Class MethodType

public final class MethodType extends Object implements Constable, TypeDescriptor.OfMethod<Class<?>,MethodType>, Serializable

A method type represents the arguments and return type accepted and returned by a method handle, or the arguments and return type passed and expected by a method handle caller. Method types must be properly matched between a method handle and all its callers, and the JVM's operations enforce this matching at, specifically during calls to MethodHandle.invokeExact and MethodHandle.invoke, and during execution of invokedynamic instructions.

The structure is a return type accompanied by any number of parameter types. The types (primitive, void, and reference) are represented by Class objects. (For ease of exposition, we treat void as if it were a type. In fact, it denotes the absence of a return type.)

All instances of MethodType are immutable. Two instances are completely interchangeable if they compare equal. Equality depends on pairwise correspondence of the return and parameter types and on nothing else.

This type can be created only by factory methods. All factory methods may cache values, though caching is not guaranteed. Some factory methods are static, while others are virtual methods which modify precursor method types, e.g., by changing a selected parameter.

Factory methods which operate on groups of parameter types are systematically presented in two versions, so that both Java arrays and Java lists can be used to work with groups of parameter types. The query methods parameterArray and parameterList also provide a choice between arrays and lists.

MethodType objects are sometimes derived from bytecode instructions such as invokedynamic, specifically from the type descriptor strings associated with the instructions in a class file's constant pool.

Like classes and strings, method types can also be represented directly in a class file's constant pool as constants. A method type may be loaded by an ldc instruction which refers to a suitable CONSTANT_MethodType constant pool entry. The entry refers to a CONSTANT_Utf8 spelling for the descriptor string. (For full details on method type constants, see sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.)

When the JVM materializes a MethodType from a descriptor string, all classes named in the descriptor must be accessible, and will be loaded. (But the classes need not be initialized, as is the case with a CONSTANT_Class.) This loading may occur at any time before the MethodType object is first derived.

Nominal Descriptors

A MethodType can be described in nominal form if and only if all of the parameter types and return type can be described with a nominal descriptor represented by ClassDesc. If a method type can be described nominally, then:

• The method type has a nominal descriptor returned by MethodType::describeConstable.
• The descriptor string returned by MethodType::descriptorString or MethodType::toMethodDescriptorString for the method type is a method descriptor (JVMS 4.3.3).

If any of the parameter types or return type cannot be described nominally, i.e. Class::describeConstable returns an empty optional for that type, then the method type cannot be described nominally:

• The method type has no nominal descriptor and MethodType::describeConstable returns an empty optional.
• The descriptor string returned by MethodType::descriptorString or MethodType::toMethodDescriptorString for the method type is not a type descriptor.