像在Java中使用内部接口一样，在Scala中实现内部特征 [英] Implementing inner traits in Scala like we do with inner interfaces in Java

问题描述

此Java代码可以正确编译:

This code in Java compiles without errors:

interface T {
    interface Q {
    }
}

class C implements T.Q {
}

Scala中的此代码不:

whereas this code in Scala does not:

trait T {
    trait Q {
    }
}

class C extends T.Q {
}

将Java代码列表正确转换为Scala的正确翻译(如果存在)?

What is the correct translation (if it exists) of the Java code listing into Scala?

欢迎对语言设计进行理论解释.

Theoretical explanations about language design are welcome.

推荐答案

内部类型Q仅为T特征的特定实例实现定义.由于scala具有与路径相关的类型，因此T的每个实例都将具有自己的子特性Q.

The inner type Q is defined only for specific instance implementation of the T trait. Since scala has path-dependent types, each instance of T will have his own subtrait Q.

scala> trait T {
     |   trait Q
     | }
defined trait T

scala> class C extends T {
     |   def getQ: this.Q = new this.Q {}
     | }
defined class C

scala> val inC = (new C).getQ
inC: C#Q = C$$anon$1@3f53073a


scala> val c = new C
c: C = C@1a7e4ff0

scala> new c.Q {}
res4: c.Q = $anon$1@36bbb2f5

---

如果您需要一个接口来实现客户端的一般行为，并且不依赖于特定的C实例，则应在Object

---

If you need an interface for a generic behavior for your clients to implement, and not dependent on a specific C instance, you should define it within an Object

scala> object T {
     |   trait Q {
     |     def implementMe: Unit
     |   }
     | }
defined module T

scala> val inT = new T.Q {
     |   def implementMe = println("implemented!")
     | }
inT: T.Q = $anon$1@20f2a08b

scala> inT.implementMe
implemented!

---

为什么要依赖路径类型?

由于设计原因，请在此处

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