Accessing "this" in gen-class constructor
CuppoJava
I've hit a stumbling block using Clojure's gen-class facility for
constructors.
Is there anyway to access "this" inside Clojure's constructor/init
function?
ie. The following type of code is quite common in Java. How would you
do the same in Clojure?
public class MyDerivedClass extends SuperClass{ //<-- Derived Class
public MyDerivedClass (){
super("derived class"); //<-- Call superclass constructor
setDescription("this is a derived class"); //<-- Call inherited
method
}
}
Thanks
-Patrick
CuppoJava
limitation. I can first instantialize the object to get a reference,
and then initialize all it's settings. This works only if I don't
expect this class to be derived from. Any subclass would expect the
class to be fully initialized in the constructor, and not have to call
another initialize_settings() function.
Emeka
Emeka
CuppoJava
I'm having problems with the setDescription() line.
Thanks for your help
-Patrick
(ns test)
(gen-class
:name test.MyDerivedClass
:extends [SuperClass]
:init init
:constructors {[] [String]})
(defn -init []
;The following line doesn't work.
;I need to call setDescription on this object
;who's reference is not available yet.
(. this setDescription "this is a derived class")
;Superclass constructor
[["derived class"] nil]
Daniel E. Renfer
Hash: SHA1
The first parameter to these functions should be the reference the object.
so try:
(defn -init [this]
(.setDescription this "this is a derived class")
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CuppoJava
methods .
The init function doesn't take a "this" parameter.
Here's an example of my problem, MyDerivedClass is a subclass of
Thread.
This doesn't work, because "this" actually refers to the first
argument passed to the constructor. It does not refer to the object
itself.
(ns test)
(gen-class
:name test.MyDerivedClass
:init init)
(defn -init [this]
(. this setName "my derived class")
;Superclass constructor
[[] nil])
Chouser
>
> I believe the first parameter must be "this", only in the case of
> methods .
> The init function doesn't take a "this" parameter.
Correct.
My understanding is that the init function is actually run before the
instance is even created. So no only is there no "this" parameter,
there's not even a "this" value yet.
Instead, your init function returns values to pass to the superclass
constructor and the value to use as your object's state. Also note
that I had to specify :constructors in order for my init to accept no
args but to use the constructor of the superclass that takes a String.
(ns net.n01se.MyThread
(:gen-class
:extends "java.lang.Thread"
:constructors {[] [String]}
:init my-init
:state myState
:exposes-methods {getId getIdSuper}))
(defn -my-init []
(prn :init-my-thread)
[["my derived class"] "this is my state"])
(defn -getId [this]
(+ 123000 (.getIdSuper this)))
This also demonstrates the use of :exposes-methods to call the
superclass's implementation of a method I'm overridding.
Note that because I didn't use a mutable object for myState, (instead
using the string "this is my state"), there's no way to change the
value of myState.
user=> (compile 'net.n01se.MyThread)
net.n01se.MyThread
user=> (def x (net.n01se.MyThread.))
:init-my-thread
#'user/x
user=> (.myState x)
"this is my state"
user=> (.getId x)
123008
user=> (.getName x)
"my derived class"
--Chouser
CuppoJava
Yeah, I figured it would be like that. No "this" value actually exists
until after the init function is called.
The reason I'm asking about this is that it's quite standard practice
to set up some parameters inside the constructor of a class.
ie. A use-case like this is quite common, and (I think) reasonable.
public class MyThread extends Thread{
public MyThread(){
setName("This is my thread");
}
}
So whenever the user creates an instance of MyThread, he can be
guaranteed that the thread's name has already been appropriately set.
How would you do this in Clojure?
Chouser
>
> The reason I'm asking about this is that it's quite standard practice
> to set up some parameters inside the constructor of a class.
>
> ie. A use-case like this is quite common, and (I think) reasonable.
>
> public class MyThread extends Thread{
> public MyThread(){
> setName("This is my thread");
> }
> }
>
> So whenever the user creates an instance of MyThread, he can be
> guaranteed that the thread's name has already been appropriately set.
>
> How would you do this in Clojure?
The example I posted does exactly this -- the name of all MyThreads
start as "my derived class" as demonstrated by:
CuppoJava
However, you accomplished this only because there exists a constructor
in Thread that takes a String name argument.
As a general case, it's not usually that convenient...
ie.
public class MyThread extends Thread{
public MyThread(){
setName("This is my thread");
setPropertyB("bla bla bla");
etc...
}
}
Chouser
Seems like that class has a rather unfortunate API. :-)
As far as I know, this is not currently possible with gen-class.
Seems like you'd need some kind of post-constructor hook, provided
just to deal with mutable state of the base class.
I wonder if Rich would accept a patch providing an :after-ctor option
to gen-class.
--Chouser
CuppoJava
I'm making do with a temporary post-constructor hook that I manually
call after instantiating the object right now. But it's tedious and
error-prone.
Thanks for the help Chouser
Chouser
>
> Haha yeah... it's a rather poor API.
>
> I'm making do with a temporary post-constructor hook that I manually
> call after instantiating the object right now. But it's tedious and
> error-prone.
I've posted a feature request:
http://code.google.com/p/clojure/issues/detail?id=45
Now you can submit your CA, submit a patch, and nobody will ever have
to suffer your pain again. :-)
--Chouser