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© 2006-2013 Andrew Cooke (site) / post authors (content).

Generics + Dynamic = bad

From: "andrew cooke" <andrew@...>

Date: Fri, 3 Mar 2006 10:39:28 -0300 (CLST)

This Java ugliness turned up at work, but I don't see how to improve it.

We have are using a messaging system (Mule) that can make routing
decisions based on the message type (messages are POJOs).  Which is cool. 
We also have a need for a generic message:

public Message<E> {
  private E payload;
  public E getPayload() {return payload;}
  public void setPayload(E payoad) {this.paylod = payload;}
}

But Java erases generic types.  So this is sent to the messaging system as
a plain old Message.  Which means we can't route on the payload.  So we
end up writing a subclass:

public StringMessage exetends Message<String> {
  public String getString() {return getPayload();}
  public void setString(String string) {setPayload(string);}
}

Yuck.

Actually, writing that, I wonder what the point of the extra setters is. 
You already inherit a correctly typed getPaload and setPayload.  So it
seems to me that, apart from the slightly less informatitve method names,
the following is equivalent:

public StringMessage extends Message<String> {}

And that raises the question - why didn't Java do this to implement
generics?  It's what C++ templates do, and while there are problems,
they're not insurmountable, AFAIK.

Andrew

Talking of POJOs

From: "andrew cooke" <andrew@...>

Date: Fri, 3 Mar 2006 10:46:08 -0300 (CLST)

Etymology - http://www.martinfowler.com/bliki/POJO.html

Via Lambda / Category Theoreticians

From: "andrew cooke" <andrew@...>

Date: Tue, 14 Mar 2006 07:28:11 -0400 (CLT)

I forgot to add that the link above was found via Lambda, with the usual
"Java programmers are therfore idiots" snark.

Which is ironic give that the place is full of people who worship a theory
best introduced as:

  Well, you know, there's stuff, right, and you do shit to it and, well
  that gives you more stuff.

  [sounds of amusement/disbelief]

  No really, its cool, because the shit can be, like, anything...

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