clojure gen-class returning own class

Question

I'm now making a class object with Clojure which has a method returning the object itself.

Written with Java, the object that I'd like to make is like,

class Point {
    public double x;
    public double y;

    public Point(double x, double y) {
        this.x = x;
        this.y = y;
    }

    public Point copy() {
        return new Point(this.x, this.y);
    }
}

The current clojure code that I wrote is like,

(ns myclass.Point
  :gen-class
  :prefix "point-"
  :init init
  :state state
  :constructors {[double double] []}
  :methods [[copy [] myclass.Point]]))

(defn point-init [x y]
   [[] {:x x :y y}])

(defn point-copy [this]
   this)

However, I got an error as follows.

java.lang.ClassNotFoundException: myclass.Point

While I have googled about this issue, I couldn't find any answers. Does anyone know the solution for this issue?

Thank you in advance for your help.

Answer 1

I'm not sure it's the only way, but, in order to use the generated class type in a method signature, you can generate the class in 2 steps - though it's still in one file and compiled in one pass:

• call gen-class with only the constructors
• call gen-class again with state, full set of constructors and methods.

I tried with various methods including forward declaration, but only the above was working eventually. I did extend your example a little bit. Note That the copy method is not very useful as-is since Point is immutable, but you may want to provide mutators to your class.

Full listing:

(ns points.Point)

;; generate a simple class with the constructors used in the copy method
(gen-class
 :name points.Point
 :init init
 :constructors {[] []
                [double double] []})

;; generate the full class 
(gen-class
 :name points.Point
 :prefix pt-
 :main true
 :state coordinates
 :init init
 :constructors {[] []
                [double double] []}
 :methods [[distance [points.Point] double]
           [copy [] points.Point]])

(defn pt-init
  ([] (pt-init 0 0))
  ([x y]
   [[] {:x x :y y}]))

(defn pt-copy 
  "Return a copy of this point"
  [this]
  (points.Point. (:x (.coordinates this)) (:y (.coordinates this))))

(defn pt-distance [^points.Point this ^points.Point p]
  (let [dx (- (:x (.coordinates this)) (:x (.coordinates p)))
        dy (- (:y (.coordinates this)) (:y (.coordinates p)))]
    (Math/sqrt (+ (* dx dx) (* dy dy)))))

(defn pt-toString [this]
  (str "Point: " (.coordinates this)))

;; Testing Java constructors and method call on Point class
(import (points Point))
(defn pt-main []
  (let [o (Point.)
        p (points.Point. 3 4)]
    (println (.toString o))
    (println (.toString p))
    (println (.distance o p))
    (println (.distance p (.copy p)))))

---

In order to generate the classes, configure project.clj with the line

:aot [points.Point]

Testing with lein gives:

tgo$ lein clean
tgo$ lein compile
Compiling points.Point
tgo$ lein run
Point: {:x 0, :y 0}
Point: {:x 3.0, :y 4.0}
5.0
0.0