Constructing a java lambda expression?

[Alan Ruttenberg]

Thanks.

(defun get-abstract-methods (classname)
  (map 'list #"getName"
       (remove-if-not
        (lambda(x)
          (let ((modifiers (#"getModifiers" x)))
            (and (#"isAbstract" 'reflect.Modifier modifiers)
                 (not (#"isStatic" 'reflect.Modifier modifiers)))))
        (#"getDeclaredMethods" (find-java-class classname)))))

(defun java-lambda-parameter-information (classname methodname position)
  (let* ((method (find methodname (#"getDeclaredMethods" (find-java-class
classname))
                       :key #"getName" :test 'equalp))
         (parameter-type (elt (#"getParameterTypes" method) position))
         (abstract-methods (get-abstract-methods parameter-type)))
    (assert (#"isInterface" parameter-type) ()
            "lambda parameter type ~a should be an interface but isn't"
            (#"getName" parameter-type))
    (assert (= (length abstract-methods) 1) ()
            "Parameter ~a type ~a has more than one abstract method:
~{~a~^, ~}"
            position (#"getName" parameter-type) abstract-methods)
    (list (#"getName" parameter-type) (car abstract-methods))))

(java-lambda-parameter-information 'Nitfsegmentsflowimpl
"forEachImagesegment" 0)
-> ("java.util.function.Consumer" "accept")

Cheers,
Alan


On Thu, Aug 25, 2022 at 6:35 AM Alessio Stalla <alessiostalla at gmail.com>
wrote:

> Because *andThen *is a *default method *in the interface, i.e. it's not
> abstract:
> https://docs.oracle.com/javase/8/docs/api/java/util/function/Consumer.html
> So, effectively, the interface only has one abstract method, and that's
> the target of lambda conversion.
> Now I have no idea if and how ABCL deals with default methods. If it's
> still stuck on Java 5/6 compatibility then I suspect that it can't, at
> least not in its Java code.
>
> On Thu, 25 Aug 2022 at 06:37, Alan Ruttenberg <alanruttenberg at gmail.com>
> wrote:
>
>> So in order to implement this I need to know the interface to use for the
>> lambda? Presumably I can determine this with reflection.
>>
>> An example of one of the functions that takes the lambda is defined:
>>
>> public final NitfSegmentsFlow forEachImageSegment(final
>> Consumer<ImageSegment> consumer)
>>
>> But consumer has two methods: accept and andThen
>>
>> Eventually it looks like accept is called on the consumer. So in this
>> case it looks like I need to use jinterface-implementation and define the
>> accept method, no lambda necessary?
>>
>> What confuses me now is if a lambda is passed as the test code I'm
>> reading does:
>>
>> forEachImageSegment(imageSegment -> {do something})
>>
>> How does java know that the lambda is the implementation of accept rather
>> than andThen. Or more practically, how do I figure out which method to
>> implement without reading the source code?
>>
>> Thanks,
>> Alan
>>
>>
>> On Wed, Aug 24, 2022 at 4:14 AM Alessio Stalla <alessiostalla at gmail.com>
>> wrote:
>>
>>> Function is a convenient interface for cases when a more specific
>>> interface does not exist. But any Java interface with a single abstract
>>> method can be the target of a lambda expression:
>>>
>>> new Thread(() -> { System.out.println("foo"); }).run();
>>>
>>> The Thread constructor takes a Runnable argument, not a Function.
>>>
>>> Lambda is just syntax sugar for interfaces with a single method, so you
>>> can reproduce them on ABCL with the jinterface thing + some macrology.
>>>
>>> On Wed, 24 Aug 2022 at 05:10, Vibhu Mohindra <vibhu.mohindra at gmail.com>
>>> wrote:
>>>
>>>> On 23/08/2022 05:27, Alan Ruttenberg wrote:
>>>> > There's a library I want to use that takes a lambda as an argument.
>>>> > Anyone know how to construct one in ABCL?
>>>>
>>>> I assume it's a Java library taking a Java Lambda that you want to call
>>>> from ABCL.
>>>>
>>>> A Java Lambda is really an instance of one of the classes in
>>>> java.util.function. Which one depends on how many parameters it has and
>>>> whether it returns a value. Let's assume your library wants a Java
>>>> Lambda that has one parameter and returns a value. That's a
>>>> java.util.function.Function. Say it looks like this:
>>>>
>>>> //Lib.java
>>>> public class Lib {
>>>>    public static void f(java.util.function.Function f) {
>>>>      System.out.println("You answered: " + f.apply(5));
>>>>    }
>>>> }
>>>>
>>>> such that it can be used from Java with a Java Lambda like this:
>>>>
>>>> Lib.f((Object x) -> (Integer)x * (Integer)x);
>>>> => You answered: 25
>>>>
>>>> You'd like to give it a Lisp Lambda from ABCL as follows:
>>>>
>>>> (jstatic "f" "Lib" #'(lambda (x) (* x x)))
>>>>
>>>> but that's not allowed because although Java can implicitly convert
>>>> from
>>>> a Java Lambda to a java.util.function.Function, it can't convert from a
>>>> Lisp Lambda to a java.util.function.Function.
>>>>
>>>> A Lisp Lambda is really an org.armedbear.lisp.Function. So one solution
>>>> is to adapt that.
>>>>
>>>> //Adaptor.java
>>>> import org.armedbear.lisp.*;
>>>> public class Adaptor implements java.util.function.Function {
>>>>    private org.armedbear.lisp.Function lispFn;
>>>>    public Adaptor(org.armedbear.lisp.Function lispFunction) {
>>>>      this.lispFn = lispFunction;
>>>>    }
>>>>    public Object apply(Object input) {
>>>>      return lispFn.execute(
>>>>        JavaObject.getInstance(input, true)).javaInstance();
>>>>    }
>>>> }
>>>>
>>>> and use it from ABCL like this:
>>>>
>>>> (jstatic "f" "Lib" (jnew "Adaptor" #'(lambda (x) (* x x))))
>>>> => You answered: 25
>>>> => NIL
>>>>
>>>> ----
>>>> Notes:
>>>>
>>>> I'm on Java 10, ABCL-1.4.0 (which is old).
>>>> I did this to build and run, starting with the jar and two java files
>>>> in
>>>> the current directory:
>>>> javac -classpath abcl-1.4.0.jar:. Adaptor.java Lib.java
>>>> java -classpath abcl-1.4.0.jar:. org.armedbear.lisp.Main
>>>>  > (jstatic "f" "Lib" (jnew "Adaptor" #'(lambda (x) (* x x))))
>>>>
>>>> You can probably create the Adaptor class from within ABCL if you don't
>>>> like that it's written in Java, but I don't remember how to. ABCL's
>>>> documentation might describe such bytecode generation somewhere.
>>>>
>>>> Pointers:
>>>> Java: java.util.function.*
>>>> ABCL: In org.armedbear.lisp, LispObject, JavaObject, Function
>>>>
>>>>
>>>> --
>>>> Vibhu
>>>>
>>>>
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