Scala类型约束不允许为空

Question

我有以下Scala代码:

class X[T1 <: AnyRef] {
   var _x : T1 = null 
}

代码_x = null突出显示为错误:

error: type mismatch;
found   : Null(null)
required: T1
var _x : T1 = null : T1

如果我添加Null类型约束，则一切正常。为什么会这样？ Scala将AnyRef定义为与java.lang.Object等效，当然可以为空。

Answer 1

代替

var _x : T1 = null

用

var _x : T1 = _

Scala语言规范中的解释:

A variable definition var x: T = _ can appear only as a member of a template. It introduces a mutable field with type T and a default initial value. The default value depends on the type T as follows:

0 if T is Int or one of its subrange types,
0L if T is Long,
0.0f if T is Float,
0.0d if T is Double,
false if T is Boolean,
() if T is Unit,
null for all other types T.