如何将类型化的集合从clojure传递给java？

Question

I know the basics of clojure/java interop: calling java from clojure and vice versa. However, I was not able to return a typed collection from clojure to java. I am trying to see something of that nature List<TypedObject> from the java code which is calling into clojure.

Java Object:

public class TypedObject {
    private OtherType1 _prop1;
    public OtherType1 getProp1() {
        return _prop1;
    }
    public void setProp1(OtherType1 prop1) {
        _prop1 = prop1;
    }
}

CLojure method:

(defn -createListOfTypedObjects
      "Creates and returns a list of TypedObjects"
      [input]
      ;Do work here  to create and return list of TypedObjects
      [typedObj1, typedObj2, typedObj3])

(:gen-class
 :name some.namespace
 :methods [createListofTypedObjects[String] ????])

Let us consider that I am writing an API using clojure, which is to be distributed as a jar file, to be used from java. My question was really how to what to pass in place of the ???? questions marks above inside the :gen-class for AOT, so that a programmer writing a piece of code in java using my api, can have the appropriate intellisense / code completion (ie: createListofTypedObjects() returns List<TypedObject> ) from within eclipse for example.

Answer 1

The others are right that Clojure doesn't ensure the types of elements in returned collections, etc. (Actually, the JVM doesn't ensure the types of elements in collections, either – that's handled entirely by javac.)

However, I can see the value of providing an API to other Java programmers that specifies an interface that declares that return values (or parameters) parameterized in various ways; this is especially attractive if one is looking to use Clojure in an existing Java environment without making waves.

This currently requires a two step process:

• define a separate interface (in Java!) that specifies the parameterized types as you like
• define your gen-class namespace (or proxy or reify instance) such that it implements that interface

(Clojure does provide a definterface form that would allow you to avoid the separate Java interface definition, but definterface , just like the rest of Clojure, does not provide for specifying parameterized types. Maybe someday... :-))

eg

public interface IFoo {
    List<TypedObject> createListOfTypedObjects ();
}

and then your gen-class namespace:

(ns your.ns.FooImpl
  (:gen-class
    :implements [IFoo]))
(defn -createListOfTypedObjects
  []
  [typedObj1, typedObj2, typedObj3])

When your users create instances of FooImpl , they'll eg get code completion indicating that the method returns List<TypedObject> rather than Object or the unparameterized List type.

If you're using sane build tools (eg maven , gradle, or properly-configured ant), then you can put the Java interface in your Clojure project, and the cross-language dependency will be taken care of.

Answer 2

If you're trying to pass something like List<String> to a java method, then you don't need to worry about it. The type parameter (eg, String ) is only used to by the javac compiler, so any List will work just fine at runtime.

On the other hand if you're trying to pass an array of a particular object type (eg, String[] ), then you can use the various -array functions:

user=> (make-array String 10)            ; an empty String array
#<String[] [Ljava.lang.String;@78878c4c>
user=> (into-array ["foo" "bar"])        ; array type inferred from first element
#<String[] [Ljava.lang.String;@743fbbfc>
user=> (into-array Number [1.2 5 7N])    ; explicit type array
#<Number[] [Ljava.lang.Number;@7433b121>

Answer 3

You don't need to worry about generics (typed collections) in Clojure. Generics are really just type hints to the Java compiler. In a running Java program, List<String> is effectively the same as List<Object> .

So, for example, a Clojure vector containing Strings is already a List<String> with no conversion needed.