Module java.base

Package java.lang.constant

Interface Constable

All Known Implementing Classes:

Character.UnicodeScript, Class, DirectMethodHandleDesc.Kind, Double, Enum, Float, Integer, Long, MethodHandle, MethodType, ProcessBuilder.Redirect.Type, StackWalker.Option, String, System.Logger.Level, Thread.State, VarHandle, VarHandle.AccessMode

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.)

Since:

12

Method Summary

Modifier and Type	Method	Description
java.util.Optional<? extends ConstantDesc>	describeConstable()	Return a nominal descriptor for this instance, if one can be constructed, or an empty Optional if one cannot be.

Method Detail

describeConstable

java.util.Optional<? extends ConstantDesc> describeConstable()

Return a nominal descriptor for this instance, if one can be constructed, or an empty Optional if one cannot be.

Returns:

An Optional containing the resulting nominal descriptor, or an empty Optional if one cannot be constructed.