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I wonder if there is a special reason in Java for using always " extends " rather than " implements " for defining bounds of typeparameters. 我想知道在Java中是否有一个特殊的原因总是使用“ extends ”而不是“ implements ”来定义类型参数的界限。

Example: 例：

public interface C {}public class A{}

is prohibited but 被禁止但是

public class A{}

is correct. 是正确的。 What is the reason for that? 是什么原因呢？

#1楼

参考：

#2楼

这是一个允许在何处进行扩展以及可能需要的扩展的示例：

public class A<T1 extends Comparable<T1>>

#3楼

It's sort of arbitrary which of the terms to use. 使用哪种术语是任意的。 It could have been either way. 可能是任何一种方式。 Perhaps the language designers thought of "extends" as the most fundamental term, and "implements" as the special case for interfaces. 也许语言设计师认为“扩展”是最基本的术语，而“实现”则是接口的特殊情况。

But I think implements would make slightly more sense. 但是我认为implements会更有意义。 I think that communicates more that the parameter types don't have to be in an inheritance relationship, they can be in any kind of subtype relationship. 我认为传达更多信息的是，参数类型不必处于继承关系中，它们可以处于任何类型的子类型关系中。

The Java Glossary expresses a . Java术语表表达了 。

#4楼

The answer is in : 答案在 ：

To declare a bounded type parameter, list the type parameter's name, followed by the extends keyword, followed by its upper bound […]. 要声明一个有界的类型参数，请列出该类型参数的名称，然后列出extends关键字，然后列出其上限 […]。 Note that, in this context, extends is used in a general sense to mean either extends (as in classes) or implements (as in interfaces). 请注意，在这种情况下，extends通常用于表示extends （如在类中）或implements （如在接口中）。

So there you have it, it's a bit confusing, and Oracle knows it. 因此，有了它，这有点令人困惑，Oracle知道了。

#5楼

We are used to 我们习惯了

class ClassTypeA implements InterfaceTypeA {}class ClassTypeB extends ClassTypeA {}

and any slight deviation from these rules greatly confuses us. 与这些规则的任何细微差异都会使我们感到困惑。

The syntax of a type bound is defined as 类型绑定的语法定义为

TypeBound:    extends TypeVariable     extends ClassOrInterfaceType {AdditionalBound}

^{( ) （ ）}

If we were to change it, we would surely add the implements case 如果我们要改变它，我们肯定会添加implements的情况下

TypeBound:    extends TypeVariable     extends ClassType {AdditionalBound}    implements InterfaceType {AdditionalBound}

and end up with two identically processed clauses 最后有两个相同处理的子句

ClassOrInterfaceType:    ClassType     InterfaceType

^{( ) （ ）}

except we would also need to take care of implements , which would complicate things further. 除了我们还需要照顾implements ，这会使事情进一步复杂化。

I believe it's the main reason why extends ClassOrInterfaceType is used instead of extends ClassType and implements InterfaceType - to keep things simple within the complicated concept. 我认为，这是使用extends ClassOrInterfaceType而不是extends ClassType并implements InterfaceType主要原因-使事情在复杂的概念中保持简单。 The problem is we don't have the right word to cover both extends and implements and we definitely don't want to introduce one. 问题是我们没有一个合适的词来涵盖extends和implements ，我们绝对不想引入一个。

Although extends brings some mess when it goes along with an interface, it's a broader term and it can be used to describe both cases. 虽然extends在与接口一起使用时会带来一些混乱，但它是一个广义术语，可以用来描述这两种情况。 Try to tune your mind to the concept of extending a type (not

#6楼

There is no semantic difference in the generic constraint language between whether a class 'implements' or 'extends'. 在类“实现”还是“扩展”之间，通用约束语言没有语义差异。 The constraint possibilities are 'extends' and 'super' - that is, is this class to operate with assignable to that other one (extends), or is this class assignable from that one (super). 约束可能性是“扩展”和“超级”-也就是说，该类是可分配给其他类的对象（扩展），还是该类可从该类分配（超级）。

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