[newbie] Can an abstract method have a signature?

[Wes Harrison]

I am getting an error about abstract methods not being allowed to have a
body but all I am doing is declaring the method's signature in an abstract
class like:

public abstract String format(String s, int i)
{
}


Wes

 

[ShadowMan]

I am getting an error about abstract methods not being allowed to
have a body but all I am doing is declaring the method's signature in
an abstract class like:

public abstract String format(String s, int i)
{
}

the right way should be without braces (empty braces ARE braces! ;-) )

 

[Olivier Merigon]

I am getting an error about abstract methods not being allowed to have a
body but all I am doing is declaring the method's signature in an abstract
class like:

public abstract String format(String s, int i)
{
}


Wes

Declare your method like this:

public abstract String format(String s, int i);

 

[jAnO!]

I am getting an error about abstract methods not being allowed to have a
body but all I am doing is declaring the method's signature in an abstract
class like:

public abstract String format(String s, int i)
{
}

No, you're also implementing the method.
Try to remove the body... hint {}... replace them for a ;

 

[Oliver Fels]

I am getting an error about abstract methods not being allowed to have a
body but all I am doing is declaring the method's signature in an abstract
class like:

public abstract String format(String s, int i)
{
}

No abstract bodies are allowed.
It is good practice to declare the abstract method in an interface:
public String format(String s, int i);

and let your (abstract) class implement it so you don't have to care much
about it.

The advantage: You can even implement a generic implementation in your
(abstract) class being overriden by children if necessary so you have a
fallback method for children which do not need the method themselves and
handle it in a generic way thus avoiding the NoSuchMethodException.

Oliver

 

[Cid]

No abstract bodies are allowed.
It is good practice to declare the abstract method in an interface:
public String format(String s, int i);

and let your (abstract) class implement it so you don't have to care much
about it.

When a method is declared abstract, it must not have a body and the
class containing it must also be declared abstract.

If the class is declared abstract but none of its methods are
abstract, then they must have bodies. But the class itself cannot be
instantiated. This presumably is what Oliver is referring to. (?)

 

[Oliver Fels]

Cid wrote:

When a method is declared abstract, it must not have a body and the
class containing it must also be declared abstract.

If the class is declared abstract but none of its methods are
abstract, then they must have bodies. But the class itself cannot be
instantiated. This presumably is what Oliver is referring to. (?)

Right.
To make it clear, two examples:

public abstract class A
{
public void methodA {...}; //not abstract
public abstract void methodB();
}


public interface B
{
public void methodB(); //abstract interface declaration
public void methodC(); //abstract interface declaration
}

public abstract class C implements B
{
public void methodB {...}; //not abstract and implements as default
behavior
}


In class C methodB is implemented with a default behavior which can be
overridden by children with a class specific implementation if required.
methodC *must* be implemented.
I mostly use this approach as it is flexible, though it requires more
discipline as forgetting the concrete chield implementation of methodB is
not immediately noticed at compile time..

Oliver