🚀 go-pugleaf

"Carlos Zin" <nomailplease@hotmail.com> wrote in message 
news:41afa7d8$1@newsgroups.borland.com...
> Please suppose I have the following case where Tool is a class defined 
> somewhere else:
>
> public class ToolFactory {
>    private class A extends Tool {}
>    private class B extends Tool {}
>    public static Tool makeTool(String toolName) {
>        if (toolName == "A")
>            return new A();
>        else if (toolName == "B")
>            return new B();
>        else
>            return null;
>    }
> }
>
> I am trying to build a static factory method named "makeTool()" which 
> produces instances of classes A and B but the compiler complain telling 
> that non-static variable this cannot be referenced from a static context.
>
> Making the method non-static eliminate the error but then I need to make 
> an instance of Toolfactory in order to access method makeTool(s) which is 
> not what I want. I need to call "ToolFactory.makeTool(s)", i.e. a static 
> method with no object instance.
> How can I solve this problem?

You can make A and B static classes:

<code>
  private static class A extends Tool {}
  private static class B extends Tool {}
</code>

Otherwise, you must have a singleton instance of your ToolFactory class 
which provides the environment (ie class variables) which can be accessed by 
your subclasses, if desired.
viz:
<code>
public class ToolFactory
{
    private class A extends Tool {}
    private class B extends Tool {}

    // place class variables here, if desired,
    // and objects of A and B can access them.

    // static singleton instance
    private static ToolFactory factoryInstance = null;

    // private constructor enforces Singleton pattern
    private ToolFactory ()
    {
        // Initialize class variables here
    }

    public static Tool makeTool(String toolName)
   {
        // singleton auto-constructor
        if (factoryInstance ==  null)
            factoryInstance = new ToolFactory ();

        if (toolName == "A")
            return new factoryInstance.A();
        else if (toolName == "B")
            return new factoryInstance.B();
        else
            return null;
    }  // makeTool
} // ToolFactory
</code>


regards,
    Dar7yl