Java Lambda Expressions and Method References to Generic Methods

Question

I have a functional interface

import java.util.concurrent.Callable;
import java.util.concurrent.CompletableFuture;

@FunctionalInterface
public interface SubmitterCompletable extends Submitter {
    @Override
    <T> CompletableFuture<T> submit(Callable<T> task);
}

and two functions

import java.util.concurrent.Callable;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Executor;

public final class CompletableFutureUtils {
    public static <U> CompletableFuture<U> runAsync(Callable<U> callable) {
        // ...
    }

    public static <U> CompletableFuture<U> runAsync(Callable<U> callable, Executor executor) {
        // ...
    }
}

and I want to create SubmitterCompletables from these functions using lambda expressions or method references. The first one works fine by using method references.

SubmitterCompletable submitterCompletable = CompletableFutureUtils::runAsync;

For the second one, however, I have to use a lambda expression to pass an Executor and it doesn't work.

Executor executor = /* ... */;
SubmitterCompletable submitterCompletable = c -> CompletableFutureUtils.runAsync(c, executor);
// Illegal lambda expression: Method submit of type SubmitterCompletable is generic

My question is whether there is a valid lambda expression for this case, or do I have to create an anonymous class in this case?

Answer 1

The issue there is that "a lambda expression can be used for a functional interface only if the method in the functional interface has NO type parameters". (JLS11, 15.27.3 Type of a Lambda Expression) with one exception - this is not the case for congruent method references.

That is why it works in your first example and doesn't in the second:

SubmitterCompletable submitterCompletable = CompletableFutureUtils::runAsync; (OK)
SubmitterCompletable submitterCompletable = c -> <anything> (NOT OK)

---

There aren't many options out there I could think of to achieve what you want:

1. Implement the interface (either in-place using an anonymous class as you've mentioned or as a standalone class).

2. Use an intermediate helper class inside your CompletableFutureUtils that would keep a ref to the executor and expose a method congruent with your Submitter's functional method which will delegate the call to the underlying runAsync(Callable<U> callable, Executor executor) util's method.

Example code:

public final static class CompletableFutureUtils {
    public static <U> CompletableFuture<U> runAsync(Callable<U> callable) {
        ...
    }

    public static <U> CompletableFuture<U> runAsync(Callable<U> callable, Executor executor) {
        ...
    }
    
    public static ExecutorRunnerProxy using(Executor executor) {
        return new ExecutorRunnerProxy(executor);
    }

    public static final class ExecutorRunnerProxy {
        private final Executor executor;

        private ExecutorRunnerProxy(Executor executor) {
            this.executor = executor;
        }

        public <T> CompletableFuture<T> runAsync(Callable<T> task) {
            return CompletableFutureUtils.runAsync(task, executor);
        }
    }
}

Example usage:

SubmitterCompletable submitterCompletable = CompletableFutureUtils::runAsync; 
SubmitterCompletable submitterWithExecutor = CompletableFutureUtils.using(executor)::runAsync;