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impossible to create a Factory Method in Java?

5 replies on 1 page. Most recent reply: Nov 6, 2002 12:17 PM by Anil Philip

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Anil Philip

Posts: 19
Nickname: anilphilip
Registered: Nov, 2002

impossible to create a Factory Method in Java?

Posted: Nov 4, 2002 12:48 PM

I would like to create a factory method as described in Design Patterns [Gang Of Four 95] page 107. This seems to be impossible to implement in Java?

I created an interface that has a static method eg.

public interface Task {
public static Task create();
public void doWork();
}

of course, the compiler complains. So I try to convert it into an abstract class - no luck again :(

But I really do need to have the interface so that concrete subclasses are forced to implement the factory method and also doWork().
And I really do need to have the method as static so that
it can function as a factory method.

Any way out?

Anil Philip

Posts: 19
Nickname: anilphilip
Registered: Nov, 2002

Re: impossible to create a Factory Method in Java?

Posted: Nov 4, 2002 12:54 PM

Of course, another reason to keep the interface is - the instantiator must be ignorant of the concrete subclass.

Anil Philip

Posts: 19
Nickname: anilphilip
Registered: Nov, 2002

Re: impossible to create a Factory Method in Java?

Posted: Nov 4, 2002 2:02 PM

Sorry, I intended to mean the factory method paradigm in C++ where
Task t = Task::create();

and create() can return ConcreteTask.

I am wondering if the solution is to call clone()
eg.
interface Task {
public void doWork();
}

assuming that tt is an instance of LogTask that implements Task,
Task t = tt.clone();

but then I wonder if t.doWork() will run Task.doWork() or LogTask.doWork()?
Also the original intent of a factory method is lost.
thanks,
Anil.

Anil Philip

Posts: 19
Nickname: anilphilip
Registered: Nov, 2002

one solution for Factory Method problem

Posted: Nov 5, 2002 11:17 AM

/*
* Task.java
*
* Created on November 5, 2002, 12:52 PM
*/

/**
*
* @author aphili01
*/
public abstract class Task {

protected abstract Object clone() throws java.lang.CloneNotSupportedException;

public abstract void doWork();

public Task makeCopy() throws java.lang.CloneNotSupportedException { return (Task) clone();};

}
**
*
* @author aphili01
*/
public class Task1 extends Task {

/** Creates a new instance of Task1 */
public Task1() {
}
protected Object clone() throws java.lang.CloneNotSupportedException {
return new Task1();
}

public void doWork() {
System.out.println("Task1.doWork()");
}

public static void main(String[] args) throws java.lang.CloneNotSupportedException {
Task1 t = new Task1();
Task tt = t.makeCopy();
tt.doWork();
}
}

Joe Cheng

Posts: 65
Nickname: jcheng
Registered: Oct, 2002

Re: impossible to create a Factory Method in Java?

Posted: Nov 6, 2002 9:38 AM

Hi Anil,

I don't think the factory method is supposed to be implemented as a static. Then you lose the whole advantage of the factory method pattern, i.e., let polymorphism choose the correct implementation of the factory method. Polymorphism doesn't work with static methods.

What you want instead is an instance method on some other object, for example, if you have some notion of a TaskManager. If there isn't a natural place for that instance method to live, then, well... are you sure you want a Factory Method? Maybe you just want a Factory.

-jmc

Anil Philip

Posts: 19
Nickname: anilphilip
Registered: Nov, 2002

Re: impossible to create a Factory Method in Java?

Posted: Nov 6, 2002 12:17 PM

> Hi Anil,
>
> I don't think the factory method is supposed to be
> implemented as a static. Then you lose the whole
> advantage of the factory method pattern, i.e., let
> polymorphism choose the correct implementation of the
> factory method. Polymorphism doesn't work with static
> methods.
>
> What you want instead is an instance method on some other
> object, for example, if you have some notion of a
> TaskManager. If there isn't a natural place for that
> instance method to live, then, well... are you sure you
> want a Factory Method? Maybe you just want a Factory.
>
> -jmc

Yes, I have a TaskManager server that accepts requests via socket and performs tasks (rather like a simple webserver). However I require that new kinds of tasks "new services" be painlessly added to the task server (like in the Interceptor pattern). They just need to register themselves for callback, and to implement a task interface. Task Manager should be able to invoke doWork() on a task and if there are fewer task objects than requests, to clone a task for the pool.

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