如何确保Clojure变量是正确的Java类型

Question

java-code XSSFRow row = sheet.getRow(p);
clojure-code (def row (.getRow sheet p))
How do we make sure that row is of type XSSFRow ?

Answer 1

Clojure is dynamically typed, so the type of a var/value is determined at runtime.

If you want to make sure a value is of a certain class, you could make the following assertion:

(assert (= (class row) XSSFRow))

Or, more concisely (thanks to opqdonut):

(assert (instance? XSSFRow row))

This does not check for exact class, but for any superclass:

(instance? Object row) ;=> true

Answer 2

Notice that there is a way to specify type hints for the compiler using metadata. In some forms you can insert the following constructions:

(let [^String str-val (do-something)] ...)
(def ^double y)
(defn f [^int int-arg ^"[[I" 2d-int-array] ...)

note the ^type expression. This is shorthand for specifying one element of metadata, that is, :tag key in metadata map. Now the compiler can optimize the code, directly resolving calls to java methods instead of reflection now when it knows exact type of var.

There is an example on clojure.org: http://clojure.org/java_interop#Java%20Interop-Type%20Hints . It suggests, and I think it is right, not to overuse type hints, since they just clutter the code and are generally not needed for adequate performance.