Class: java.lang.Class<T>
- public final class Class<T>
- implements Serializable, GenericDeclaration, Type, AnnotatedElement
Instances of the class Class represent classes and
interfaces in a running Java application. An enum is a kind of
class and an annotation is a kind of interface. 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. The primitive Java types (boolean,
byte, char, short,
int, long, float, and
double), and the keyword void are also
represented as Class objects.
Class has no public constructor. Instead Class objects are constructed automatically by the Java Virtual Machine as classes are loaded and by calls to the defineClass method in the class loader.
The following example uses a Class object to print the class name of an object:
void printClassName(Object obj) {
System.out.println("The class of " + obj +
" is " + obj.getClass().getName());
}
It is also possible to get the Class object for a named type (or for void) using a class literal (JLS Section 15.8.2). For example:
System.out.println("The name of class Foo is: "+Foo.class.getName());
Methods
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asSubclasstop
Casts this Class object to represent a subclass of the class represented by the specified class object. Checks that that the cast is valid, and throws a ClassCastException if it is not. If this method succeeds, it always returns a reference to this class object.This method is useful when a client needs to "narrow" the type of a Class object to pass it to an API that restricts the Class objects that it is willing to accept. A cast would generate a compile-time warning, as the correctness of the cast could not be checked at runtime (because generic types are implemented by erasure).
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casttop
public T cast(Object obj)Casts an object to the class or interface represented by this Class object. -
desiredAssertionStatustop
public boolean desiredAssertionStatus()Returns the assertion status that would be assigned to this class if it were to be initialized at the time this method is invoked. If this class has had its assertion status set, the most recent setting will be returned; otherwise, if any package default assertion status pertains to this class, the most recent setting for the most specific pertinent package default assertion status is returned; otherwise, if this class is not a system class (i.e., it has a class loader) its class loader's default assertion status is returned; otherwise, the system class default assertion status is returned.Few programmers will have any need for this method; it is provided for the benefit of the JRE itself. (It allows a class to determine at the time that it is initialized whether assertions should be enabled.) Note that this method is not guaranteed to return the actual assertion status that was (or will be) associated with the specified class when it was (or will be) initialized.
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forNametop
Returns the Class object associated with the class or interface with the given string name. Invoking this method is equivalent to:Class.forName(className, true, currentLoader)
where currentLoader denotes the defining class loader of the current class.For example, the following code fragment returns the runtime Class descriptor for the class named java.lang.Thread:
Class t = Class.forName("java.lang.Thread")
A call to forName("X") causes the class named X to be initialized.
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forNametop
public static Class<?> forName(String name, boolean initialize, ClassLoader loader) throws ClassNotFoundExceptionReturns the Class object associated with the class or interface with the given string name, using the given class loader. Given the fully qualified name for a class or interface (in the same format returned by getName) this method attempts to locate, load, and link the class or interface. The specified class loader is used to load the class or interface. If the parameter loader is null, the class is loaded through the bootstrap class loader. The class is initialized only if the initialize parameter is true and if it has not been initialized earlier.If name denotes a primitive type or void, an attempt will be made to locate a user-defined class in the unnamed package whose name is name. Therefore, this method cannot be used to obtain any of the Class objects representing primitive types or void.
If name denotes an array class, the component type of the array class is loaded but not initialized.
For example, in an instance method the expression:
Class.forName("Foo")
is equivalent to:Class.forName("Foo", true, this.getClass().getClassLoader())
Note that this method throws errors related to loading, linking or initializing as specified in Sections 12.2, 12.3 and 12.4 of The Java Language Specification. Note that this method does not check whether the requested class is accessible to its caller.If the loader is null, and a security manager is present, and the caller's class loader is not null, then this method calls the security manager's checkPermission method with a RuntimePermission("getClassLoader") permission to ensure it's ok to access the bootstrap class loader.
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getAnnotationtop
public <A extends Annotation> A getAnnotation(Class<A> annotationClass)Returns this element's annotation for the specified type if such an annotation is present, else null.- Specified by:
- getAnnotation from AnnotatedElement
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getAnnotationstop
public Annotation[] getAnnotations()Returns all annotations present on this element. (Returns an array of length zero if this element has no annotations.) The caller of this method is free to modify the returned array; it will have no effect on the arrays returned to other callers.- Specified by:
- getAnnotations from AnnotatedElement
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getCanonicalNametop
public String getCanonicalName()Returns the canonical name of the underlying class as defined by the Java Language Specification. Returns null if the underlying class does not have a canonical name (i.e., if it is a local or anonymous class or an array whose component type does not have a canonical name). -
getClassLoadertop
public ClassLoader getClassLoader()Returns the class loader for the class. Some implementations may use null to represent the bootstrap class loader. This method will return null in such implementations if this class was loaded by the bootstrap class loader.If a security manager is present, and the caller's class loader is not null and the caller's class loader is not the same as or an ancestor of the class loader for the class whose class loader is requested, then this method calls the security manager's checkPermission method with a RuntimePermission("getClassLoader") permission to ensure it's ok to access the class loader for the class.
If this object represents a primitive type or void, null is returned.
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getClassestop
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. This method returns an array of length 0 if this Class object has no public member classes or interfaces. This method also returns an array of length 0 if this Class object represents a primitive type, an array class, or void. -
getComponentTypetop
public native Class<?> getComponentType()Returns the Class representing the component type of an array. If this class does not represent an array class this method returns null. -
getConstructortop
public Constructor<T> getConstructor(Class<?>... parameterTypes) throws NoSuchMethodException, SecurityExceptionReturns 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. If this Class object represents an inner class declared in a non-static context, the formal parameter types include the explicit enclosing instance as the first parameter.The constructor to reflect is the public constructor of the class represented by this Class object whose formal parameter types match those specified by parameterTypes.
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getConstructorstop
public Constructor<?>[] getConstructors() throws SecurityExceptionReturns an array containing Constructor objects reflecting all the public constructors of the class represented by this Class object. An array of length 0 is returned if the class has no public constructors, or if the class is an array class, or if the class reflects a primitive type or void. Note that while this method returns an array of Constructor<T> objects (that is an array of constructors from this class), the return type of this method is Constructor<?>[] and not Constructor<T>[] as might be expected. This less informative return type is necessary since after being returned from this method, the array could be modified to hold Constructor objects for different classes, which would violate the type guarantees of Constructor<T>[]. -
getDeclaredAnnotationstop
public Annotation[] getDeclaredAnnotations()Returns all annotations that are directly present on this element. Unlike the other methods in this interface, this method ignores inherited annotations. (Returns an array of length zero if no annotations are directly present on this element.) The caller of this method is free to modify the returned array; it will have no effect on the arrays returned to other callers.- Specified by:
- getDeclaredAnnotations from AnnotatedElement
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getDeclaredClassestop
public Class<?>[] getDeclaredClasses() throws SecurityExceptionReturns 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. This method 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, an array class, or void. -
getDeclaredConstructortop
public Constructor<T> getDeclaredConstructor(Class<?>... parameterTypes) throws NoSuchMethodException, SecurityExceptionReturns 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. If this Class object represents an inner class declared in a non-static context, the formal parameter types include the explicit enclosing instance as the first parameter. -
getDeclaredConstructorstop
public Constructor<?>[] getDeclaredConstructors() throws SecurityExceptionReturns 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. The elements in the array returned are not sorted and are not in any particular order. If the class has a default constructor, it is included in the returned array. This method returns an array of length 0 if this Class object represents an interface, a primitive type, an array class, or void.See The Java Language Specification, section 8.2.
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getDeclaredFieldtop
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. Note that this method will not reflect the length field of an array class. -
getDeclaredFieldstop
public Field[] getDeclaredFields() throws SecurityExceptionReturns 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. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if the class or interface declares no fields, or if this Class object represents a primitive type, an array class, or void.See The Java Language Specification, sections 8.2 and 8.3.
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getDeclaredMethodtop
public Method getDeclaredMethod(String name, Class<?>... parameterTypes) throws NoSuchMethodException, SecurityExceptionReturns 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. If more than one method with the same parameter types is declared in a class, and one of these methods has a return type that is more specific than any of the others, that method is returned; otherwise one of the methods is chosen arbitrarily. If the name is "<init>"or "<clinit>" a NoSuchMethodException is raised. -
getDeclaredMethodstop
public Method[] getDeclaredMethods() throws SecurityExceptionReturns 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. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if the class or interface declares no methods, or if this Class object represents a primitive type, an array class, or void. The class initialization method <clinit> is not included in the returned array. If the class declares multiple public member methods with the same parameter types, they are all included in the returned array.See The Java Language Specification, section 8.2.
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getDeclaringClasstop
public native Class<?> getDeclaringClass()If the class or interface represented by this Class object is a member of another class, returns the Class object representing the class in which it was declared. This method returns null if this class or interface is not a member of any other class. If this Class object represents an array class, a primitive type, or void,then this method returns null. -
getEnclosingClasstop
public Class<?> getEnclosingClass()Returns the immediately enclosing class of the underlying class. If the underlying class is a top level class this method returns null. -
getEnclosingConstructortop
public Constructor<?> getEnclosingConstructor()If this Class object represents a local or anonymous class within a constructor, returns a Constructor object representing the immediately enclosing constructor of the underlying class. Returns null otherwise. In particular, this method returns null if the underlying class is a local or anonymous class immediately enclosed by a type declaration, instance initializer or static initializer. -
getEnclosingMethodtop
public Method getEnclosingMethod()If this Class object represents a local or anonymous class within a method, returns a Method object representing the immediately enclosing method of the underlying class. Returns null otherwise. In particular, this method returns null if the underlying class is a local or anonymous class immediately enclosed by a type declaration, instance initializer or static initializer. -
getEnumConstantstop
public T[] getEnumConstants()Returns the elements of this enum class or null if this Class object does not represent an enum type. -
getFieldtop
Returns a Field object that reflects the specified public member 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 determined by the algorithm that follows. Let C be the class represented by this object:
- If C declares a public field with the name specified, that is the field to be reflected.
- If no field was found in step 1 above, this algorithm is applied recursively to each direct superinterface of C. The direct superinterfaces are searched in the order they were declared.
- If no field was found in steps 1 and 2 above, and C has a superclass S, then this algorithm is invoked recursively upon S. If C has no superclass, then a NoSuchFieldException is thrown.
See The Java Language Specification, sections 8.2 and 8.3.
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getFieldstop
public Field[] getFields() throws SecurityExceptionReturns an array containing Field objects reflecting all the accessible public fields of the class or interface represented by this Class object. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if the class or interface has no accessible public fields, or if it represents an array class, a primitive type, or void.Specifically, if this Class object represents a class, this method returns the public fields of this class and of all its superclasses. If this Class object represents an interface, this method returns the fields of this interface and of all its superinterfaces.
The implicit length field for array class is not reflected by this method. User code should use the methods of class Array to manipulate arrays.
See The Java Language Specification, sections 8.2 and 8.3.
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getGenericInterfacestop
public Type[] getGenericInterfaces()Returns the Types representing the interfaces directly implemented by the class or interface represented by this object.If a superinterface is a parameterized type, the Type object returned for it must accurately reflect the actual type parameters used in the source code. The parameterized type representing each superinterface is created if it had not been created before. See the declaration of ParameterizedType for the semantics of the creation process for parameterized types.
If this object represents a class, the return value is an array containing objects representing all interfaces implemented by the class. The order of the interface objects in the array corresponds to the order of the interface names in the implements clause of the declaration of the class represented by this object. In the case of an array class, the interfaces Cloneable and Serializable are returned in that order.
If this object represents an interface, the array contains objects representing all interfaces directly extended by the interface. The order of the interface objects in the array corresponds to the order of the interface names in the extends clause of the declaration of the interface represented by this object.
If this object represents a class or interface that implements no interfaces, the method returns an array of length 0.
If this object represents a primitive type or void, the method returns an array of length 0.
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getGenericSuperclasstop
public Type getGenericSuperclass()Returns the Type representing the direct superclass of the entity (class, interface, primitive type or void) represented by this Class.If the superclass is a parameterized type, the Type object returned must accurately reflect the actual type parameters used in the source code. The parameterized type representing the superclass is created if it had not been created before. See the declaration of ParameterizedType for the semantics of the creation process for parameterized types. If this Class represents either the Object class, an interface, a primitive type, or void, then null is returned. If this object represents an array class then the Class object representing the Object class is returned.
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getInterfacestop
public native Class<?>[] getInterfaces()Determines the interfaces implemented by the class or interface represented by this object.If this object represents a class, the return value is an array containing objects representing all interfaces implemented by the class. The order of the interface objects in the array corresponds to the order of the interface names in the implements clause of the declaration of the class represented by this object. For example, given the declaration:
class Shimmer implements FloorWax, DessertTopping { ... }
suppose the value of s is an instance of Shimmer; the value of the expression:s.getClass().getInterfaces()[0]
is the Class object that represents interface FloorWax; and the value of:s.getClass().getInterfaces()[1]
is the Class object that represents interface DessertTopping.If this object represents an interface, the array contains objects representing all interfaces extended by the interface. The order of the interface objects in the array corresponds to the order of the interface names in the extends clause of the declaration of the interface represented by this object.
If this object represents a class or interface that implements no interfaces, the method returns an array of length 0.
If this object represents a primitive type or void, the method returns an array of length 0.
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getMethodtop
public Method getMethod(String name, Class<?>... parameterTypes) throws NoSuchMethodException, SecurityExceptionReturns 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 of the desired method. The parameterTypes parameter is an array of Class objects that identify the method's formal parameter types, in declared order. If parameterTypes is null, it is treated as if it were an empty array.If the name is "<init>;"or "<clinit>" a NoSuchMethodException is raised. Otherwise, the method to be reflected is determined by the algorithm that follows. Let C be the class represented by this object:
- C is searched for any matching methods. If no matching method is found, the algorithm of step 1 is invoked recursively on the superclass of C.
- If no method was found in step 1 above, the superinterfaces of C are searched for a matching method. If any such method is found, it is reflected.
Note that there may be more than one matching method in a class because while the Java language forbids a class to declare multiple methods with the same signature but different return types, the Java virtual machine does not. This increased flexibility in the virtual machine can be used to implement various language features. For example, covariant returns can be implemented with bridge methods; the bridge method and the method being overridden would have the same signature but different return types.
See The Java Language Specification, sections 8.2 and 8.4.
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getMethodstop
public Method[] getMethods() throws SecurityExceptionReturns 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 those inherited from superclasses and superinterfaces. Array classes return all the (public) member methods inherited from the Object class. The elements in the array returned are not sorted and are not in any particular order. This method returns an array of length 0 if this Class object represents a class or interface that has no public member methods, or if this Class object represents a primitive type or void.The class initialization method <clinit> is not included in the returned array. If the class declares multiple public member methods with the same parameter types, they are all included in the returned array.
See The Java Language Specification, sections 8.2 and 8.4.
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getModifierstop
public native 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, static, abstract and interface; they should be decoded using the methods of class Modifier.If the underlying class is an array class, then its public, private and protected modifiers are the same as those of its component type. If this Class represents a primitive type or void, its public modifier is always true, and its protected and private modifiers are always false. If this object represents an array class, a primitive type or void, then its final modifier is always true and its interface modifier is always false. The values of its other modifiers are not determined by this specification.
The modifier encodings are defined in The Java Virtual Machine Specification, table 4.1.
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getNametop
public String getName()Returns the name of the entity (class, interface, array class, primitive type, or void) represented by this Class object, as a String.If this class object represents a reference type that is not an array type then the binary name of the class is returned, as specified by the Java Language Specification, Second Edition.
If this class object represents a primitive type or void, then the name returned is a String equal to the Java language keyword corresponding to the primitive type or void.
If this class object represents a class of arrays, then the internal form of the name consists of the name of the element type preceded by one or more '[' characters representing the depth of the array nesting. The encoding of element type names is as follows:
Element Type Encoding boolean Z byte B char C class or interface Lclassname; double D float F int I long J short S The class or interface name classname is the binary name of the class specified above.
Examples:
String.class.getName() returns "java.lang.String" byte.class.getName() returns "byte" (new Object[3]).getClass().getName() returns "[Ljava.lang.Object;" (new int[3][4][5][6][7][8][9]).getClass().getName() returns "[[[[[[[I" -
getPackagetop
public Package getPackage()Gets the package for this class. The class loader of this class is used to find the package. If the class was loaded by the bootstrap class loader the set of packages loaded from CLASSPATH is searched to find the package of the class. Null is returned if no package object was created by the class loader of this class.Packages have attributes for versions and specifications only if the information was defined in the manifests that accompany the classes, and if the class loader created the package instance with the attributes from the manifest.
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getProtectionDomaintop
public ProtectionDomain getProtectionDomain()Returns the ProtectionDomain of this class. If there is a security manager installed, this method first calls the security manager's checkPermission method with a RuntimePermission("getProtectionDomain") permission to ensure it's ok to get the ProtectionDomain. -
getResourcetop
Finds a resource with a given name. The rules for searching resources associated with a given class are implemented by the defining class loader of the class. This method delegates to this object's class loader. If this object was loaded by the bootstrap class loader, the method delegates to ClassLoader.getSystemResource(java.lang.String).Before delegation, an absolute resource name is constructed from the given resource name using this algorithm:
- If the name begins with a '/' ('\u002f'), then the absolute name of the resource is the portion of the name following the '/'.
- Otherwise, the absolute name is of the following form:
modified_package_name/name
Where the modified_package_name is the package name of this object with '/' substituted for '.' ('\u002e').
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getResourceAsStreamtop
public InputStream getResourceAsStream(String name)Finds a resource with a given name. The rules for searching resources associated with a given class are implemented by the defining class loader of the class. This method delegates to this object's class loader. If this object was loaded by the bootstrap class loader, the method delegates to ClassLoader.getSystemResourceAsStream(java.lang.String).Before delegation, an absolute resource name is constructed from the given resource name using this algorithm:
- If the name begins with a '/' ('\u002f'), then the absolute name of the resource is the portion of the name following the '/'.
- Otherwise, the absolute name is of the following form:
modified_package_name/name
Where the modified_package_name is the package name of this object with '/' substituted for '.' ('\u002e').
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getSignerstop
public native Object[] getSigners()Gets the signers of this class. -
getSimpleNametop
public String getSimpleName()Returns the simple name of the underlying class as given in the source code. Returns an empty string if the underlying class is anonymous.The simple name of an array is the simple name of the component type with "[]" appended. In particular the simple name of an array whose component type is anonymous is "[]".
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getSuperclasstop
public native Class<? super T> getSuperclass()Returns the Class representing the superclass of the entity (class, interface, primitive type or void) represented by this Class. If this Class represents either the Object class, an interface, a primitive type, or void, then null is returned. If this object represents an array class then the Class object representing the Object class is returned. -
getTypeParameterstop
public TypeVariable<Class<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 from GenericDeclaration
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isAnnotationtop
public boolean isAnnotation()Returns true if this Class object represents an annotation type. Note that if this method returns true, Class.isInterface() would also return true, as all annotation types are also interfaces. -
isAnnotationPresenttop
public boolean isAnnotationPresent(Class<? extends Annotation> annotationClass)Returns true if an annotation for the specified type is present on this element, else false. This method is designed primarily for convenient access to marker annotations.- Specified by:
- isAnnotationPresent from AnnotatedElement
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isAnonymousClasstop
public boolean isAnonymousClass()Returns true if and only if the underlying class is an anonymous class. -
isArraytop
public native boolean isArray()Determines if this Class object represents an array class. -
isAssignableFromtop
public native boolean isAssignableFrom(Class<?> cls)Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter. It returns true if so; otherwise it returns false. If this Class object represents a primitive type, this method returns true if the specified Class parameter is exactly this Class object; otherwise it returns false.Specifically, 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. See The Java Language Specification, sections 5.1.1 and 5.1.4 , for details.
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isEnumtop
public boolean isEnum()Returns true if and only if this class was declared as an enum in the source code. -
isInstancetop
public native boolean isInstance(Object obj)Determines if the specified Object is assignment-compatible with the object represented by this Class. 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.Specifically, if this Class object represents a declared class, this method returns true if the specified Object argument is an instance of the represented class (or of any of its subclasses); it returns false otherwise. If this Class object represents an array class, this method returns true if the specified Object argument can be converted to an object of the array class by an identity conversion or by a widening reference conversion; it returns false otherwise. If this Class object represents an interface, this method returns true if the class or any superclass of the specified Object argument implements this interface; it returns false otherwise. If this Class object represents a primitive type, this method returns false.
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isInterfacetop
public native boolean isInterface()Determines if the specified Class object represents an interface type. -
isLocalClasstop
public boolean isLocalClass()Returns true if and only if the underlying class is a local class. -
isMemberClasstop
public boolean isMemberClass()Returns true if and only if the underlying class is a member class. -
isPrimitivetop
public native boolean isPrimitive()Determines if the specified Class object represents a primitive type.There are nine predefined Class objects to represent the eight primitive types and void. These are created by the Java Virtual Machine, and have the same names as the primitive types that they represent, namely boolean, byte, char, short, int, long, float, and double.
These objects may only be accessed via the following public static final variables, and are the only Class objects for which this method returns true.
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isSynthetictop
public boolean isSynthetic()Returns true if this class is a synthetic class; returns false otherwise. -
newInstancetop
public T newInstance() throws InstantiationException, IllegalAccessExceptionCreates a new instance of the class represented by this Class object. The class is instantiated as if by a new expression with an empty argument list. The class is initialized if it has not already been initialized.Note that this method propagates any exception thrown by the nullary constructor, including a checked exception. Use of this method effectively bypasses the compile-time exception checking that would otherwise be performed by the compiler. The Constructor.newInstance method avoids this problem by wrapping any exception thrown by the constructor in a (checked) java.lang.reflect.InvocationTargetException.
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toStringtop
public String toString()Converts the object to a string. The string representation is the string "class" or "interface", followed by a space, and then by the fully qualified name of the class in the format returned by getName. If this Class object represents a primitive type, this method returns the name of the primitive type. If this Class object represents void this method returns "void".