Java中通配符泛型的继承

2024年1月8日 280次阅读

既然我们能做到

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

我们可以说ArrayList<？>是ArrayList的超类< Integer> ？
上面的例子是否描述了多态性？

更新：通常,如果A< T> B是超T类的T>. , 然后

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

然后说A<？>是正确的.是超级B ？

最佳答案 TLDR：

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

不,但我们可以说它是ArrayList的超类型

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

是的,我们也可以说它是B 的超类型.

另请注意,继承仅在子类或超类的上下文中使用,而不是在子类型或超类型中使用.

Quick Reference

您需要了解的第一件事是java中的class is a type.
我们将在下面看到,无论在哪里使用术语sub / super class,使用sub / super类型都是有效的,但反之亦然

现在让我定义超类.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.

用简单的单词来解释这个问题,请考虑一个更简单的例子：
℃下F1,…,FN>是ArrayList< T>和C< T1,…,Tn>将说ArrayList< Integer>在上面的定义. T是类型参数,我们用Integer实例化它,它是类型参数.

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

现在,A<？>来自A< Integer>的扩展条款？ (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,在扩展中,我们一共提到了不同的类类型.

现在,让我们看看sub / super类型的定义.到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.

现在通过这个定义,根据第2点

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

？包含整数(Reference).因此,这使得A<？> A 的超型.

您可以轻松地看到此定义的第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>. B T延伸. ,使它属于两种定义.

最后,我们看到继承意味着什么.到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: […]

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