为什么Scala语言要求您初始化实例变量而不是依赖默认值？ [英] Why does Scala language require you initialize a instance variable instead of relying on a default value?

问题描述

Scala语言要求您在使用实例变量之前对其进行初始化。但是，Scala不会为您的变量提供默认值。相反，您必须使用通配符下划线手动设置其值，该通配符下划线的作用类似于默认值，如下所示：

Scala language requires you initialize your instance variable before using it. However, Scala does not provide a default value for your variable. Instead, you have to set up its value manually by using the wildcard underscore, which acts like a default value, as follows

var name:String = _

我知道，我知道...我可以在类中定义构造函数定义，它以我们的实例变量作为参数，因此Scala不会如下所示强制其初始化

I know, i know... I can define a constructor in the class definition, which takes as parameter our instance variable, so Scala does not force its initialization as shown below

class Person(var name:String) 

但是，我需要在主体中声明它，因为我需要使用Java注释， ElementType是FIELD或METHOD；也就是说，它只能应用于实例变量或在我们的类主体中声明的方法。

However, i need to declare it in the body because i need to use a Java annotation whose ElementType is FIELD or METHOD; that is, it can just be applied to either a instance variable or method declared in the body of our class.

问题：为什么Scala语言要求您初始化一个实例变量-无论是默认值_还是所需的任何东西-都在类的主体中声明而不是依赖默认值？

Question: Why does Scala language require you initialize a instance variable - be it a default value _ or whatever you want - declared in the body of a class instead of relying on a default value ?

推荐答案

将注释指定为构造函数参数时，您可以可以应用注释。另外，您可能需要使用元注释来限制将您正在使用的注释应用于哪个目标-请参见 http://www.scala-lang.org/api/2.10.2-RC2/index.html#scala.annotation.meta.package

You can apply the annotation when you specify it as a constructor argument. Also, you may need to use a meta-annotation to restrict which target the annotation you're using is applied to - see http://www.scala-lang.org/api/2.10.2-RC2/index.html#scala.annotation.meta.package

关于依赖默认值的问题尚不清楚。使用下划线进行初始化对应于将变量的值分配为null。您还在想其他什么默认值？

Your question about "relying on a default value" is somewhat unclear, though. Initialization using an underscore corresponds to assigning the value of the variable to null. What other default are you thinking of?

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