java.lang.Object
public class Object
Class Object is the root of the class hierarchy. Every class has Object as a superclass. All objects, including arrays, implement the methods of this class.
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public
Object
()
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public final
Class
<?>
getClass
()
The actual result type is Class<? extends |X|> where |X| is the erasure of the static type of the expression on which getClass is called.
For example, no cast is required in this code fragment:
Number n = 0;
Class<? extends Number> c = n.getClass();
public int
hashCode
()
The general contract of hashCode is:
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 Java
TM
programming language.)
public boolean
equals
(
Object
obj)
The equals method implements an equivalence relation on non-null object references:
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.
protected
Object
clone
() throws
CloneNotSupportedException
x.clone() != x
x.clone().getClass() == x.getClass()
x.clone().equals(x)
By convention, the returned object should be obtained by calling
super.clone
. If a class and all of its superclasses (except
Object
) obey this convention, it will be the case that
x.clone().getClass() == x.getClass()
.
By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by
super.clone
before returning it. Typically, this means copying any mutable objects that comprise the internal "deep structure" of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by
super.clone
need to be modified.
The method
clone
for class
Object
performs a specific cloning operation. First, if the class of this object does not implement the interface
Cloneable
, then a
CloneNotSupportedException
is thrown. Note that all arrays are considered to implement the interface
Cloneable
. Otherwise, this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.
The class
Object
does not itself implement the interface
Cloneable
, so calling the
clone
method on an object whose class is
Object
will result in throwing an exception at run time.
public
String
toString
()
The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
public final void
notify
()
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:
Only one thread at a time can own an object's monitor.
public final void
notifyAll
()
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.
public final void
wait
(long timeout) throws
InterruptedException
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 }
If the current thread is
interrupted
by any thread before or 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.
public final void
wait
(long timeout, int nanos) throws
InterruptedException
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
wait(long)
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:
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 }
public final void
wait
() throws
InterruptedException
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 }
protected void
finalize
() throws
Throwable
The general contract of
finalize
is that it is invoked if and when the Java
TM
virtual machine has determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, except as a result of an action taken by the finalization of some other object or class which is ready to be finalized. The
finalize
method may take any action, including making this object available again to other threads; the usual purpose of
finalize
, however, is to perform cleanup actions before the object is irrevocably discarded. For example, the finalize method for an object that represents an input/output connection might perform explicit I/O transactions to break the connection before the object is permanently discarded.
The
finalize
method of class
Object
performs no special action; it simply returns normally. Subclasses of
Object
may override this definition.
The Java programming language does not guarantee which thread will invoke the
finalize
method for any given object. It is guaranteed, however, that the thread that invokes finalize will not be holding any user-visible synchronization locks when finalize is invoked. If an uncaught exception is thrown by the finalize method, the exception is ignored and finalization of that object terminates.
After the
finalize
method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded.
The
finalize
method is never invoked more than once by a Java virtual machine for any given object.
Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored.
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publicObject()
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public finalClass<? extendsObject>getClass()
public inthashCode()
The general contract of hashCode is:
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 Java
TM
programming language.)
public booleanequals(Objectobj)
The equals method implements an equivalence relation on non-null object references:
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.
protectedObjectclone() throwsCloneNotSupportedException
x.clone() != x
x.clone().getClass() == x.getClass()
x.clone().equals(x)
By convention, the returned object should be obtained by calling
super.clone
. If a class and all of its superclasses (except
Object
) obey this convention, it will be the case that
x.clone().getClass() == x.getClass()
.
By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by
super.clone
before returning it. Typically, this means copying any mutable objects that comprise the internal "deep structure" of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by
super.clone
need to be modified.
The method
clone
for class
Object
performs a specific cloning operation. First, if the class of this object does not implement the interface
Cloneable
, then a
CloneNotSupportedException
is thrown. Note that all arrays are considered to implement the interface
Cloneable
. Otherwise, this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.
The class
Object
does not itself implement the interface
Cloneable
, so calling the
clone
method on an object whose class is
Object
will result in throwing an exception at run time.
publicStringtoString()
The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:
getClass().getName() + '@' + Integer.toHexString(hashCode())
public final voidnotify()
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:
Only one thread at a time can own an object's monitor.
public final voidnotifyAll()
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.
public final voidwait(long timeout) throwsInterruptedException
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 }
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.
public final voidwait(long timeout, int nanos) throwsInterruptedException
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
wait(long)
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:
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 }
public final voidwait() throwsInterruptedException
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 }
protected voidfinalize() throwsThrowable
The general contract of
finalize
is that it is invoked if and when the Java
TM
virtual machine has determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, except as a result of an action taken by the finalization of some other object or class which is ready to be finalized. The
finalize
method may take any action, including making this object available again to other threads; the usual purpose of
finalize
, however, is to perform cleanup actions before the object is irrevocably discarded. For example, the finalize method for an object that represents an input/output connection might perform explicit I/O transactions to break the connection before the object is permanently discarded.
The
finalize
method of class
Object
performs no special action; it simply returns normally. Subclasses of
Object
may override this definition.
The Java programming language does not guarantee which thread will invoke the
finalize
method for any given object. It is guaranteed, however, that the thread that invokes finalize will not be holding any user-visible synchronization locks when finalize is invoked. If an uncaught exception is thrown by the finalize method, the exception is ignored and finalization of that object terminates.
After the
finalize
method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded.
The
finalize
method is never invoked more than once by a Java virtual machine for any given object.
Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored.