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Calling a method

 
Ranch Hand
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Hi!
class Base {}
class Agg extends Base{
public String getFields(){
String name = "Agg";
return name;
}
}
public class Avf{
public static void main(String argv[]){
Base a = new Agg();
//Here
}
}
What code placed after the comment //Here will result in calling the getFields method resulting in the output of the string "Agg"?
1) System.out.println(a.getFields());
2) System.out.println(a.name);
3) System.out.println((Base) a.getFields());
4) System.out.println( ((Agg) a).getFields());
The correct answer is 4, why is not 1??
Thank in advance.
 
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I don't think 1) will compile. Since a is of type Base, the compiler will look in class Base for the method, but will not find it.
 
mister krabs
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That is correct.
a is an object of type Base and Base does not have a getFields() method.
 
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Can you explain the difference between
a.getFields() and
( (Agg) a.getFields() )
In this programm, both method calls work, but whats the difference?
class Base
{
public String getFields(){ String name = "Base"; return name; }
}
class Agg extends Base
{
public String getFields(){ String name = "Agg"; return name; }
}
public class Avf{
public static void main(String argv[]){
Base a = new Agg();
//Here
System.out.println( a.getFields());
//System.out.println( ((Agg) a).getFields()); Works, too
}
}
Result: "Agg" is printed
 
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Hello Thomas,
The code posted by Jordi will not compile as the there is no method named getFields() defined in the class Base.It will compile if we cast the reference variable a for the object of class Agg like ((Agg) a).getFields().

It will make you understand dynamic binding or polymorphism much much better.
If you remove getFields() from Base class, you actually take getFields() out of the polymorphism between Base and Agg class. Compiler only know b is refer to a Base class, which does not have the method getFields(), compile time error!!!
Remember Agg ISA Base. Compiler does not care which child/grandchild/grandgrandchild down the hierarchy b is actually referring to, and which getFields() it should call. However, they all have an getFields() (polymorphism). It is a runtime decision. Dynamic Binding.
Dynamic binding, or binding at runtime only for those methods defined in Base class, either inherited or overrided by the subclass. The override can be already happened now or will happen in the future, the compiler does not care.

I hope it will help.
Gurpreet Sachdeva
Please visit for all type of info. about SCJP (Including mock exams) at: http://www.go4java.20m.com
 
Thomas Paul
mister krabs
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In Thomas' example, there is no difference between these two:
a.getFields() and
( (Agg) a.getFields() )
They will both run the Agg getFields() method.
------------------
Tom - SCJP --- Co-Moderator of the Programmer Certification Forums
 
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