overriding a concrete method by abstract method

[shakunthala Divakar]

Hi Javaranchers
The below quetion is from a Mock exam
Which of the following statements is true?

a. An abstract method can not be overridden by an abstract method.
b. An instance method that is not abstract can not be overridden by an abstract method.
c. An abstract method declaration can not include a throws clause.
d. The body of an abstract method is represented by a set of empty brackets.
e. None of the above

Answer Given is e.None of the above.
but i expected b.
And i tried with some code to test

  class Superclass{ protected  void print(){}}
 
abstract class Subclass extends Superclass
{ 
protected void print();//1
}
  

the above shows error at line 1 saying,This metthod requires body instead of ;
Then the anwer given in mock exam is wrong.....am i right???

thanks
Shakunthala Divakar

[Neha Bhattad]

What about option a?
>> a. An abstract method can not be overridden by an abstract method.

I thought that was true. Can someone please explain why not?

[shakunthala Divakar]

Hi javaranchers
I am sorry.The code i was trying was wrong.
i have missed abtract modifier for print method.
The statement
An instance method that is not abstract can not be overridden by an abstract method...is right

[shakunthala Divakar]

sorry again
The statement is An instance method that is not abstract can not be overridden by an abstract method...is NOT right.
Hi Neha Bhattad ,
i think even option a. is not true so the given anwer in mock exam is right.
Try the below code

 abstract class Superclass{ abstract protected void print(int a);}
 
abstract class Subclass extends Superclass
{ 
abstract protected void print(int a);
}   

Thanks
shakunthala Divakar

[Neha Bhattad]

Shakunthala,

In your code:

abstract class Superclass{ abstract protected void print(int a);}
 
abstract class Subclass extends Superclass
{ 
abstract protected void print(int a);
}   

The abstract method in Subclass is the same as the one in the superclass...is it really overridden?? To me it looks like the method is plain inherited.

[Burkhard Hassel]

Howdy,

yes, it compiles. It even compiles with an annotation:

abstract class Superclass{
abstract protected void print(int a);
}
 
 
 
abstract class Subclass extends Superclass {
@Override
abstract protected void print(int a);
}

But is this really an override?


Bu.

[Satya Maheshwari]

Hi Bu

To me method "print" doesn't seem to be overridden as neither of them have an implementation. Hence there is no behavior in the base class "print" which can be overridden in the sub class. My doubt is, if this is not overriding then what it actually is ? Is it something like base and sub classes having same named private methods which is again not a case of overriding.

[Neha Bhattad]

From Sun Java

>> An abstract class can override an abstract method by providing another abstract method declaration.

Consider the case of an abstract method in an interface being overriden by an implementing abstract class.

public interface Buffer {
char get() throws BufferEmpty, BufferError;
}
public abstract class InfiniteBuffer implements Buffer {
abstract char get() throws BufferError;
}

>>An instance method that is not abstract can be overridden by an abstract method.

abstract class Point {
int x, y;
public abstract String toString();
}
class ColoredPoint extends Point {
int color;
public String toString() {
return super.toString() + ": color " + color; // error
}
}

[Neha Bhattad]

Forgot to mention the second code:
>> results in a compile-time error because the invocation super.toString() refers to method toString in class Point, which is abstract and therefore cannot be invoked.

[Neha Bhattad]

Hi Bu,

So is that truly an override? Does using the @Override change anything?

Thanks
Neha.