Java Core Reflection

The class java.lang.Class

    package java.lang;

    import java.lang.reflect.Field;

    import java.lang.reflect.Method;

    import java.lang.reflect.Constructor;

    public final class Class extends Object

Instances of the class Class represent Java types in a way that allows them to be manipulated by a running Java program. Every array also belongs to a class that is reflected as a Class object that is shared by all arrays with the same element type and number of dimensions. Finally, the eight primitive Java types and void are also represented by unique Class objects.

There are no public constructors for class Class. The Java Virtual Machine automatically constructs Class objects when new classes are loaded; such objects cannot be created by user programs.

While the class Class properly belongs in the java.lang.reflect package, it remains in java.lang for backwards compatibility.

Methods

The class Class is augmented with new methods to:

determine if a Class object represents an array type
determine if a Class object represents a primitive type
determine the Java language modifiers of the represented class type
reflect the members and constructors of the represented type
determine if an object is an instance of the represented class, or implements the represented interface
determine if a class or interface is a superclass or superinterface of a class or interface
get the component type for a represented array type

The existing and new methods of class Class are described below.

toString

    public String toString()

Returns a String consisting of the word class, a space, and the fully-qualified name of the class if this Class object represents a class (either a declared class or an array class). If this Class object represents an interface, then this method returns a String consisting of the word interface, followed by a space, followed by the fully-qualified name of the interface. If this Class object represents a primitive type, then this method returns the name of the primitive type. If this Class object represents void, returns the String "void".

This method overrides the toString method of class Object.

forName

    public static Class forName(String className) 

        throws ClassNotFoundException

Given the fully-qualified name for a class, this method attempts to locate, load, and link the specified class. If it succeeds, returns the Class object representing the class. If it fails, the method throws a ClassNotFoundException.

Class objects for array types may be obtained via this method. These Class objects are automatically constructed by the Java Virtual Machine.

Class objects that represent the primitive Java types or void cannot be obtained via this method.

newInstance

    public Object newInstance()

        throws InstantiationException, IllegalAccessException

Creates and initializes a new instance of the class represented by this Class object, provided it represents an instantiable class. This is done exactly as if by an instance creation expression with an empty argument list. If evaluation of such an instance creation expression would complete abruptly, then the invocation of newInstance will complete abruptly for the same reason. Otherwise, it returns the newly created and initialized instance.

The method throws an IllegalAccessException if the class or initializer is not accessible to the calling class. The method throws an InstantiationException if it attempts to instantiate an abstract class or an interface, or if it is invoked on a Class object that represents a primitive type or void.

isInstance

    public boolean isInstance(Object obj)

This method is the dynamic equivalent of the Java language instanceof operator. The method returns true if the specified Object argument is non- null and can be cast to the reference type represented by this Class object without raising a ClassCastException. It returns false otherwise.

If this Class object represents a primitive type or void, returns false.

See The Java Language Specification, section 15.19.2.

isAssignableFrom

    public boolean isAssignableFrom(Class fromClass)

This method tests whether the type represented by the specified Class parameter can be converted to the type represented by this Class object via an identity conversion or via a widening reference conversion. It returns true if so, false otherwise.

If this Class object represents a primitive type, returns true if the specified Class parameter is exactly this Class object, false otherwise.

This method throws a NullPointerException if the specified Class parameter is null.

See The Java Language Specification, sections 5.1.1, 5.1.4 and 5.2.

isInterface

    public boolean isInterface()

If this Class object represents an interface type, returns true, otherwise returns false.

isArray

    public boolean isArray()

If this Class object represents an array type, returns true; otherwise returns false.

isPrimitive

    public boolean isPrimitive()

If this Class object represents a primitive Java type, returns true; otherwise returns false.

There are nine predefined Class objects that represent theprimitive Java types and void. These are created by the Java Virtual Machine, and have the same names as the primitive types that they represent. These objects may only be accessed via the following public static final variables:

    

        java.lang.Boolean.TYPE

        java.lang.Character.TYPE

        java.lang.Byte.TYPE

        java.lang.Short.TYPE

        java.lang.Integer.TYPE

        java.lang.Long.TYPE

        java.lang.Float.TYPE

        java.lang.Double.TYPE

        java.lang.Void.TYPE

These are the only Class objects for which this method returns true.

getName

    public String getName()

Returns the fully-qualified name of the class (declared or array), interface, primitive type or void represented by this Class object, as a String.

getModifiers

    public int getModifiers()

Returns the Java language modifiers for this class or interface, encoded in an integer. The modifiers consist of the Java Virtual Machine's constants for public, protected, private, final, and interface; they should be decoded using the methods of class Modifier.

The modifier encodings are defined in The Java Virtual Machine Specification, table 4.1.

getClassLoader

    public ClassLoader getClassLoader()

Returns the class loader object that loaded this Class. Returns null if this Class was not loaded by a class loader.

getSuperclass

    public Class getSuperclass()

If this Class object represents a class other than Object, returns the Class that represents the superclass of the class. Returns null if this Class represents the class Object, or if it represents an interface type or a primitive type.

getInterfaces

    public Class[] getInterfaces()

Returns an array of Class objects representing the interfaces of the class or interface represented by this Class object. If this Class object represents a class, returns an array containing objects representing the interfaces directly implemented by the class. If this Class object represents an interface, returns an array containing objects representing the direct superinterfaces of the interface. Returns an array of length 0 if this Class implements no interfaces or if it represents a primitive type.

getComponentType

    public Class getComponentType()

If this Class object represents an array type, returns the Class object representing the component type of the array; otherwise returns null.

getDeclaringClass

    public Class getDeclaringClass()

If this class or interface is a member of another class, returns the Class object representing the class of which it is a member (its declaring class). Returns a null reference if this class or interface is not a member of any other class.

getClasses

    public Class[] getClasses()

Returns an array containing Class objects representing all the public classes and interfaces that are members of the class represented by this Class object. This includes public class and interface members inherited from superclasses and public class and interface members declared by the class. Returns an array of length 0 if the class has no public member classes or interfaces, or if this Class object represents a primitive type.

getFields

    public Field[] getFields()

        throws SecurityException

Returns an array containing Field objects reflecting all the public accessible fields of the class or interface represented by this Class object, including those declared by the class or interface and those declared by superclasses and superinterfaces. (Thus, the array includes the public member fields of the class as well as any additional public hidden fields.) Returns an array of length 0 if the class or interface has no public accessible fields.

Note that the implicit length field for array types is not reflected by this method. User code should use the methods of class Array to manipulate arrays.

The method throws a SecurityException if access to this information is denied.

See The Java Language Specification, sections 8.2 and 8.3.

getMethods

    public Method[] getMethods()

        throws SecurityException

Returns an array containing Method objects reflecting all the public member methods of the class or interface represented by this Class object, including those declared by the class or interface and and those inherited from superclasses and superinterfaces. Returns an array of length 0 if the class or interface has no public member methods.

The method throws a SecurityException if access to this information is denied.

See The Java Language Specification, sections 8.2 and 8.4.

getConstructors

    public Constructor[] getConstructors()

        throws SecurityException

Returns an array containing Constructor objects that reflect all the public constructors of the class represented by this Class object. Returns an array of length 0 if the class has no public constructors, or if this Class object represents an interface or a primitive type.

The method throws a SecurityException if access to this information is denied.

getField

    public Field getField(String name)

        throws NoSuchFieldException, SecurityException

Returns a Field object that reflects the specified public accessible field of the class or interface represented by this Class object. The name parameter is a String specifying the simple name of the desired field.

The field to be reflected is located by searching all the accessible fields of the class or interface represented by this Class object for a public field with the specified name.

The method throws a NoSuchFieldException if a matching field is not found.

The method throws a SecurityException if access to the underlying field is denied.

See The Java Language Specification, sections 8.2 and 8.3.

getMethod

    public Method getMethod(String name, Class[] parameterTypes)

        throws NoSuchMethodException, SecurityException

Returns a Method object that reflects the specified public member method of the class or interface represented by this Class object. The name parameter is a String specifying the simple name the desired method, and the parameterTypes parameter is an array of Class objects that identify the method's formal parameter types, in declared order.

The method to reflect is located by searching all the member methods of the class or interface represented by this Class object for a public method with the specified name and exactly the same formal parameter types.

The method throws a NoSuchMethodException a matching method is not found.

The method throws a SecurityException if access to the underlying method is denied.

See The Java Language Specification, sections 8.2 and 8.4.

getConstructor

    public Constructor getConstructor(Class[] parameterTypes)

        throws NoSuchMethodException, SecurityException

Returns a Constructor object that reflects the specified public constructor of the class represented by this Class object. The parameterTypes parameter is an array of Class objects that identify the constructor's formal parameter types, in declared order.

The constructor to reflect is located by searching all the constructors of the class represented by this Class object for a public constructor with the exactly the same formal parameter types.

The method throws a NoSuchMethodException if a matching constructor is not found.

The method throws a SecurityException if access to the underlying constructor is denied.

getDeclaredClasses

    public Class[] getDeclaredClasses()

        throws SecurityException

Returns an array of Class objects reflecting all the classes and interfaces declared as members of the class represented by this Class object. This includes public, protected, default (package) access, and private classes and interfaces declared by the class, but excludes inherited classes and interfaces. Returns an array of length 0 if the class declares no classes or interfaces as members, or if this Class object represents a primitive type.

The method throws a SecurityException if access to this information is denied.

getDeclaredFields

    public Field[] getDeclaredFields()

        throws SecurityException

Returns an array of Field objects reflecting all the fields declared by the class or interface represented by this Class object. This includes public, protected, default (package) access, and private fields, but excludes inherited fields. Returns an array of length 0 if the class or interface declares no fields, or if this Class object represents a primitive type.

The method throws a SecurityException if access to this information is denied.

See The Java Language Specification, sections 8.2 and 8.3.

getDeclaredMethods

    public Method[] getDeclaredMethods()

        throws SecurityException

Returns an array of Method objects reflecting all the methods declared by the class or interface represented by this Class object. This includes public, protected, default (package) access, and private methods, but excludes inherited methods. Returns an array of length 0 if the class or interface declares no methods, or if this Class object represents a primitive type.

The method throws a SecurityException if access to this information is denied.

See The Java Language Specification, sections 8.2 and 8.4.

getDeclaredConstructors

    public Constructor[] getDeclaredConstructors()

        throws SecurityException

Returns an array of Constructor objects reflecting all the constructors declared by the class represented by this Class object. These are public, protected, default (package) access, and private constructors. Returns an array of length 0 if this Class object represents an interface or a primitive type.

The method throws a SecurityException if access to this information is denied.

getDeclaredField

    public Field getDeclaredField(String name)

        throws NoSuchFieldException, SecurityException

Returns a Field object that reflects the specified declared field of the class or interface represented by this Class object. The name parameter is a String that specifies the simple name of the desired field.

The method throws a NoSuchFieldException if a field with the specified name is not found.

The method throws a SecurityException if access to the underlying field is denied.

See The Java Language Specification, sections 8.2 and 8.3.

getDeclaredMethod

    public Method getDeclaredMethod(String name,

                                Class[] parameterTypes)

        throws NoSuchMethodException, SecurityException

Returns a Method object that reflects the specified declared method of the class or interface represented by this Class object. The name parameter is a String that specifies the simple name of the desired method, and the parameterTypes parameter is an array of Class objects that identify the method's formal parameter types, in declared order.

The method throws a NoSuchMethodException if a matching method is not found.

The method throws a SecurityException if access to the underlying method is denied.

See The Java Language Specification, sections 8.2 and 8.4.

getDeclaredConstructor

    public Constructor getDeclaredConstructor(Class[] parameterTypes)

        throws NoSuchMethodException, SecurityException

Returns a Constructor object that reflects the specified constructor of the class or interface represented by this Class object. The parameterTypes parameter is an array of Class objects that identify the constructor's formal parameter types, in declared order.

The method throws a NoSuchMethodException if a matching constructor is not found.

The method throws a SecurityException if access to the underlying constructor is denied.

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