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());
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).
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.
The equals method implements an equivalence relation
on non-null object references:
x, x.equals(x) should return
true.
x and y, x.equals(y)
should return true if and only if
y.equals(x) returns true.
x, y, and z, if
x.equals(y) returns true and
y.equals(z) returns true, then
x.equals(z) should return true.
x and y, multiple invocations of
x.equals(y) consistently return true
or consistently return false, provided no
information used in equals comparisons on the
objects is modified.
x,
x.equals(null) should return false.
The equals method for class Object implements
the most discriminating possible equivalence relation on objects;
that is, for any non-null reference values x and
y, this method returns true if and only
if x and y refer to the same object
(x == y has the value true).
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
Class object associated with the class or
interface with the given string name. Invoking this method is
equivalent to:
whereClass.forName(className, true, currentLoader)
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.
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")
Class.forName("Foo", true, this.getClass().getClassLoader())
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.
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. 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.
Class representing the component type of an
array. If this class does not represent an array class this method
returns null.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 the public constructor of the class
represented by this Class object whose formal parameter
types match those specified by parameterTypes.
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.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.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.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.
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.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.
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.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.
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.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:
NoSuchFieldException
is thrown.See The Java Language Specification, sections 8.2 and 8.3.
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.
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.
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.
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 { ... }
s is an instance of
Shimmer; the value of the expression:
is thes.getClass().getInterfaces()[0]
Class object that represents interface
FloorWax; and the value of:
is thes.getClass().getInterfaces()[1]
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.
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. 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:
See The Java Language Specification, sections 8.2 and 8.4.
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.
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.
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"
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.
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.Before delegation, an absolute resource name is constructed from the given resource name using this algorithm:
modified_package_name/name
Where the modified_package_name is the package name of this object with '/' substituted for '.' ('\u002e').
Before delegation, an absolute resource name is constructed from the given resource name using this algorithm:
modified_package_name/name
Where the modified_package_name is the package name of this object with '/' substituted for '.' ('\u002e').
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 "[]".
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.java.util.Hashtable.
The general contract of hashCode is:
hashCode method on each of
the two objects must produce the same integer result.
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)
Class object represents an array class.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.
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.
Class object represents an
interface type.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.
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 .
wait methods.
The awakened thread will not be able to proceed until the current thread relinquishes the lock on this object. The awakened thread will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened thread enjoys no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner of this object's monitor. A thread becomes the owner of the object's monitor in one of three ways:
synchronized statement
that synchronizes on the object.
Class, by executing a
synchronized static method of that class.
Only one thread at a time can own an object's monitor.
wait methods.
The awakened threads will not be able to proceed until the current thread relinquishes the lock on this object. The awakened threads will compete in the usual manner with any other threads that might be actively competing to synchronize on this object; for example, the awakened threads enjoy no reliable privilege or disadvantage in being the next thread to lock this object.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
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".
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up
either through a call to the notify method or the
notifyAll method. The thread then waits until it can
re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.The current thread must own this object's monitor.
This method causes the current thread (call it T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Thread T becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout);
... // Perform action appropriate to condition
}
(For more information on this topic, see Section 3.2.3 in Doug Lea's
"Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
2000), or Item 50 in Joshua Bloch's "Effective Java Programming
Language Guide" (Addison-Wesley, 2001).
If the current thread is interrupted by another thread while it is waiting, then an InterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
Note that the wait method, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.
This method is similar to the wait method of one
argument, but it allows finer control over the amount of time to
wait for a notification before giving up. The amount of real time,
measured in nanoseconds, is given by:
1000000*timeout+nanos
In all other respects, this method does the same thing as the method of one argument. In particular, wait(0, 0) means the same thing as wait(0).
The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until either of the following two conditions has occurred:
notify method
or the notifyAll method.
timeout
milliseconds plus nanos nanoseconds arguments, has
elapsed.
The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop:
synchronized (obj) {
while (<condition does not hold>)
obj.wait(timeout, nanos);
... // Perform action appropriate to condition
}
This method should only be called by a thread that is the owner
of this object's monitor. See the notify method for a
description of the ways in which a thread can become the owner of
a monitor.