Data Structures Containing Polymorphic Method References

Question

The line

associations.put("test1",B::setBeta);

below does not compile. I'm not clear why it won't work since B extends A. Is there a way to make this work? I'm trying to build a map of method references from an inheritance family.

import java.util.HashMap;
import java.util.Map;
import java.util.function.BiConsumer;

public enum Test {
    ENUM0() {
        @Override
        public void init() {
            associations.put("test0",A::setAlpha);
        }
    },
    ENUM1() {
        @Override
        public void init() {
            associations.put("test1",B::setBeta);
        }
    };
    public abstract void init();

    Map<String, BiConsumer<? extends A, String>> associations = new HashMap<>();
}

class A {
    public String getAlpha() {
        return alpha;
    }
    public void setAlpha(String alpha) {
        this.alpha = alpha;
    }

    String alpha;
}

class B extends A {
    public String getBeta() {
        return beta;
    }
    public void setBeta(String beta) {
        this.beta = beta;
    }

    String beta;
}

You need to use `Consumer` instead of `BiConsumer extends A, String>` since `setBeta` accepts only one argument. — vsminkov, Sep 13 '16 at 22:03
@vsminkov It will accept 2, since it is a statically referenced instance method. — Jorn Vernee, Sep 13 '16 at 22:03
Well... Then it does not work because `setBeta` is not declared in `A` class. Because you can't reference `setBeta` using `A` class. — vsminkov, Sep 13 '16 at 22:05
@JornVernee but it's impossible to pass instance of `A` as a parameter to `BiConsumer` — vsminkov, Sep 13 '16 at 22:09
You've got a separate `associations` instance for each enum value. Do you need a map, or can you just have a key and a value field? — Andy Turner, Sep 13 '16 at 22:09
@AndyTurner for the actual application a map is necessary because there are multiple key-value pairs. — KevinRethwisch, Sep 14 '16 at 04:52

score 1 · Accepted Answer · answered Sep 13 '16 at 22:09

1

This seems like a curious case of type inference, explicitly tagging the expression resolves the compilation errors:

associations.put("test1", (BiConsumer<B, String>) B::setBeta);

answered Sep 13 '16 at 22:09

Jorn Vernee

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Awesome! I didn't think about using a cast in that context. – KevinRethwisch Sep 14 '16 at 04:49
But don’t be surprised that the type inference doesn’t handle this case—a `BiConsumer extends A, String>` is useless as there is no correct way to invoke its `accept` method… – Holger Sep 14 '16 at 15:01
Actually it does work. The first parameter is a method call returning the proper type because Java cannot declare the variable properly AFAICT. – KevinRethwisch Sep 16 '16 at 02:43

Data Structures Containing Polymorphic Method References

1 Answers1