polymorphism.md

Polymorphism in Java

"One Interface, multiple methods"

There are two types of polymorphism in java -

• Compile time polymorphism (or Static polymorphism)
• Runtime polymorphism (or Dynamic Polymorphism)

We can perform polymorphism in java by

• Method overloading (Compile time polymorphism)
• Method overriding (Runtime polymorphism)

Overriding

An ability of a subclass to re-implement an instance method inherited from a superclass.

Rules for method overriding

1. Only inherited methods can be overridden. public, protected and default (in the same package) can be overridden and private methods cannot be overridden.
2. Final and static methods cannot be overridden.

• A final method means that it cannot be re-implemented by a subclass, thus it cannot be overridden. -
• A static method is available to all instances of the superclass and its subclasses, so it’s not permissible to re-implement the static method in a particular subclass.

3. The overriding method must have same argument list.
4. The overriding method must have same return type (or subtype).
5. The overriding method must not have more restrictive access modifier.
6. The overriding method must not throw new or broader checked exceptions.
7. Use the super keyword to invoke the overridden method from a subclass.
8. Constructors cannot be overridden. Because constructors are not methods and a subclass’ constructor cannot have same name as a superclass’ one, so there’s nothing relates between constructors and overriding.
9. Abstract methods must be overridden by the first concrete (non-abstract) subclass.
10. A static method in a subclass may hide another static one in a superclass, and that’s called hiding.

public class Animal {
    public static void testClassMethod() {
        System.out.println("The static method in Animal");
    }
}

class Dog extends Animal {
    public static void testClassMethod() {
        System.out.println("The static method in Dog");
    }
    
    public void DoSomething {
      testClassMethod(); // prints The static method in Dog
      Animal.testClassMethod(); // calls hidden method
    }
    }
}

11. The synchronized modifier has no effect on the rules of overriding.
12. The strictfp modifier(float point related) has no effect on the rules of overriding.