RE: Digester and inheritance

[Galbreath, Mark]

It appears from your listings and descriptions that Component is basically a
JavaBean and your helper classes should have "has a" relationships with it,
not "is a."  Essentially, what I am suggestion is that you wrap Component in
the helper classes.  Wrappers are far more functional than subclasses (also
known as the "decorator" pattern) and ensure that adding methods to
Component will never affect the helpers (loose coupling), making ongoing
maintenance easier.

IMHO, implementation (class) inheritance is way overused in Java (even
within the Java API).

As for Digester, it is indifferent to the relationships between your
classes; I am merely suggesting a refactoring of your code.

Mark

-----Original Message-----
From: Stacy Weng [mailto:[EMAIL PROTECTED]]
Sent: Friday, March 01, 2002 3:13 PM
To: Struts Users Mailing List
Subject: Re: Digester and inheritance


Wouldn't that be a "has-a" relationship?  If I do that, than I would end up
with this (since originally I have many child classes extending from
Component):

public class Component {
    public String name = null;
    public String getName() {
        return this.name;
    }
    public void setName(String name) {
        this.name=name;
    }
}

public class HelperClass1 {
    private final Component c;
    public String getName() {
        return c.getName;
    }
    public void setName(String name) {
        c.setName(name);
    }
}

public class HelperClass2 {
    private final Component c;
    public String getName() {
        return c.getName;
    }
    public void setName(String name) {
        c.setName(name);
    }
}

public class HelperClass3 {
    private final Component c;
    public String getName() {
        return c.getName;
    }
    public void setName(String name) {
        c.setName(name);
    }
}

The reason I'm extending from Component is because these helper classes "is
- a" Component.  They only have slight variations.  Does digester not handle
"is-a" relationships?

--Stacy
ps - Thanks again for taking the time to respond.  I can't stress how much I
appreciate it.

  ----- Original Message ----- 
  From: Galbreath, Mark 
  To: 'Struts Users Mailing List' 
  Sent: Friday, March 01, 2002 2:40 PM
  Subject: RE: Digester and inheritance


  You are misusing inheritance.  Instead of extending Component, give
  HelperClass (was ChildClass) a private field that references an instance
of
  Component (a design pattern known as "composition").Each instance method
in
  HelperClass invokes the corresponding method on the contained instance of
  Component and returns the results (a design pattern known as
"forwarding"):

  public class HelperClass {
      private final Component c;
      public void setName( String name) {
          c.setName( name);
      }
      public String getName() {
          return c.getName();
      }
  }

  Cheers!
  Mark

  -----Original Message-----
  From: Stacy Weng [mailto:[EMAIL PROTECTED]]
  Sent: Friday, March 01, 2002 2:12 PM

  I have a parent class called Component, and several child classes
extending
  from it.   All the common get/set methods are defined in Component.  So,
  after each child is getting created from addObjectCreate() method, I call
  addSetProperties() method to initialize their values (values are from the
  xml file), however, digester can't find the set methods from the child
  class, cause they're defined in the parent class (Component).  The
temporary
  solution I had was defining those set methods in the child classes, and
have
  them call the super methods.  But that seems to defeat the whole purpose
of
  inheritance.  Here's an example with the problem:

  public class Component {
      public String name = null;
      public void setName(String name) {
          this.name=name;
      }
      public void getname() {
          return this.name;
      }
  }

  public class ChildClass extend Component {
      //Other methods that differ from parent class
  }

  for the digester rules:

  digester.addObjectCreate("ChildClassPattern", "some.package.ChildClass");
  digester.addSetProperties("ChildClassPattern");

  so, as it gets to addSetProperties(), it doesn't see any of the get/set
  methods in ChildClass, and yet, it doesn't know to look its parent class,
  Component.

  I hope I'm able to get my problem across.  Thanks again for taking the
time
  to respond.

  --Stacy



  ----- Original Message ----- 
    From: Fernando Esteban Barril Otero 
    To: Struts Users Mailing List 
    Sent: Friday, March 01, 2002 1:48 PM
    Subject: Re: Digester and inheritance


    I'm using digester and inheritance without problems. What kind
    of exception are you getting?

    Fernando

    ----- Original Message -----
    From: "Stacy Weng" <[EMAIL PROTECTED]>
    To: "Struts Users Mailing List" <[EMAIL PROTECTED]>
    Sent: Friday, March 01, 2002 1:20 PM
    Subject: Digester and inheritance


    Has anyone encountered problems with inheritance using Digester?  It
seems
    to me that the addSetProperties() does not when its object is inherited
  from
    a parent class...hence it assumes the get/set methods aren't defined,
when
    they are actually defined in its parent class.  I've looked into the
  source,
    and it seems that the getMethod call that digester uses does not handle
    parent methods.  Is there a patch out there for inheritance with
digester?

    Once again, thanks again in advance for any input.  It is greatly
    appreciated.  =)

    --Stacy



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