added in API level 26

MethodType

public final class MethodType
extends Object implements Serializable

java.lang.Object
   ↳ java.lang.invoke.MethodType


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.

Summary

Public methods

MethodType appendParameterTypes(Class...<?> ptypesToInsert)

Finds or creates a method type with additional parameter types.

MethodType appendParameterTypes(List<Class<?>> ptypesToInsert)

Finds or creates a method type with additional parameter types.

MethodType changeParameterType(int num, Class<?> nptype)

Finds or creates a method type with a single different parameter type.

MethodType changeReturnType(Class<?> nrtype)

Finds or creates a method type with a different return type.

MethodType dropParameterTypes(int start, int end)

Finds or creates a method type with some parameter types omitted.

boolean equals(Object x)

Compares the specified object with this type for equality.

MethodType erase()

Erases all reference types to Object.

static MethodType fromMethodDescriptorString(String descriptor, ClassLoader loader)

Finds or creates an instance of a method type, given the spelling of its bytecode descriptor.

MethodType generic()

Converts all types, both reference and primitive, to Object.

static MethodType genericMethodType(int objectArgCount, boolean finalArray)

Finds or creates a method type whose components are Object with an optional trailing Object[] array.

static MethodType genericMethodType(int objectArgCount)

Finds or creates a method type whose components are all Object.

boolean hasPrimitives()

Reports if this type contains a primitive argument or return value.

boolean hasWrappers()

Reports if this type contains a wrapper argument or return value.

int hashCode()

Returns the hash code value for this method type.

MethodType insertParameterTypes(int num, Class...<?> ptypesToInsert)

Finds or creates a method type with additional parameter types.

MethodType insertParameterTypes(int num, List<Class<?>> ptypesToInsert)

Finds or creates a method type with additional parameter types.

static MethodType methodType(Class<?> rtype, MethodType ptypes)

Finds or creates a method type with the given components.

static MethodType methodType(Class<?> rtype, Class[]<?> ptypes)

Finds or creates an instance of the given method type.

static MethodType methodType(Class<?> rtype, Class<?> ptype0)

Finds or creates a method type with the given components.

static MethodType methodType(Class<?> rtype)

Finds or creates a method type with the given components.

static MethodType methodType(Class<?> rtype, List<Class<?>> ptypes)

Finds or creates a method type with the given components.

static MethodType methodType(Class<?> rtype, Class<?> ptype0, Class...<?> ptypes)

Finds or creates a method type with the given components.

Class[]<?> parameterArray()

Presents the parameter types as an array (a convenience method).

int parameterCount()

Returns the number of parameter types in this method type.

List<Class<?>> parameterList()

Presents the parameter types as a list (a convenience method).

Class<?> parameterType(int num)

Returns the parameter type at the specified index, within this method type.

Class<?> returnType()

Returns the return type of this method type.

String toMethodDescriptorString()

Produces a bytecode descriptor representation of the method type.

String toString()

Returns a string representation of the method type, of the form "(PT0,PT1...)RT".

MethodType unwrap()

Converts all wrapper types to their corresponding primitive types.

MethodType wrap()

Converts all primitive types to their corresponding wrapper types.

Inherited methods

Public methods

appendParameterTypes

added in API level 26
public MethodType appendParameterTypes (Class...<?> ptypesToInsert)

Finds or creates a method type with additional parameter types. Convenience method for methodType.

Parameters
ptypesToInsert Class: zero or more new parameter types to insert after the end of the parameter list

Returns
MethodType the same type, except with the selected parameter(s) appended

Throws
IllegalArgumentException if any element of ptypesToInsert is void.class or if the resulting method type would have more than 255 parameter slots
NullPointerException if ptypesToInsert or any of its elements is null

appendParameterTypes

added in API level 26
public MethodType appendParameterTypes (List<Class<?>> ptypesToInsert)

Finds or creates a method type with additional parameter types. Convenience method for methodType.

Parameters
ptypesToInsert List: zero or more new parameter types to insert after the end of the parameter list

Returns
MethodType the same type, except with the selected parameter(s) appended

Throws
IllegalArgumentException if any element of ptypesToInsert is void.class or if the resulting method type would have more than 255 parameter slots
NullPointerException if ptypesToInsert or any of its elements is null

changeParameterType

added in API level 26
public MethodType changeParameterType (int num, 
                Class<?> nptype)

Finds or creates a method type with a single different parameter type. Convenience method for methodType.

Parameters
num int: the index (zero-based) of the parameter type to change

nptype Class: a new parameter type to replace the old one with

Returns
MethodType the same type, except with the selected parameter changed

Throws
IndexOutOfBoundsException if num is not a valid index into parameterArray()
IllegalArgumentException if nptype is void.class
NullPointerException if nptype is null

changeReturnType

added in API level 26
public MethodType changeReturnType (Class<?> nrtype)

Finds or creates a method type with a different return type. Convenience method for methodType.

Parameters
nrtype Class: a return parameter type to replace the old one with

Returns
MethodType the same type, except with the return type change

Throws
NullPointerException if nrtype is null

dropParameterTypes

added in API level 26
public MethodType dropParameterTypes (int start, 
                int end)

Finds or creates a method type with some parameter types omitted. Convenience method for methodType.

Parameters
start int: the index (zero-based) of the first parameter type to remove

end int: the index (greater than start) of the first parameter type after not to remove

Returns
MethodType the same type, except with the selected parameter(s) removed

Throws
IndexOutOfBoundsException if start is negative or greater than parameterCount() or if end is negative or greater than parameterCount() or if start is greater than end

equals

added in API level 26
public boolean equals (Object x)

Compares the specified object with this type for equality. That is, it returns true if and only if the specified object is also a method type with exactly the same parameters and return type.

Parameters
x Object: object to compare

Returns
boolean true if this object is the same as the obj argument; false otherwise.

erase

added in API level 26
public MethodType erase ()

Erases all reference types to Object. Convenience method for methodType. All primitive types (including void) will remain unchanged.

Returns
MethodType a version of the original type with all reference types replaced

fromMethodDescriptorString

added in API level 26
public static MethodType fromMethodDescriptorString (String descriptor, 
                ClassLoader loader)

Finds or creates an instance of a method type, given the spelling of its bytecode descriptor. Convenience method for methodType. Any class or interface name embedded in the descriptor string will be resolved by calling ClassLoader.loadClass(java.lang.String) on the given loader (or if it is null, on the system class loader).

Note that it is possible to encounter method types which cannot be constructed by this method, because their component types are not all reachable from a common class loader.

This method is included for the benefit of applications that must generate bytecodes that process method handles and invokedynamic.

Parameters
descriptor String: a bytecode-level type descriptor string "(T...)T"

loader ClassLoader: the class loader in which to look up the types

Returns
MethodType a method type matching the bytecode-level type descriptor

Throws
NullPointerException if the string is null
IllegalArgumentException if the string is not well-formed
TypeNotPresentException if a named type cannot be found

generic

added in API level 26
public MethodType generic ()

Converts all types, both reference and primitive, to Object. Convenience method for genericMethodType. The expression type.wrap().erase() produces the same value as type.generic().

Returns
MethodType a version of the original type with all types replaced

genericMethodType

added in API level 26
public static MethodType genericMethodType (int objectArgCount, 
                boolean finalArray)

Finds or creates a method type whose components are Object with an optional trailing Object[] array. Convenience method for methodType. All parameters and the return type will be Object, except the final array parameter if any, which will be Object[].

Parameters
objectArgCount int: number of parameters (excluding the final array parameter if any)

finalArray boolean: whether there will be a trailing array parameter, of type Object[]

Returns
MethodType a generally applicable method type, for all calls of the given fixed argument count and a collected array of further arguments

Throws
IllegalArgumentException if objectArgCount is negative or greater than 255 (or 254, if finalArray is true)

genericMethodType

added in API level 26
public static MethodType genericMethodType (int objectArgCount)

Finds or creates a method type whose components are all Object. Convenience method for methodType. All parameters and the return type will be Object.

Parameters
objectArgCount int: number of parameters

Returns
MethodType a generally applicable method type, for all calls of the given argument count

Throws
IllegalArgumentException if objectArgCount is negative or greater than 255

hasPrimitives

added in API level 26
public boolean hasPrimitives ()

Reports if this type contains a primitive argument or return value. The return type void counts as a primitive.

Returns
boolean true if any of the types are primitives

hasWrappers

added in API level 26
public boolean hasWrappers ()

Reports if this type contains a wrapper argument or return value. Wrappers are types which box primitive values, such as Integer. The reference type java.lang.Void counts as a wrapper, if it occurs as a return type.

Returns
boolean true if any of the types are wrappers

hashCode

added in API level 26
public int hashCode ()

Returns the hash code value for this method type. It is defined to be the same as the hashcode of a List whose elements are the return type followed by the parameter types.

Returns
int the hash code value for this method type

insertParameterTypes

added in API level 26
public MethodType insertParameterTypes (int num, 
                Class...<?> ptypesToInsert)

Finds or creates a method type with additional parameter types. Convenience method for methodType.

Parameters
num int: the position (zero-based) of the inserted parameter type(s)

ptypesToInsert Class: zero or more new parameter types to insert into the parameter list

Returns
MethodType the same type, except with the selected parameter(s) inserted

Throws
IndexOutOfBoundsException if num is negative or greater than parameterCount()
IllegalArgumentException if any element of ptypesToInsert is void.class or if the resulting method type would have more than 255 parameter slots
NullPointerException if ptypesToInsert or any of its elements is null

insertParameterTypes

added in API level 26
public MethodType insertParameterTypes (int num, 
                List<Class<?>> ptypesToInsert)

Finds or creates a method type with additional parameter types. Convenience method for methodType.

Parameters
num int: the position (zero-based) of the inserted parameter type(s)

ptypesToInsert List: zero or more new parameter types to insert into the parameter list

Returns
MethodType the same type, except with the selected parameter(s) inserted

Throws
IndexOutOfBoundsException if num is negative or greater than parameterCount()
IllegalArgumentException if any element of ptypesToInsert is void.class or if the resulting method type would have more than 255 parameter slots
NullPointerException if ptypesToInsert or any of its elements is null

methodType

added in API level 26
public static MethodType methodType (Class<?> rtype, 
                MethodType ptypes)

Finds or creates a method type with the given components. Convenience method for methodType. The resulting method has the same parameter types as ptypes, and the specified return type.

Parameters
rtype Class: the return type

ptypes MethodType: the method type which supplies the parameter types

Returns
MethodType a method type with the given components

Throws
NullPointerException if rtype or ptypes is null

methodType

added in API level 26
public static MethodType methodType (Class<?> rtype, 
                Class[]<?> ptypes)

Finds or creates an instance of the given method type.

Parameters
rtype Class: the return type

ptypes Class: the parameter types

Returns
MethodType a method type with the given components

Throws
NullPointerException if rtype or ptypes or any element of ptypes is null
IllegalArgumentException if any element of ptypes is void.class

methodType

added in API level 26
public static MethodType methodType (Class<?> rtype, 
                Class<?> ptype0)

Finds or creates a method type with the given components. Convenience method for methodType. The resulting method has the single given parameter type.

Parameters
rtype Class: the return type

ptype0 Class: the parameter type

Returns
MethodType a method type with the given return value and parameter type

Throws
NullPointerException if rtype or ptype0 is null
IllegalArgumentException if ptype0 is void.class

methodType

added in API level 26
public static MethodType methodType (Class<?> rtype)

Finds or creates a method type with the given components. Convenience method for methodType. The resulting method has no parameter types.

Parameters
rtype Class: the return type

Returns
MethodType a method type with the given return value

Throws
NullPointerException if rtype is null

methodType

added in API level 26
public static MethodType methodType (Class<?> rtype, 
                List<Class<?>> ptypes)

Finds or creates a method type with the given components. Convenience method for methodType.

Parameters
rtype Class: the return type

ptypes List: the parameter types

Returns
MethodType a method type with the given components

Throws
NullPointerException if rtype or ptypes or any element of ptypes is null
IllegalArgumentException if any element of ptypes is void.class

methodType

added in API level 26
public static MethodType methodType (Class<?> rtype, 
                Class<?> ptype0, 
                Class...<?> ptypes)

Finds or creates a method type with the given components. Convenience method for methodType. The leading parameter type is prepended to the remaining array.

Parameters
rtype Class: the return type

ptype0 Class: the first parameter type

ptypes Class: the remaining parameter types

Returns
MethodType a method type with the given components

Throws
NullPointerException if rtype or ptype0 or ptypes or any element of ptypes is null
IllegalArgumentException if ptype0 or ptypes or any element of ptypes is void.class

parameterArray

added in API level 26
public Class[]<?> parameterArray ()

Presents the parameter types as an array (a convenience method). Changes to the array will not result in changes to the type.

Returns
Class[]<?> the parameter types (as a fresh copy if necessary)

parameterCount

added in API level 26
public int parameterCount ()

Returns the number of parameter types in this method type.

Returns
int the number of parameter types

parameterList

added in API level 26
public List<Class<?>> parameterList ()

Presents the parameter types as a list (a convenience method). The list will be immutable.

Returns
List<Class<?>> the parameter types (as an immutable list)

parameterType

added in API level 26
public Class<?> parameterType (int num)

Returns the parameter type at the specified index, within this method type.

Parameters
num int: the index (zero-based) of the desired parameter type

Returns
Class<?> the selected parameter type

Throws
IndexOutOfBoundsException if num is not a valid index into parameterArray()

returnType

added in API level 26
public Class<?> returnType ()

Returns the return type of this method type.

Returns
Class<?> the return type

toMethodDescriptorString

added in API level 26
public String toMethodDescriptorString ()

Produces a bytecode descriptor representation of the method type.

Note that this is not a strict inverse of fromMethodDescriptorString. Two distinct classes which share a common name but have different class loaders will appear identical when viewed within descriptor strings.

This method is included for the benefit of applications that must generate bytecodes that process method handles and invokedynamic. fromMethodDescriptorString, because the latter requires a suitable class loader argument.

Returns
String the bytecode type descriptor representation

toString

added in API level 26
public String toString ()

Returns a string representation of the method type, of the form "(PT0,PT1...)RT". The string representation of a method type is a parenthesis enclosed, comma separated list of type names, followed immediately by the return type.

Each type is represented by its simple name.

Returns
String a string representation of the object.

unwrap

added in API level 26
public MethodType unwrap ()

Converts all wrapper types to their corresponding primitive types. Convenience method for methodType. All primitive types (including void) will remain unchanged. A return type of java.lang.Void is changed to void.

Returns
MethodType a version of the original type with all wrapper types replaced

wrap

added in API level 26
public MethodType wrap ()

Converts all primitive types to their corresponding wrapper types. Convenience method for methodType. All reference types (including wrapper types) will remain unchanged. A void return type is changed to the type java.lang.Void. The expression type.wrap().erase() produces the same value as type.generic().

Returns
MethodType a version of the original type with all primitive types replaced