6

I am trying to find a Clojure-idiomatic way to initialize a Java object. I have the following code:

(let [url-connection
      (let [url-conn (java.net.HttpURLConnection.)]
        (doto url-conn
          (.setDoInput true)
          ; more initialization on url-conn
          )
        url-conn)]
  ; use the url-connection
  )

but it seems very awkward.

What is a better way to create the HttpURLConnection object and initialize it before using it later in the code?

UPDATE: It seems that (doto ...) may come in handy here:

(let [url-connection
        (doto (java.net.HttpURLConnection.)
          (.setDoInput true)
          ; more initialization
          ))]
  ; use the url-connection
  )

According to the doto docs, it returns the value to which it is "doing".

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2 Answers 2

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4

As explained in the update to my question, here is the answer I came up with:

(let [url-connection
        (doto (java.net.HttpURLConnection.)
          (.setDoInput true)
          ; more initialization
          ))]
  ; use the url-connection
  )

Maybe someone can come up with a better one.

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11 Comments

What else do you want more? The code is as minimal as you can get.
I haven't read Barski's book, but CL does have arrays and hash tables. And I think that, at least at this point, Clojure might have a hard time matching the efficiency of the faster CL implementations.
But I would also say that Clojure's approach to data types is a lot more consistent than CL's. IMHO that part of Clojure is better designed than the corresponding bits of CL.
You're right that for a lot of things being ultra-efficient isn't all that important (though there are times when it is, and I'm glad to see that a lot of effort is going into making Clojure efficient.) I commented mainly because it's possible to get the wrong impression of CL from many books about it. You can program in a very list-oriented way in CL (and this will be pretty inefficient for some things,) but that's not the whole story. Production CL usually looks pretty different.
And I should add that I certainly agree with you that things like the literal syntax, and particularly the nice sequence abstraction in Clojure can indeed make many things a lot nicer in Clojure. I don't think anyone who has used CL a lot would associate it with the word "perfection" ;). CL has a lot of warts, and though many of them become pretty unimportant once you are experienced with the language, some remain annoying- very much so in some cases. But that said, for me at least, neither Clojure nor CL strictly dominates the other. CL has a lot to offer, despite the warts.
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3

Assuming that there is no constructor that accepts all the initialization parameters needed, then the way you did it is the only one I know.

The one thing you could do is wrap it all in a function like this:

(defn init-url-conn [doInput ...other params..] 
     (let [url-conn (java.net.HttpURLConnection.)]
        (doto url-conn
          (.setDoInput true)
          ; more initialization on url-conn
          )
        url-conn))

And call with:

(let [url-connection
      (let [url-conn (init-url-con true ...other params..)]
  ; use the url-connection
  )

However, this is specific per object and it is really useful only if you are initializing object of that class more than once.

Also you could write a macro that accepts all method names, and params and does this. But, when called, that call wouldn't be much shorter than your first example.

If anyone has a better idea, I'd like to see it, since I was asking myself the same just the other day..

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1 Comment

Yes. And doc for doto has a similar example: (doto (new java.util.HashMap) (.put "a" 1) (.put "b" 2))

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