateId() {}
}

class Student extends University {
    void generateId() {
        System.out.println("Student ID starts with S");
    }
}

class Teacher extends University {
    void generateId() {
        System.out.println("Teacher ID starts with T");
    }
}

class Admin extends University {
    void generateId() {
        System.out.println("Admin ID starts with A");
    }
}

class Soft {
    public static void main(String[] args) {
        University u;

        u = new Student();
        u.generateId();

        u = new Teacher();
        u.generateId();

        u = new Admin();
        u.generateId();
    }
}

Why this is important?

Same method generateId()
Different outputs
Clean and scalable code