Java 如何处理泛型的模糊类型推断？

Question

这段代码中，T可以是A、B、C、D，但是Eclipse显示是D。

static class A { }
static class B extends A { }
static class C extends B { }
static class D extends C { }
static <T> void copy(List<? super T> dst, List<? extends T> src) {
    for (T t : src)
        dst.add(t);
}
public static void main(String[] args) {
    List<A> dst = new ArrayList<>();
    List<D> src = new ArrayList<>();
    copy(dst, src); // Eclipse shows T is D
}

类型推断是如何进行的以及为什么选择 D 是否有任何规则？

Answer 1

Is there any rule for how type inference is done

是的，entire 18th chapter of the Java Language Specification 专门讨论这个话题:-)

and why it selects D?

我认为以下规则对此负责：

If the bound set does not contain a bound of the form G<..., αi, ...> = capture(G<...>) for all i (1 ≤ i ≤ n), then a candidate instantiation Ti is defined for each αi:

If αi has one or more proper lower bounds, L1, ..., Lk, then Ti = lub(L1, ..., Lk) (§4.10.4).

Otherwise, if the bound set contains throws αi, and the proper upper bounds of αi are, at most, Exception, Throwable, and Object, then Ti = RuntimeException.

Otherwise, where αi has proper upper bounds U1, ..., Uk, Ti = glb(U1, ..., Uk) (§5.1.10).

The bounds α1 = T1, ..., αn = Tn are incorporated with the current bound set.

If the result does not contain the bound false, then the result becomes the new bound set, and resolution proceeds by selecting a new set of variables to instantiate (if necessary), as described above.

用简单的英语来说，在尝试类型参数的可能值时，编译器首先尝试下限，如果合适则使用下限。

在我们的例子中，约束集表示 D extends T and D extends A，因此 T 的下限是 D，因此 D 是第一个候选替换。

编译器然后通过假设 T = D 来验证 D 是否适合，这简化了将约束设置为 D extends D and D extends A，两者都为真。

因此，编译器使用D。

Java 如何处理泛型的模糊类型推断？

How does Java handle ambiguous type inference for generics?

java

generics

type-inference

generic-programming

inferred-type