Module java.base
Package java.lang.invoke

Class MethodType

java.lang.Object
java.lang.invoke.MethodType
All Implemented Interfaces:
Serializable, Constable, TypeDescriptor, TypeDescriptor.OfMethod<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:

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:

Since:
1.7
See Also:

  • Method Details

    • methodType

      public static MethodType methodType(Class<?> rtype, Class<?>[] ptypes)
      Finds or creates an instance of the given method type.
      Parameters:
      rtype - the return type
      ptypes - the parameter types
      Returns:
      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

      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 - the return type
      ptypes - the parameter types
      Returns:
      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

      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 - the return type
      ptype0 - the first parameter type
      ptypes - the remaining parameter types
      Returns:
      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

    • methodType

      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 - the return type
      Returns:
      a method type with the given return value
      Throws:
      NullPointerException - if rtype is null

    • methodType

      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 - the return type
      ptype0 - the parameter type
      Returns:
      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

      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 - the return type
      ptypes - the method type which supplies the parameter types
      Returns:
      a method type with the given components
      Throws:
      NullPointerException - if rtype or ptypes is null

    • genericMethodType

      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 - number of parameters (excluding the final array parameter if any)
      finalArray - whether there will be a trailing array parameter, of type Object[]
      Returns:
      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)
      See Also:

    • genericMethodType

      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 - number of parameters
      Returns:
      a generally applicable method type, for all calls of the given argument count
      Throws:
      IllegalArgumentException - if objectArgCount is negative or greater than 255
      See Also:

    • changeParameterType

      public MethodType changeParameterType(int num, Class<?> nptype)
      Finds or creates a method type with a single different parameter type. Convenience method for methodType.
      Specified by:
      changeParameterType in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Parameters:
      num - the index (zero-based) of the parameter type to change
      nptype - a new parameter type to replace the old one with
      Returns:
      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

    • insertParameterTypes

      public MethodType insertParameterTypes(int num, Class<?>... ptypesToInsert)
      Finds or creates a method type with additional parameter types. Convenience method for methodType.
      Specified by:
      insertParameterTypes in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Parameters:
      num - the position (zero-based) of the inserted parameter type(s)
      ptypesToInsert - zero or more new parameter types to insert into the parameter list
      Returns:
      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

    • appendParameterTypes

      public MethodType appendParameterTypes(Class<?>... ptypesToInsert)
      Finds or creates a method type with additional parameter types. Convenience method for methodType.
      Parameters:
      ptypesToInsert - zero or more new parameter types to insert after the end of the parameter list
      Returns:
      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

    • insertParameterTypes

      public MethodType insertParameterTypes(int num, List<Class<?>> ptypesToInsert)
      Finds or creates a method type with additional parameter types. Convenience method for methodType.
      Parameters:
      num - the position (zero-based) of the inserted parameter type(s)
      ptypesToInsert - zero or more new parameter types to insert into the parameter list
      Returns:
      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

    • appendParameterTypes

      public MethodType appendParameterTypes(List<Class<?>> ptypesToInsert)
      Finds or creates a method type with additional parameter types. Convenience method for methodType.
      Parameters:
      ptypesToInsert - zero or more new parameter types to insert after the end of the parameter list
      Returns:
      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

    • dropParameterTypes

      public MethodType dropParameterTypes(int start, int end)
      Finds or creates a method type with some parameter types omitted. Convenience method for methodType.
      Specified by:
      dropParameterTypes in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Parameters:
      start - the index (zero-based) of the first parameter type to remove
      end - the index (greater than start) of the first parameter type after not to remove
      Returns:
      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

    • changeReturnType

      public MethodType changeReturnType(Class<?> nrtype)
      Finds or creates a method type with a different return type. Convenience method for methodType.
      Specified by:
      changeReturnType in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Parameters:
      nrtype - a return parameter type to replace the old one with
      Returns:
      the same type, except with the return type change
      Throws:
      NullPointerException - if nrtype is null

    • hasPrimitives

      public boolean hasPrimitives()
      Reports if this type contains a primitive argument or return value. The return type void counts as a primitive.
      Returns:
      true if any of the types are primitives

    • hasWrappers

      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:
      true if any of the types are wrappers

    • erase

      public MethodType erase()
      Erases all reference types to Object. Convenience method for methodType. All primitive types (including void) will remain unchanged.
      Returns:
      a version of the original type with all reference types replaced

    • generic

      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:
      a version of the original type with all types replaced

    • wrap

      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:
      a version of the original type with all primitive types replaced

    • unwrap

      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:
      a version of the original type with all wrapper types replaced

    • parameterType

      public Class<?> parameterType(int num)
      Returns the parameter type at the specified index, within this method type.
      Specified by:
      parameterType in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Parameters:
      num - the index (zero-based) of the desired parameter type
      Returns:
      the selected parameter type
      Throws:
      IndexOutOfBoundsException - if num is not a valid index into parameterArray()

    • parameterCount

      public int parameterCount()
      Returns the number of parameter types in this method type.
      Specified by:
      parameterCount in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Returns:
      the number of parameter types

    • returnType

      public Class<?> returnType()
      Returns the return type of this method type.
      Specified by:
      returnType in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Returns:
      the return type

    • parameterList

      public List<Class<?>> parameterList()
      Presents the parameter types as a list (a convenience method). The list will be immutable.
      Specified by:
      parameterList in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Returns:
      the parameter types (as an immutable list)

    • lastParameterType

      public Class<?> lastParameterType()
      Returns the last parameter type of this method type. If this type has no parameters, the sentinel value void.class is returned instead.
      API Note:

      The sentinel value is chosen so that reflective queries can be made directly against the result value. The sentinel value cannot be confused with a real parameter, since void is never acceptable as a parameter type. For variable arity invocation modes, the expression lastParameterType().getComponentType() is useful to query the type of the "varargs" parameter.

      Returns:
      the last parameter type if any, else void.class
      Since:
      10

    • parameterArray

      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.
      Specified by:
      parameterArray in interface TypeDescriptor.OfMethod<Class<?>,MethodType>
      Returns:
      the parameter types (as a fresh copy if necessary)

    • equals

      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.
      Overrides:
      equals in class Object
      Parameters:
      x - object to compare
      Returns:
      true if this object is the same as the obj argument; false otherwise.
      See Also:

    • hashCode

      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.
      Overrides:
      hashCode in class Object
      Returns:
      the hash code value for this method type
      See Also:

    • toString

      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.

      Overrides:
      toString in class Object
      Returns:
      a string representation of the object.

    • fromMethodDescriptorString

      public static MethodType fromMethodDescriptorString(String descriptor, ClassLoader loader) throws IllegalArgumentException, TypeNotPresentException
      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 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 - a bytecode-level type descriptor string "(T...)T"
      loader - the class loader in which to look up the types
      Returns:
      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
      SecurityException - if the security manager is present and loader is null and the caller does not have the RuntimePermission("getClassLoader")

    • toMethodDescriptorString

      public String toMethodDescriptorString()
      Returns a descriptor string for the method type. This method is equivalent to calling MethodType::descriptorString.

      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:
      the descriptor string for this method type
      See Java Virtual Machine Specification:
      4.3.3 Method Descriptors
      See Also:

    • descriptorString

      public String descriptorString()
      Returns a descriptor string for this method type.

      If this method type can be described nominally, then the result is a method type descriptor (JVMS 4.3.3). MethodTypeDesc for this method type can be produced by calling MethodTypeDesc::ofDescriptor with the result descriptor string.

      If this method type cannot be described nominally and the result is a string of the form:

      "(<parameter-descriptors>)<return-descriptor>"
      where <parameter-descriptors> is the concatenation of the descriptor string of all of the parameter types and the descriptor string of the return type. No MethodTypeDesc can be produced from the result string.
      Specified by:
      descriptorString in interface TypeDescriptor
      Returns:
      the descriptor string for this method type
      See Java Virtual Machine Specification:
      4.3.3 Method Descriptors
      Since:
      12
      See Also:

    • describeConstable

      public Optional<MethodTypeDesc> describeConstable()
      Returns a nominal descriptor for this instance, if one can be constructed, or an empty Optional if one cannot be.
      Specified by:
      describeConstable in interface Constable
      Returns:
      An Optional containing the resulting nominal descriptor, or an empty Optional if one cannot be constructed.
      Since:
      12
      See Also: