Interface Constable

Represents a type which is constable . A constable type is one whose values are constants that can be represented in the constant pool of a Java classfile as described in JVMS Moved out of a link with destination https://docs.oracle.com/javase/specs/jvms/se20/html/jvms-4.html#jvms-4.4.Moved to a link with destination https://docs.oracle.com/javase/specs/jvms/se21/html/jvms-4.html#jvms-4.4.4.4, and whose instances can describe themselves nominally as a ConstantDesc .

Some constable types have a native representation in the constant pool: String , Integer , Long , Float , Double , Class , MethodType , and MethodHandle . The types String , Integer , Long , Float , and Double serve as their own nominal descriptors; Class , MethodType , and MethodHandle have corresponding nominal descriptors ClassDesc , MethodTypeDesc , and MethodHandleDesc .

Other reference types can be constable if their instances can describe themselves in nominal form as a ConstantDesc . Examples in the Java SE Platform API are types that support Java language features such as Enum , and runtime support classes such as VarHandle . These are typically described with a DynamicConstantDesc , which describes dynamically generated constants (JVMS Moved out of a link with destination https://docs.oracle.com/javase/specs/jvms/se20/html/jvms-4.html#jvms-4.4.10.Moved to a link with destination https://docs.oracle.com/javase/specs/jvms/se21/html/jvms-4.html#jvms-4.4.10.4.4.10).

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.)