在Scala中，“ null == last”和“ null eq last”有什么区别？ [英] What's the difference between `null == last` and `null eq last`, in Scala?

问题描述

我在scala 2.7.7的内置类 MessageQueue.scala 中看到，在第164行附近：

I see in the built-in class MessageQueue.scala of scala 2.7.7, around line 164, it's:

def extractFirst(p: Any => Boolean): MessageQueueElement = {
changeSize(-1) // assume size decreases by 1

val msg = if (null eq last) null
else {
    ...
  }
}

我不明白 val msg = if（null eq last）null 很好，为什么它使用 eq ，但不是 null 。如果我写 if（last == null）null ，是否正确？有什么区别吗？

I don't understand val msg = if (null eq last) null well, why it uses eq, but not null. If I write if (last==null) null, is it correct? Is there any difference?

推荐答案

当 == 的任何一侧时是 null 或，如果的第一个操作数== 计算为为空，那么Scala将不会调用等于。因此，在这种情况下，是的， x == null 与 x eq null 相同； 不会被调用 equals 方法。请注意以下情况。

When either side of the == is null or if the first operand of == evaluates to null, then Scala will not invoke equals. So, in this case, yes, x == null is the same as x eq null; the equals method is not invoked. Pay attention to the cases below.

请考虑以下问题：

class X {
   // this is just for testing
   // any equals that returns true when the other object is null
   // is arguably broken. thus even though it may be invoked
   // the end semantics should remain the same
   override def equals(x: Any) = true
}
var x = new X()
x == null // false -- compiler optimization?
null == x // false
var y = null
y == x // false -- see documentation below, y is null, x is not
x == y // true  -- x does not evaluate to null, equals invokes
x eq y // false

并注意：

(new X()) == null

导致警告说新鲜物体将永远不等于（为空）。

Results in a warning saying a "a fresh object" will never be equal (to null).

我怀疑 x == y 发出的代码可能比 x == null 发出的代码略多/不同

I suspect there may be slightly more/different code emitted for x == y than x == null (in case the equals must be invoked), but have not checked.

快乐编码。

Scala语言规范的6.3节（空值）说：

Section 6.3 (The Null Value) of the Scala Language Specification has this to say:

null值的类型为scala.Null，因此与每个引用
类型都兼容。它表示参考值，该参考值引用特殊的空对象。此对象
在scala.AnyRef类中实现方法，如下所示：

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:

• [null] eq（x）和[ null] ==（x）如果参数x也是空对象，则返回true。


• ne（x）和！=（x）当true时返回true参数x也不是 null对象。


• isInstanceOf [T]始终返回false。


• asInstanceOf [T]返回 null如果T符合
  scala.AnyRef，则对象本身，否则抛出NullPointerException。

• [null] eq( x ) and [null] ==( 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 "null" object itself if T conforms to scala.AnyRef, and throws a NullPointerException otherwise.

对任何其他成员的引用 null对象将引发
NullPointerException。

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

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