scala - 覆盖 'val'时出现意外结果

Question

在Scala 2.10.4中，给出以下类:

scala> class Foo { 
     |   val x = true
     |   val f = if (x) 100 else 200
     | }
defined class Foo

以下两个示例对我来说很有意义:

scala> new Foo {}.f
 res0: Int = 100

scala> new Foo { override val x = false}.f
res1: Int = 200

但是，为什么此调用不返回 100？

scala> new Foo { override val x = true }.f
res2: Int = 200

Answer 1

由于val不会被多次初始化，因此x实际上是null初始化期间的false(或默认Boolean的Foo)，然后在扩展您的示例中的Foo的匿名类中进行初始化。

我们可以使用AnyRef更加轻松地对其进行测试:

class Foo { 
    val x = ""
    val f = if (x == null) "x is null" else "not null"
}

scala> new Foo { override val x = "a" }.f
res10: String = x is null

scala> new Foo {}.f
res11: String = not null

Scala FAQ中有完整的解释。摘抄:

Naturally when a val is overridden, it is not initialized more than once. So though x2 in the above example is seemingly defined at every point, this is not the case: an overridden val will appear to be null during the construction of superclasses, as will an abstract val.

避免这种情况的一种简单方法是使用延迟val或def，如果所引用的val可能会被覆盖。

另外，您可以使用 -Xcheckinit编译器标志来警告您潜在的初始化错误，例如这样。