Polymorphism

[Raj Kumar Bindal]

The output of below shown code is:

test
Base constructor
der
Derived constructor
der
Baseprintsec
der

class base
{
       public void printMe()
       {
               System.out.println("Base");
       }
       public base()
       {
               System.out.println("Base constructor");
               printMe();              // Please explain this
       }
       public void printMesec()
       {
               System.out.println("Baseprintsec");
               printMe();
       }
}
class Derived extends base
{
       public void printMe()
       {
               System.out.println("der");
       }
       public Derived()
       {
               System.out.println("Derived constructor");
               printMe();
       }
}
public class Test {


       public static void main(String[] args) {
               System.out.println("test");
               base b = new Derived();
               b.printMesec();


       }

}

If i am calling printMe() from constructor of base class, it is still calling printMe() of derived. Even though there is no object of derived class till this line, still method of derived class is getting called.

Can anybody please clarify?

[Henry Wong]

Basically, polymorphism applies -- even before instantiation of the object has completed. Since the derived class overridden the base printme() method, it is always called.

Henry

[Raj Kumar Bindal]

I really doubt if polymorphism is applied before instance creation.

I thought first objects gets created and then there binding is done using polymorphism.

Any more thoughts?

[Abhi Deshmukh]

I am agreeing with Henry.
Have a look at below code snippet where we are using "this" explicitly to clear picture.

 
class base  
{  
	public void printMe()  
	{  
		System.out.println("Base");  
	}  
	public base()  
	{  
		System.out.println("Base constructor");  
		this.printMe();              // We are using this explicitly   
	}  
	public void printMesec()  
	{  
		System.out.println("Baseprintsec");  
		this.printMe();  
	}  
}   

And we know that within an instance method or a constructor, this is a reference to the current object (Derived object in our case) — the object whose method or constructor is being called. We can refer to any member of the current object from within an instance method or a constructor by using this.
Since printMe() method is overridden in Derived class it get executed.

[Raza Mohd]

if you call the derived class constructor . first it goes to the super class constructer if any.
once it get it executes the code thats what you are getting.
and then it runs its own constructor that is the derived constructor.Since the printMe() is a overridden method it will always be executed.

[Rob Spoor]

Raj Kumar Bindal wrote:I really doubt if polymorphism is applied before instance creation.

Really? Because everybody here, and your own example too, clearly state otherwise.

[YogeshS Pandit]

Greetings all. I've just joined coderanch and this is my first post. This is a good topic to understand polymorphism. But correct me if I am wrong, isn't it a bad idea to call overridable instance methods from a constructor, because of the reason mentioned by Henry?

Basically, polymorphism applies -- even before instantiation of the object has completed. Since the derived class overridden the base printme() method, it is always called.

As the overridden method in derived class can use an uninitialized instance variable.

[Rob Spoor]

YogeshS Pandit wrote:Greetings all. I've just joined coderanch and this is my first post. This is a good topic to understand polymorphism. But correct me if I am wrong, isn't it a bad idea to call overridable instance methods from a constructor, because of the reason mentioned by Henry?

Basically, polymorphism applies -- even before instantiation of the object has completed. Since the derived class overridden the base printme() method, it is always called.

As the overridden method in derived class can use an uninitialized instance variable.

Welcome to the Ranch! And you are completely right - it is very dangerous to do this. If you ever choose to do this you must document it so people implementing sub classes know that they cannot use any instance variables. Better is to avoid it altogether.

Note that Sun ignores both advices. They call several methods in constructors (for example setModel in several Swing components) without mentioning this.

[YogeshS Pandit]

Thanks for confirming Rob.

[Campbell Ritchie]

Rob Prime wrote: . . . Note that Sun ignores both advices. They call several methods in constructors (for example setModel in several Swing components) without mentioning this.

Was that before people realised what a bad idea it is to call methods from constructors?

[Rob Spoor]

If it was then people have not though things through early enough

[Campbell Ritchie]

There are lots of things hiding in the API which people obviously didn't think about in time.

[Neil Cartmell]

Campbell Ritchie wrote:

Rob Prime wrote: . . . Note that Sun ignores both advices. They call several methods in constructors (for example setModel in several Swing components) without mentioning this.

Was that before people realised what a bad idea it is to call methods from constructors?

Wait a minute. It's a bad idea to call methods from constructors? I've just started learning about how to make a GUI and the book i'm reading tells you to call LOADS of methods from the constructor of the JFrame class. Like all the add button methods. Is this something i should avoid?

[Seetharaman Venkatasamy]

Neil Cartmell wrote: It's a bad idea to call methods from constructors?

the method *which is designed for overriding* by subclass . otherwise, there is no harm to call a method[say static method] from constructor

[Neil Cartmell]

Seetharaman Venkatasamy wrote:

Neil Cartmell wrote: It's a bad idea to call methods from constructors?

the method *which is designed for overriding* by subclass . otherwise, there is no harm to call a method[say static method] from constructor

ok thanks!

[Seetharaman Venkatasamy]

Neil Cartmell wrote:ok thanks!

you are welcome

[Campbell Ritchie]

Seetharaman Venkatasamy wrote: . . . there is no harm to call a method[say static method] from constructor

Disagree. Probably best only to call private or final instance methods from a constructor.