Cast java.lang.reflect.Method to a functional inte

It is difficult to find any clues for the topic. All I could find is questions about converting one functional interface to another and some articles on type casting in Java. Not what I was looking for.

This question is about converting lambda → Method and I want the opposite, to convert Method to any functional interface, for example, to Consumer.

The way I found is to create a lambda adapter around the Method#invoke method:

    public void registerCallbacks(final Object annotated) {
        Class clazz = annotated.getClass();
        for (Method method : clazz.getDeclaredMethods()) {
            if (method.isAnnotationPresent(Callback.class)) {
                Callback registration = method.getAnnotation(Callback.class);

                List<String> warnings = new ArrayList<>(3);

                if (!Modifier.isPublic(method.getModifiers()))
                    warnings.add(String.format("Method %s must be public", method));
                if (method.getParameterCount() != 1)
                    warnings.add(String.format("Method %s must consume only one argument", method));
                if (method.getParameterCount() == 1 && !method.getParameterTypes()[0].equals(Integer.class))
                    warnings.add(String.format("Method %s must consume %s", method, Integer.class));

                if (!warnings.isEmpty()) {
                    warnings.forEach(log::warn);
                    continue;
                }

                CALLBACKS_MAPPER.registerCallback((param) -> {
                    try {
                        method.invoke(annotated, param);
                    } catch (IllegalAccessException | InvocationTargetException e) {
                        // Should not happen due to checks before.
                        log.warn(String.format("Could not invoke %s on %s with %s", method, annotated, param), e);
                    }
                });
                log.info("Registered {} as a callback", method);
            }
        }
    }

However I want to avoid writing

    CALLBACKS_MAPPER.registerCallback((param) -> {
        try {
            method.invoke(annotated, param);
        } catch (IllegalAccessException | InvocationTargetException e) {
            // Should not happen due to checks before.
            log.warn(String.format("Could not invoke %s on %s with %s", method, annotated, param), e);
        }
    });

in favor of something simpler, like

    CALLBACKS_MAPPER.registerCallback(SomeApacheLib.methodToFunction(annotated, method));

➥ So, is there a way to map old Java 1.1 reflection library to newer Java 8 functional interfaces, or it is me being stupid and the abovementioned solution with lambda is fine as it is?

If you're content with using reflection under the hood, just don't like the try/catch around the invoke, you can just make a simple utility function like:

public static <T> Consumer<T> toConsumer(Object annotated, Method m) {
    return param -> {
        try {
            m.invoke(annotated, param);
        } catch (IllegalAccessException | InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    };
}

And that gives you exactly the syntax you wanted:

CALLBACKS_MAPPER.registerCallback(toConsumer(annotated, method));

But if you want to actually avoid reflection altogether, you can use LambdaMetafactory to create a Consumer:

static Consumer<String> toConsumer(MethodHandles.Lookup lookup, Object annotated, Method method) throws Throwable {
    MethodType consumeString = MethodType.methodType(void.class, String.class);
    MethodHandle handle = lookup.unreflect(method);
    final CallSite site = LambdaMetafactory.metafactory(lookup, "accept",
            MethodType.methodType(Consumer.class, annotated.getClass()),
            consumeString.changeParameterType(0, Object.class),
            handle,
            consumeString);
    return (Consumer<String>) site.getTarget().invoke(annotated);
}

Change String to whatever your callbacks are expected to accept. And then:

CALLBACKS_MAPPER.registerCallback(toConsumer(MethodHandles.lookup(), annotated, method));

Of course, the only proper solution here is that you refactor your code to use a known callback interface on which you can normally call a defined method, instead of passing Methods around.