Question

I’m getting a Null Pointer Exception while in executing a macro to do some Java interop, and I can’t figure out why.

I’m setting the fields of a Java object using a map with nested data, and since the names of the map keys are the same as the field names of the object, I created a macro:

(defmacro set-keys! [pose m k klst]
  `(set! (. ~pose ~(symbol (name k)))
         (double-array (map #(% (~(keyword (name k)) ~m)) ~klst))))

For testing, I defined pse to be an initialized Java object with the fields of interest set to zero, and I defined mp as a simplified map that would still work:

(def mp {:pos {:x 1 :y 2})

Now comes the odd behavior. Executing set-keys! with explicitly typed arguments works:

user> (set-keys! pse mp :pos [:x :y])
#<double[] [D@691ba57a>

But if I use run the code in a inside a let call:

user> (let [x :pos y [:x :y]]
        (set-keys! pse mp x y))

I get a Null Pointer Exception with “No message.” as the message. What is the issue here?

EDIT: I defined pse with (def pse (pose_t.)), pose_t is a generated class, and the pose_t() constructor initializes all the arrays in its fields, but doesn’t initialize the values. The relevant lines of the class code for the simplified example are:

public double pos[];

public pose_t() {
    pos = new double[3];
}

Answer 1

Macro arguments are not evaluated, so ~(keyword (name k)) expands to – roughly – (keyword (name (quote x))) – which is :x, not :pos

At least I guess that’s the problem. If not, it would be nice if you could include a definition for pse.

Addendum: you can inspect what the macro is actually expanding to by evaluating:

user> (macroexpand-1 '(set-keys! pse mp x y))
(set! (. pse x) (clojure.core/double-array (clojure.core/map
    (fn* [p1__2066__2067__auto__] (p1__2066__2067__auto__ (:x mp))) y)))

Note the 😡

Addendum 2: (. obj field) actually also doesn’t evaluate field, meaning you’d have to use java reflection to make the field argument dynamic.