为什么这个分支打破类型推断？

Question

I'm using a home-grown implementation of Either in Java, which has a methods like this:

public static <L, R> Either<L, R> left(final L value);

public static <L, R> Either<L, R> right(final R value);

public <T> T fold(
        final Function<? super L, ? extends T> leftFunction,
        final Function<? super R, ? extends T> rightFunction); 

These two methods compile and work fine:

Either<Foo, Bar> rightToLeft() {
    Either<Foo, Bar> input = Either.right(new Bar());
    return input.fold(
        l -> null, 
        r -> Either.left(new Foo())
    );
}

Either<Foo, Bar> rightToRight() {
    Either<Foo, Bar> input = Either.right(new Bar());
    return input.fold(
        l -> null,
        r -> Either.right(new Bar())
    );
}

This method does not compile:

Either<Foo, Bar> rightToLeftOrRightConditionally() {
    Either<Foo, Bar> input = Either.right(new Bar());
    return input.fold(
        l -> null, 
        r -> {
            if (r.equals("x")) {
               return Either.left(new Foo());
            }
            return Either.right(new Bar());
        });
}

Error:

incompatible types: inferred type does not conform to upper bound(s)
    inferred: Either<? extends Object,? extends Object>
    upper bound(s): Either<Foo,Bar>,java.lang.Object

(I've trimmed out the package qualifiers to make the error more readable)

I can make it compile by specifying the types:

if (r.equals("x")) {
    return Either.<Foo, Bar> left(new Foo());
}
return Either.<Foo, Bar> right(new Bar());

But why do I need to? And how can I avoid this code clutter?

Answer 1

This code should work.

It compiles on the latest JDK, 1.8.0_121.

It fails to compile on JDK 1.8.0-51.

This means it's most likely a bug in this version of the JDK, since later versions should not change the behaviour of the compiler unless to fix a bug. It may be bug JDK-8055963 .

So, the solutions are:

1. Upgrade your compiler
2. If you can't upgrade the compiler (eg someone else, stubborn, owns the build system), stick with your existing workaround of making the types explicit.

Answer 2

I do not see your entire class but this code compiles for me:

class Foo{}
class Bar{}

class Either<L,R> {

    private L left;
    private R right;

    public Either(L left, R right) {
        this.left = left;
        this.right = right;
    }

    public static <L, R> Either<L,R> left(L l) {
        return new Either<>(l, null);
    }

    public static <L, R> Either<L,R> right(R r) {
        return new Either<>(null, r);
    }


    public <T> T fold(
            final Function<? super L, ? extends T> leftFunction,
            final Function<? super R, ? extends T> rightFunction) {
        return null;
    }

    Either<Foo, Bar> rightToLeft() {
        Either<Foo, Bar> input = Either.right(new Bar());
        return input.fold(
                l -> null,
                r -> Either.left(new Foo())
        );
    }

    Either<Foo, Bar> rightToRight() {
        Either<Foo, Bar> input = Either.right(new Bar());
        return input.fold(
                l -> null,
                r -> Either.right(new Bar())
        );
    }

    Either<Foo, Bar> rightToLeftOrRightConditionally() {
        Either<Foo, Bar> input = Either.right(new Bar());
        return input.fold(l -> null, r -> {
            if (r.equals("x")) {
                return Either.left(new Foo());
            }
            return Either.right(new Bar());
        });
    }
}