Module java.base

Interface Constable

public 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 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 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.)

See Java Virtual Machine Specification:
4.4 The Constant Pool
4.4.10 The CONSTANT_Dynamic_info and CONSTANT_InvokeDynamic_info Structures