java - 为什么describeConstable是可选的？

Question

在什么情况下，someEnum.describeConstable()会无法返回EnumDesc？

即它将返回一个空的Optional。

Answer 1

describeConstable() 是从接口(interface) Constable 继承的方法，其中描述为:

Returns an Optional containing the nominal descriptor for this instance, if one can be constructed, or an empty Optional if one cannot be constructed.

当然，在某些类(class)中，例如Integer ，它永远不能为空:

Returns an Optional containing the nominal descriptor for this instance, which is the instance itself.

但返回值是一个可选，因为并非所有接口(interface)实现都如此。

Constable 的 javadoc有这样说:

A Constable need not be able to (or may choose not to) describe all its instances in the form of a ConstantDesc; this method returns an Optional that can be empty to indicate that a nominal descriptor could not be created for an instance. (For example, MethodHandle will produce nominal descriptors for direct method handles, but not necessarily those produced by method handle combinators.)