泛型自引用类型的 Java 子类化问题答案

【问题标题】：Java subclassing issue with generic type of self-referential type泛型自引用类型的 Java 子类化问题
【发布时间】：2012-02-16 15:30:23
【问题描述】：

import java.util.*;

// Let's define a self-referential type:
class SelfReferential<T extends SelfReferential<T>> {}

//A complete (i.e. not parameterized) subtype of SelfReferential:
class SubclassA extends SelfReferential<SubclassA> {}

//A partial (i.e. parameterized) subtype of SelfReferential:
class SubclassB<T extends SubclassB<T>> extends SelfReferential<T> {}

//Two complete subtypes of SubclassB
class SubclassB1 extends SubclassB<SubclassB1> {}    
class SubclassB2 extends SubclassB<SubclassB2> {}

//Now let's define a generic type over SelfReferential:
class Generic<T extends SelfReferential<T>> {}

//No problem creating a subtype for A, B1 or B2
class GenericA extends Generic<SubclassA> {}   
class GenericB1 extends Generic<SubclassB1> {}    
class GenericB2 extends Generic<SubclassB2> {}

//We can also defined a parameterize type for specific types extending SubclassB
class GenericB<T extends SubclassB<T>> extends Generic<T> {}

//However, it does not seem possible to define a non-parameterized subtype of Generic of ANY subtype of SublassB
//My goal is to provide a type alias for GenericB<? extends SubclassB<?>> to avoid 
//having to mention it everywhere in the code. This is like providing an alias for ArrayList<String> using 
class ArrayListOfString extends ArrayList<String> {}

//Unsucessful attempts:
//class GenericAnyB extends Generic<SubclassB> {} //ERROR: bound mismatch
//class GenericAnyB extends Generic<SubclassB<?>> {} //ERROR: bound mismatch
//class GenericAnyB extends Generic<? extends SubclassB<?>> {} //ERROR: invalid syntax: a supertype cannot specify any wildcard
//class GenericAnyB extends Generic<SubclassB<? extends SubclassB>> {} //ERROR: bound mismatch
//class GenericAnyB extends Generic<SubclassB<SubclassB<SubclassB<SubclassB<SubclassB<SubclassB>>>>>> {} // well...
//class GenericAnyB extends <T extends SubclassB<T>> Generic<T> {} //ERROR: this syntax is illegal

归根结底，我无法在 extends 子句中指定“引用循环”。

问题：这是 Java 语言限制吗？

【问题讨论】：

标签： java generics

【解决方案1】：

你是对的，这是不可能的，就像声明一个具有自引用类型的变量没有通配符或原始类型是不可能的。您将无法直接实例化 SubclassB，原因与没有自引用类型参数的情况下无法将其用作绑定一样。

有关此限制的更多讨论，请参阅此帖子：Self bound generic type with fluent interface and inheritance

底线是GenericAnyB 本身需要是通用的才能使用SubclassB 作为绑定：

class GenericAnyB<T extends SubclassB<T>> extends Generic<T> { }

这只是在任何东西可用之前在层次结构中添加了一个额外的步骤：

class GenericB1 extends GenericAnyB<SubclassB1> { }   
class GenericB2 extends GenericAnyB<SubclassB2> { }

【讨论】：