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Java™ Platform Standard Ed. 6 |
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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.
Class| Constructor Summary | |
|---|---|
Object()
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| Method Summary | |
|---|---|
protected Object |
clone()
Creates and returns a copy of this object. |
boolean |
equals(Object obj)
Indicates whether some other object is "equal to" this one. |
protected void |
finalize()
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. |
Class<?> |
getClass()
Returns the runtime class of this Object. |
int |
hashCode()
Returns a hash code value for the object. |
void |
notify()
Wakes up a single thread that is waiting on this object's monitor. |
void |
notifyAll()
Wakes up all threads that are waiting on this object's monitor. |
String |
toString()
Returns a string representation of the object. |
void |
wait()
Causes the current thread to wait until another thread invokes the notify() method or the
notifyAll() method for this object. |
void |
wait(long timeout)
Causes the current thread to wait until either another thread invokes the notify() method or the
notifyAll() method for this object, or a
specified amount of time has elapsed. |
void |
wait(long timeout,
int nanos)
Causes the current thread to wait until another thread invokes the notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed. |
| Constructor Detail |
|---|
public Object()
| Method Detail |
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public final Class<?> getClass()
Object. The returned
Class object is the object that is locked by static synchronized methods of the represented class.
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();
Class object that represents the runtime
class of this object.public int hashCode()
java.util.Hashtable.
The general contract of hashCode is:
hashCode method on each of
the two objects must produce the same integer result.
equals(java.lang.Object)
method, then calling the hashCode method on each of the
two objects must produce distinct integer results. However, the
programmer should be aware that producing distinct integer results
for unequal objects may improve the performance of hashtables.
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.)
equals(java.lang.Object),
Hashtablepublic boolean equals(Object obj)
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.
obj - the reference object with which to compare.
true if this object is the same as the obj
argument; false otherwise.hashCode(),
Hashtable
protected Object clone()
throws CloneNotSupportedException
will be true, and that the expression:x.clone() != x
will be true, but these are not absolute requirements. While it is typically the case that:x.clone().getClass() == x.getClass()
will be true, this is not an absolute requirement.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.
CloneNotSupportedException - if the object's class does not
support the Cloneable interface. Subclasses
that override the clone method can also
throw this exception to indicate that an instance cannot
be cloned.Cloneablepublic String toString()
toString method returns a string that
"textually represents" this object. The result should
be a concise but informative representation that is easy for a
person to read.
It is recommended that all subclasses override this method.
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()
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.
IllegalMonitorStateException - if the current thread is not
the owner of this object's monitor.notifyAll(),
wait()public final void notifyAll()
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.
IllegalMonitorStateException - if the current thread is not
the owner of this object's monitor.notify(),
wait()
public final void wait(long timeout)
throws InterruptedException
notify() method or the
notifyAll() method for this object, or a
specified amount of time has elapsed.
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 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.
timeout - the maximum time to wait in milliseconds.
IllegalArgumentException - if the value of timeout is
negative.
IllegalMonitorStateException - if the current thread is not
the owner of the object's monitor.
InterruptedException - if any thread interrupted the
current thread before or while the current thread
was waiting for a notification. The interrupted
status of the current thread is cleared when
this exception is thrown.notify(),
notifyAll()
public final void wait(long timeout,
int nanos)
throws InterruptedException
notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.
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:
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.
timeout - the maximum time to wait in milliseconds.nanos - additional time, in nanoseconds range
0-999999.
IllegalArgumentException - if the value of timeout is
negative or the value of nanos is
not in the range 0-999999.
IllegalMonitorStateException - if the current thread is not
the owner of this object's monitor.
InterruptedException - if any thread interrupted the
current thread before or while the current thread
was waiting for a notification. The interrupted
status of the current thread is cleared when
this exception is thrown.
public final void wait()
throws InterruptedException
notify() method or the
notifyAll() method for this object.
In other words, this method behaves exactly as if it simply
performs the call wait(0).
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.
IllegalMonitorStateException - if the current thread is not
the owner of the object's monitor.
InterruptedException - if any thread interrupted the
current thread before or while the current thread
was waiting for a notification. The interrupted
status of the current thread is cleared when
this exception is thrown.notify(),
notifyAll()
protected void finalize()
throws Throwable
finalize method to dispose of
system resources or to perform other cleanup.
The general contract of finalize is that it is invoked if and when the JavaTM 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.
Throwable - the Exception raised by this method
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Java™ Platform Standard Ed. 6 |
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