Java in Clojure, Evaluation question

开发者_开发问答Is there a detailed explanation for results obtained on evaluating the following on REPL.

(.PI Math)

gives

IllegalArgument Exception

while

(. Math PI)

evaluates to

3.141592653589793

The explanation is at http://clojure.org/java_interop.

user> (macroexpand '(.PI Math))
(. (clojure.core/identity Math) PI)

(identity Math) returns the Class object representing the Math class. You're trying to access an instance member called PI in this Class object, but it doesn't exist. (This is different from accessing a static member called PI in the Math class.) You would only ever use this Class object for reflection, or to pass a Class around to other methods as an Object, or those sorts of things.

user> (class (identity Math))
java.lang.Class
user> (.getName (identity Math))
"java.lang.Math"
user> (.getName Math)
"java.lang.Math"
user> (.getMethods Math)
#<Method[] [Ljava.lang.reflect.Method;@12344e8>
user> (vec (.getMethods Math))
[#<Method public static int java.lang.Math.abs(int)> #<Method public static long java.lang.Math.abs(long)> #<Method public static float java.lang.Math.abs(float)> ...]
user> (.getField Math "PI")
#<Field public static final double java.lang.Math.PI>
user> (.getDouble (.getField Math "PI") Math)
3.141592653589793

The shortest way to do what you want is probably Math/PI.

user> (macroexpand '(Math/PI))
(. Math PI)
user> Math/PI
3.141592653589793
user> (. Math PI)
3.141592653589793