Module java.base
Package java.lang.reflect

Class Constructor<T>

java.lang.Object
java.lang.reflect.AccessibleObject
java.lang.reflect.Executable
java.lang.reflect.Constructor<T>
Type Parameters:
T - the class in which the constructor is declared
All Implemented Interfaces:
AnnotatedElement, GenericDeclaration, Member
public final class Constructor<T> extends Executable
Constructor provides information about, and access to, a single constructor for a class.

Constructor permits widening conversions to occur when matching the actual parameters to newInstance() with the underlying constructor's formal parameters, but throws an IllegalArgumentException if a narrowing conversion would occur.

Since:
1.1
See Also:
Member, Class, Class.getConstructors(), Class.getConstructor(Class[]), Class.getDeclaredConstructors()

  • Method Details

    • setAccessible

      public void setAccessible​(boolean flag)
      Set the accessible flag for this reflected object to the indicated boolean value. A value of true indicates that the reflected object should suppress checks for Java language access control when it is used. A value of false indicates that the reflected object should enforce checks for Java language access control when it is used, with the variation noted in the class description.

      This method may be used by a caller in class C to enable access to a member of declaring class D if any of the following hold:

      • C and D are in the same module.
      • The member is public and D is public in a package that the module containing D exports to at least the module containing C.
      • The member is protected static, D is public in a package that the module containing D exports to at least the module containing C, and C is a subclass of D.
      • D is in a package that the module containing D opens to at least the module containing C. All packages in unnamed and open modules are open to all modules and so this method always succeeds when D is in an unnamed or open module.

      This method cannot be used to enable access to private members, members with default (package) access, protected instance members, or protected constructors when the declaring class is in a different module to the caller and the package containing the declaring class is not open to the caller's module.

      This method cannot be used to enable write access to a non-modifiable final field. The following fields are non-modifiable:

      • static final fields declared in any class or interface
      • final fields declared in a hidden class
      • final fields declared in a record

      The accessible flag when true suppresses Java language access control checks to only enable read access to these non-modifiable final fields.

      If there is a security manager, its checkPermission method is first called with a ReflectPermission("suppressAccessChecks") permission.

      A SecurityException is also thrown if this object is a Constructor object for the class Class and flag is true.

      Overrides:
      setAccessible in class AccessibleObject
      Parameters:
      flag - the new value for the accessible flag
      Throws:
      InaccessibleObjectException - if access cannot be enabled
      SecurityException - if the request is denied by the security manager or this is a constructor for java.lang.Class
      See Also:
      AccessibleObject.trySetAccessible(), MethodHandles.privateLookupIn(java.lang.Class<?>, java.lang.invoke.MethodHandles.Lookup)

    • getDeclaringClass

      public Class<T> getDeclaringClass()
      Returns the Class object representing the class that declares the constructor represented by this object.
      Specified by:
      getDeclaringClass in interface Member
      Specified by:
      getDeclaringClass in class Executable
      Returns:
      an object representing the declaring class of the underlying member

    • getName

      public String getName()
      Returns the name of this constructor, as a string. This is the binary name of the constructor's declaring class.
      Specified by:
      getName in interface Member
      Specified by:
      getName in class Executable
      Returns:
      the simple name of the underlying member

    • getModifiers

      public int getModifiers()
      Returns the Java language modifiers for the executable represented by this object.
      Specified by:
      getModifiers in interface Member
      Specified by:
      getModifiers in class Executable
      Returns:
      the Java language modifiers for the underlying member
      See Also:
      Modifier

    • getTypeParameters

      public TypeVariable<Constructor<T>>[] getTypeParameters()
      Returns an array of TypeVariable objects that represent the type variables declared by the generic declaration represented by this GenericDeclaration object, in declaration order. Returns an array of length 0 if the underlying generic declaration declares no type variables.
      Specified by:
      getTypeParameters in interface GenericDeclaration
      Specified by:
      getTypeParameters in class Executable
      Returns:
      an array of TypeVariable objects that represent the type variables declared by this generic declaration
      Throws:
      GenericSignatureFormatError - if the generic signature of this generic declaration does not conform to the format specified in The Java Virtual Machine Specification
      Since:
      1.5

    • getParameterTypes

      public Class<?>[] getParameterTypes()
      Returns an array of Class objects that represent the formal parameter types, in declaration order, of the executable represented by this object. Returns an array of length 0 if the underlying executable takes no parameters. Note that the constructors of some inner classes may have an implicitly declared parameter in addition to explicitly declared ones.
      Specified by:
      getParameterTypes in class Executable
      Returns:
      the parameter types for the executable this object represents

    • getParameterCount

      public int getParameterCount()
      Returns the number of formal parameters (whether explicitly declared or implicitly declared or neither) for the executable represented by this object.
      Overrides:
      getParameterCount in class Executable
      Returns:
      The number of formal parameters for the executable this object represents
      Since:
      1.8

    • getGenericParameterTypes

      public Type[] getGenericParameterTypes()
      Returns an array of Type objects that represent the formal parameter types, in declaration order, of the executable represented by this object. Returns an array of length 0 if the underlying executable takes no parameters. Note that the constructors of some inner classes may have an implicitly declared parameter in addition to explicitly declared ones.

      If a formal parameter type is a parameterized type, the Type object returned for it must accurately reflect the actual type arguments used in the source code.

      If a formal parameter type is a type variable or a parameterized type, it is created. Otherwise, it is resolved.

      Overrides:
      getGenericParameterTypes in class Executable
      Returns:
      an array of Types that represent the formal parameter types of the underlying executable, in declaration order
      Throws:
      GenericSignatureFormatError - if the generic method signature does not conform to the format specified in The Java Virtual Machine Specification
      TypeNotPresentException - if any of the parameter types of the underlying executable refers to a non-existent type declaration
      MalformedParameterizedTypeException - if any of the underlying executable's parameter types refer to a parameterized type that cannot be instantiated for any reason
      Since:
      1.5

    • getExceptionTypes

      public Class<?>[] getExceptionTypes()
      Returns an array of Class objects that represent the types of exceptions declared to be thrown by the underlying executable represented by this object. Returns an array of length 0 if the executable declares no exceptions in its throws clause.
      Specified by:
      getExceptionTypes in class Executable
      Returns:
      the exception types declared as being thrown by the executable this object represents

    • getGenericExceptionTypes

      public Type[] getGenericExceptionTypes()
      Returns an array of Type objects that represent the exceptions declared to be thrown by this executable object. Returns an array of length 0 if the underlying executable declares no exceptions in its throws clause.

      If an exception type is a type variable or a parameterized type, it is created. Otherwise, it is resolved.

      Overrides:
      getGenericExceptionTypes in class Executable
      Returns:
      an array of Types that represent the exception types thrown by the underlying executable
      Throws:
      GenericSignatureFormatError - if the generic method signature does not conform to the format specified in The Java Virtual Machine Specification
      TypeNotPresentException - if the underlying executable's throws clause refers to a non-existent type declaration
      MalformedParameterizedTypeException - if the underlying executable's throws clause refers to a parameterized type that cannot be instantiated for any reason
      Since:
      1.5

    • equals

      public boolean equals​(Object obj)
      Compares this Constructor against the specified object. Returns true if the objects are the same. Two Constructor objects are the same if they were declared by the same class and have the same formal parameter types.
      Overrides:
      equals in class Object
      Parameters:
      obj - the reference object with which to compare.
      Returns:
      true if this object is the same as the obj argument; false otherwise.
      See Also:
      Object.hashCode(), HashMap

    • hashCode

      public int hashCode()
      Returns a hashcode for this Constructor. The hashcode is the same as the hashcode for the underlying constructor's declaring class name.
      Overrides:
      hashCode in class Object
      Returns:
      a hash code value for this object.
      See Also:
      Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)

    • toString

      public String toString()
      Returns a string describing this Constructor. The string is formatted as the constructor access modifiers, if any, followed by the fully-qualified name of the declaring class, followed by a parenthesized, comma-separated list of the constructor's formal parameter types. For example: 
      
    public java.util.Hashtable(int,float)

      If the constructor is declared to throw exceptions, the parameter list is followed by a space, followed by the word "throws" followed by a comma-separated list of the thrown exception types.

      The only possible modifiers for constructors are the access modifiers public, protected or private. Only one of these may appear, or none if the constructor has default (package) access.

      Overrides:
      toString in class Object
      Returns:
      a string describing this Constructor
      See Java Language Specification:
      8.8.3 Constructor Modifiers
      8.9.2 Enum Body Declarations

    • toGenericString

      public String toGenericString()
      Returns a string describing this Constructor, including type parameters. The string is formatted as the constructor access modifiers, if any, followed by an angle-bracketed comma separated list of the constructor's type parameters, if any, including informative bounds of the type parameters, if any, followed by the fully-qualified name of the declaring class, followed by a parenthesized, comma-separated list of the constructor's generic formal parameter types. If this constructor was declared to take a variable number of arguments, instead of denoting the last parameter as "Type[]", it is denoted as "Type...". A space is used to separate access modifiers from one another and from the type parameters or class name. If there are no type parameters, the type parameter list is elided; if the type parameter list is present, a space separates the list from the class name. If the constructor is declared to throw exceptions, the parameter list is followed by a space, followed by the word "throws" followed by a comma-separated list of the generic thrown exception types.

      The only possible modifiers for constructors are the access modifiers public, protected or private. Only one of these may appear, or none if the constructor has default (package) access.

      Specified by:
      toGenericString in class Executable
      Returns:
      a string describing this Constructor, include type parameters
      See Java Language Specification:
      8.8.3 Constructor Modifiers
      8.9.2 Enum Body Declarations
      Since:
      1.5

    • newInstance

      public T newInstance​(Object... initargs) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException
      Uses the constructor represented by this Constructor object to create and initialize a new instance of the constructor's declaring class, with the specified initialization parameters. Individual parameters are automatically unwrapped to match primitive formal parameters, and both primitive and reference parameters are subject to method invocation conversions as necessary.

      If the number of formal parameters required by the underlying constructor is 0, the supplied initargs array may be of length 0 or null.

      If the constructor's declaring class is an inner class in a non-static context, the first argument to the constructor needs to be the enclosing instance; see section 15.9.3 of The Java Language Specification.

      If the required access and argument checks succeed and the instantiation will proceed, the constructor's declaring class is initialized if it has not already been initialized.

      If the constructor completes normally, returns the newly created and initialized instance.

      Parameters:
      initargs - array of objects to be passed as arguments to the constructor call; values of primitive types are wrapped in a wrapper object of the appropriate type (e.g. a float in a Float)
      Returns:
      a new object created by calling the constructor this object represents
      Throws:
      IllegalAccessException - if this Constructor object is enforcing Java language access control and the underlying constructor is inaccessible.
      IllegalArgumentException - if the number of actual and formal parameters differ; if an unwrapping conversion for primitive arguments fails; or if, after possible unwrapping, a parameter value cannot be converted to the corresponding formal parameter type by a method invocation conversion; if this constructor pertains to an enum class.
      InstantiationException - if the class that declares the underlying constructor represents an abstract class.
      InvocationTargetException - if the underlying constructor throws an exception.
      ExceptionInInitializerError - if the initialization provoked by this method fails.

    • isVarArgs

      public boolean isVarArgs()
      Returns true if this executable was declared to take a variable number of arguments; returns false otherwise.
      Overrides:
      isVarArgs in class Executable
      Returns:
      true if an only if this executable was declared to take a variable number of arguments.
      Since:
      1.5

    • isSynthetic

      public boolean isSynthetic()
      Returns true if this executable is a synthetic construct; returns false otherwise.
      Specified by:
      isSynthetic in interface Member
      Overrides:
      isSynthetic in class Executable
      Returns:
      true if and only if this executable is a synthetic construct as defined by The Java Language Specification.
      See Java Language Specification:
      13.1 The Form of a Binary
      Since:
      1.5

    • getAnnotation

      public <T extends Annotation> T getAnnotation​(Class<T> annotationClass)
      Returns this element's annotation for the specified type if such an annotation is present, else null.

      Note that any annotation returned by this method is a declaration annotation.

      Specified by:
      getAnnotation in interface AnnotatedElement
      Overrides:
      getAnnotation in class Executable
      Type Parameters:
      T - the type of the annotation to query for and return if present
      Parameters:
      annotationClass - the Class object corresponding to the annotation type
      Returns:
      this element's annotation for the specified annotation type if present on this element, else null
      Throws:
      NullPointerException - if the given annotation class is null
      Since:
      1.5

    • getDeclaredAnnotations

      public Annotation[] getDeclaredAnnotations()
      Returns annotations that are directly present on this element. This method ignores inherited annotations. If there are no annotations directly present on this element, the return value is an array of length 0. The caller of this method is free to modify the returned array; it will have no effect on the arrays returned to other callers.

      Note that any annotations returned by this method are declaration annotations.

      Specified by:
      getDeclaredAnnotations in interface AnnotatedElement
      Overrides:
      getDeclaredAnnotations in class AccessibleObject
      Returns:
      annotations directly present on this element
      Since:
      1.5

    • getParameterAnnotations

      public Annotation[][] getParameterAnnotations()
      Returns an array of arrays of Annotations that represent the annotations on the formal parameters, in declaration order, of the Executable represented by this object. Synthetic and mandated parameters (see explanation below), such as the outer "this" parameter to an inner class constructor will be represented in the returned array. If the executable has no parameters (meaning no formal, no synthetic, and no mandated parameters), a zero-length array will be returned. If the Executable has one or more parameters, a nested array of length zero is returned for each parameter with no annotations. The annotation objects contained in the returned arrays are serializable. The caller of this method is free to modify the returned arrays; it will have no effect on the arrays returned to other callers. A compiler may add extra parameters that are implicitly declared in source ("mandated"), as well as parameters that are neither implicitly nor explicitly declared in source ("synthetic") to the parameter list for a method. See Parameter for more information.

      Note that any annotations returned by this method are declaration annotations.

      Specified by:
      getParameterAnnotations in class Executable
      Returns:
      an array of arrays that represent the annotations on the formal and implicit parameters, in declaration order, of the executable represented by this object
      Since:
      1.5
      See Also:
      Parameter, Parameter.getAnnotations()

    • getAnnotatedReturnType

      public AnnotatedType getAnnotatedReturnType()
      Returns an AnnotatedType object that represents the use of a type to specify the return type of the method/constructor represented by this Executable. If this Executable object represents a constructor, the AnnotatedType object represents the type of the constructed object. If this Executable object represents a method, the AnnotatedType object represents the use of a type to specify the return type of the method.
      Specified by:
      getAnnotatedReturnType in class Executable
      Returns:
      an object representing the return type of the method or constructor represented by this Executable
      Since:
      1.8

    • getAnnotatedReceiverType

      public AnnotatedType getAnnotatedReceiverType()
      Returns an AnnotatedType object that represents the use of a type to specify the receiver type of the method/constructor represented by this Executable object. The receiver type of a method/constructor is available only if the method/constructor has a receiver parameter (JLS 8.4.1). If this Executable object represents an instance method or represents a constructor of an inner member class, and the method/constructor either has no receiver parameter or has a receiver parameter with no annotations on its type, then the return value is an AnnotatedType object representing an element with no annotations. If this Executable object represents a static method or represents a constructor of a top level, static member, local, or anonymous class, then the return value is null.
      Overrides:
      getAnnotatedReceiverType in class Executable
      Returns:
      an object representing the receiver type of the method or constructor represented by this Executable or null if this Executable can not have a receiver parameter
      Since:
      1.8