scala - null.==(obj) 不会在 Scala 中抛出空指针异常

Question

我是 Scala 的新手。请帮助我理解下面的代码片段

null.==("goutam") // ---> return false
null.equals("goutam") // ---> throw NullPointerException

根据我的理解，null 是 Null tr​​ait 的唯一实例，它扩展了 Anyref 和 == 并且 equals 都是 AnyRef 的函数。那么为什么第一个语句不会抛出但第二个语句会抛出？

Answer 1

Why first statement does not throw but second one does

根据语言规范 (6.3)，在 null 上有特定的方法。这不会导致 NullReferenceException调用时发生。它们被定义为:

6.3 The Null Value

The null value is of type scala.Null, and is thus compatible with every reference type. It denotes a reference value which refers to a special “null” object. This object implements methods in class scala.AnyRef as follows:

• eq(x) and ==(x) return true iff the argument x is also the "null" object.
• ne(x) and !=(x) return true iff the argument x is not also the "null" object.
• isInstanceOf[T] always returns false.
• asInstanceOf[T] returns the default value of type T.
• ## returns 0.

A reference to any other member of the "null" object causes a NullPointerException to be thrown.

equals在 AnyRef 上定义并且不处理 null根据定义的值。 eq ，可以使用检查引用相等性(这通常不是您想要做的):

scala> null.==("goutam")
res0: Boolean = false

scala> null.eq("goutam")
res1: Boolean = false

==处理null正确，这就是你应该使用的。更多详情请见 Whats the difference between == and .equals in Scala?