regarding inheritance

[debasmita pattnayak]

hi all,
please someone expain me the following code snippet:

 class Animal{
   void method() throws Exception{}
   Animal(Srting name){
      System.out.println(name);
   } 
   class Dog extends Animal{
     void method() throws Exception{}
   }
   class Test{
      public static void main(String args[]){
         new Animal("Girrafe");
      }
   }
      

According to me it should print "Girrafe". please clarify if i am correct or not?
thanks in advance
[ July 23, 2007: Message edited by: debasmita pattnayak ]

[Ulf Dittmer]

That's easy to test, isn't it?

What I mean by that is the code contains a problem that will prevent it from compiling, but once you fix that, it will run. (Actually, it contains 3 problems, but 2 of those are just trivial typos.)

[debasmita pattnayak]

hey ulf,
to my surprise i am not able to find any errors so that the code doesnt compile..
please explain....

thanks for your support...

[Ulf Dittmer]

Well, the code you posted is missing a closing "}" before "class Dog", and it references something called "Srting". Those are the trivial problems. Once you fix those, the Dog class still won't compile. Can you tell us why that is?

[Ramu Valivarthi]

Yes,the Dog class won't compile..you need write a parameterized super() inside the Dog method.

class Animal{
void method() throws Exception{}
Animal(String name){
System.out.println(name);
}
}
class Dog extends Animal{
Dog(){
super("Gf");
}
void method() throws Exception{}
}
class Test1{
public static void main(String args[]){
new Animal("Girafee");
}
}

[Robert Horvath]

Hi Depasmita!

This is indeed a tricky question. After eliminating the typos already mentioned by Ulf you get the following three classes as a starting point:

class Animal{
   void method() throws Exception{}
      Animal(String name){
         System.out.println(name);
   }   
}
 
class Dog extends Animal{
   void method() throws Exception{}
}
   
class Test{
   public static void main(String args[]){
      new Animal("Giraffe");
   }
}

The code doesn't compile, because you've implemented a constructor with an non-empty argument list in Animal, but Dog requires a constructor with no arguments. Just to be clear: The compiler adds a Dog()-constructor along with a super()-call when Dog is compiled. This super()-call needs an Animal()-construtor, but it doesn't exist!! Note: the compiler adds a non-argument-constructor at compile-time only if there is no other constructor implemented.


Following two alternatives to solve the problem (class Test always stays unchanged):
1. Implement a constructor in Dog that explicitly calls the Animal(String)-constructor:

class Animal{ // UNCHANGED
   void method() throws Exception{}
      Animal(String name){
         System.out.println(name);
   }
}
 
class Dog extends Animal{
   void method() throws Exception{}
      Dog(String name) {
         super(name);
      }
}

Note: don't forget super(name), since otherwise the compiler would add a super() call - and Animal() doesn't exist!


2. Implement an non-argument constructor in Animal

class Animal{
   void method() throws Exception{}
      Animal(String name){
         System.out.println(name);
   }

   Animal() {}
}
 
class Dog extends Animal{ // UNCHANGED
   void method() throws Exception{}
}

Animal() will be invoked in case you instantiate Dog or if you execute
new Animal();


Cheers,
Robert