In Clojure how can I read a public member variables of an instance of a Java class?

Question

In Clojure how can I read a public member variables of an instance of a Java class? I want something like:

 (. instance publicMemberName)

I also tried:

instance/publicMemberName 

but this only works with static methods

Answer 1

If your object follows Java bean convention of getFoo to access member field foo, and you only need read access (i.e. aren't going to be mutating your object), you can use bean. That'll give you an immutable Clojure map that mimics the object, and then you can use standard keyword accessors.

user> (bean (java.awt.Point. 1.0 2.0))
{:y 2.0, :x 1.0, :location #<Point java.awt.Point[x=1,y=2]>, :class java.awt.Point}

user> (:x *1)
1.0

Answer 2

In Java, the class java.awt.Point has public fields x and y. See the javadocs here http://download.oracle.com/javase/6/docs/api/java/awt/Point.html.

In Clojure the dot macro works for fields and methods. This worked for me:

user=> (let [p (new java.awt.Point 2 4)] (.x p))
2

EDIT: The following also works (note the space between the dot and the p):

user=> (let [p (new java.awt.Point 2 4)] (. p x))
2

EDIT: I decided to make a complete example given that java.awt.Point has methods getX and getY in addition to public fields x and y. So here goes. First make a Java class like this:

public class C {
    public int x = 100;
}

Compile it

$ javac C.java

Now move C.class into your clojure directory. Next start the REPL, import the class, and watch it work:

$ java -cp clojure.jar clojure.main
Clojure 1.2.0
user=> (import C)
C
user=> (let [q (new C)] (. q x))
100

Note the other way works too:

user=> (let [q (new C)] (.x q))
100