CODEWITHSUNDEEP
javajava-12
William Entriken
William Entriken

Reputation: 39303

Why is describeConstable an optional?

Under what circumstance would someEnum.describeConstable() fail to return an EnumDesc?

i.e. it would return an empty Optional.

Upvotes: 2

Views: 3785

Answers (2)

Turing85
Turing85

Reputation: 20195

The method Optional<? extends ConstantDesc> describeConstable() stems from the interface Constable, not from Enum. From Constable's documentation:

...

The nominal form of an instance of a constable type is obtained via describeConstable(). 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.)

See The Java Virtual Machine Specification:
4.4 The Constant Pool
4.4.10 The CONSTANT_InvokeDynamic_info Structure

...

Upvotes: 6

Andreas
Andreas

Reputation: 159165

describeConstable() is a method inherited from interface Constable, where the description is:

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

Sure, in some classes, e.g. Integer, it can never be empty:

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

But the return value is an Optional because that is not true for all implementations of the interface.

The javadoc of Constable has this to say:

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.)

Upvotes: 7

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