Type inference more restrictive in JDK 7 than JDK 6?

问题

I think this might be related to Why does a generic cast of a List<? extends Set..> to List<Set..> succeed on Sun JDK 6 but fail to compile on Oracle JDK 7?

If we take the following classes, they compile fine under JDK 6:

public final class Foo<V> {

    private final V value;

    private Foo(final V value) {

        this.value = value;
    }

    public static <T, R extends T> Foo<T> of(final R value) {

        return new Foo<T>(value);
    }
}

final class Tester {

    @Test(groups="unit")

    public static void test() {

        bar(Foo.of(BigDecimal.ZERO));  // This line fails in JDK 7 but not JDK 6
    }

    private static void bar(final Foo<? extends Number> target) {

        assert target != null;
    }
}

However, under JDK 7, I receive the following error:

[ERROR] \work\fsb-core\src\test\java\com\fsb\core\Foo.java:[42,8] error:
              method bar in class Tester cannot be applied to given types;

I thought type inference was less restrictive (e.g., adding constructor inference) in JDK 7. However, here, the compiler is rejecting a type that is valid under JDK 6.

Is this a bug? Or were the rules on inference made more stringent for methods?

回答1:

Strictly according to the spec, T cannot be inferred (per 15.12.2.7), so it should be taken as Object.

This can be viewed as a failure of the spec. This is how spec infers R: first there is constraint R :> BigDecimal, where :> means is a supertype of. The inference rules then choose R=BigDecimal since it's the most specific type satisfying the constraint.

Now, since T:>R, T:>BigDecimal, one would think this should yield T=BigDecimal too.

Unfortunately the inference rules do not take T:>R into account. There is no contraint on T. T is not inferred through the same principle.

While it sucks, spec is spec. Your code should not compile. Javac6 is wrong there.

In Java 8 there's an great improvement on inference rules to make lambda expression easier to use. Hopefully your code should compile in Java 8.

来源：https://stackoverflow.com/questions/8660316/type-inference-more-restrictive-in-jdk-7-than-jdk-6