java - Java 中通配符泛型的继承

Question

因为我们可以做到

ArrayList<?> l = new ArrayList<Integer>();

我们可以说 ArrayList<?>是 ArrayList<Integer> 的父类(super class)？ 上面的例子是否因此描述了多态性？

更新:一般来说，如果A<T>是 B<T> 的父类(super class), 然后

A<?> obj = new B<Integer>();

那么说对了A<?>是 B<Integer> 的父类(super class)？

Answer 1

TLDR:

Can we say ArrayList<?> is superclass of ArrayList<Integer> ?

NO 但我们可以说它是 ArrayList<Integer> 的父类(super class)型

Then is right to say A<?> is super class of B<Integer> ?

YES 我们也可以说它是 B<Integer> 的父类(super class)型

另请注意，继承仅用于子类或父类(super class)的上下文中，而不用于子类型或父类(super class)型。

Quick Reference

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首先你需要了解的是class is a type在 java 。 我们将在下面看到，只要有效使用术语 sub/super class ，使用 sub/super type 都是有效的，但反之则不然

现在让我定义父类(super class)。根据JLS 8.1.4

Given a (possibly generic) class declaration for C<F1,...,Fn> (n ≥ 0, C ≠ Object) (Fi here is type parameter), the direct superclass of the class type C<F1,...,Fn> is the type given in the extends clause of the declaration of C if an extends clause is present, or Object otherwise.

Let C<F1,...,Fn> (n > 0) be a generic class declaration. The direct superclass of the parameterized class type C<T1,...,Tn>, where Ti (1 ≤ i ≤ n) is a type (argument), is D<U1 θ,...,Uk θ>, where D<U1,...,Uk> is the direct superclass of C<F1,...,Fn>, and θ is the substitution [F1:=T1,...,Fn:=Tn].

A class A is a subclass of class C if either of the following is true:

a) A is the direct subclass of C
b)There exists a class B such that A is a subclass of B, and B is a subclass of C, applying this definition recursively.

Class C is said to be a superclass of class A whenever A is a subclass of C.

为了用简单的词来解释这件事，考虑一个更简单的例子: C<F1,...,Fn>是ArrayList<T>和 C<T1,...,Tn>会说ArrayList<Integer>在上面的定义中。 T是类型参数，我们用 Integer 实例化它这是类型参数。

[ What did we meant by θ is the substitution [F1:=T1,...,Fn:=Tn] ? ]

现在，做A<?>出现在 A<Integer> 的扩展子句中？ (我知道问这样的结构是愚蠢的，但让我们严格定义)。不，不是的。通常，在 extends 中我们会提到完全不同的类类型。

现在，让我们看看子/父类(super class)型的定义。通过 JLS 4.10.2

Given a generic type declaration C<F1,...,Fn> (n > 0), the direct supertypes of the parameterized type C<T1,...,Tn>, where Ti (1 ≤ i ≤ n) is a type, are all of the following:

1. D<U1 θ,...,Uk θ>, where D<U1,...,Uk> is a generic type which is a direct supertype of the generic type C<T1,...,Tn> and θ is the substitution [F1:=T1,...,Fn:=Tn].

2. C<S1,...,Sn>, where Si contains Ti (1 ≤ i ≤ n) (§4.5.1).

3. The type Object, if C<F1,...,Fn> is a generic interface type with no direct superinterfaces.

4. The raw type C.

现在根据这个定义，根据第 2 点

C<S1,...,Sn>, where Si contains Ti (1 ≤ i ≤ n) (§4.5.1).

?包含 Integer (Reference)。因此，这使得 A<?> A<Integer> 的父类(super class)型.

你可以很容易地看到这个定义的第 1 点，包括子类定义本身。

问题的第二部分，我们已经说过A<T>延伸B<T> , 使其同时属于这两个定义。

最后，我们看看继承是什么意思。通过 JLS 8.4.8

A class C inherits from its direct superclass all concrete methods m (both static and instance) of the superclass for which all of the following are true: [...]