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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.Classprotected Objectclone()booleanprotected voidfinalize()Class<?>getClass()Object.inthashCode()voidnotify()voidtoString()voidwait()voidwait(long timeoutMillis)voidwait(long timeoutMillis,
int nanos)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.HashMap.
The general contract of hashCode is:
hashCode method
must consistently return the same integer, provided no information
used in equals comparisons on the object is modified.
This integer need not remain consistent from one execution of an
application to another execution of the same application.
equals(Object)
method, then calling the 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 hash tables.
hashCode method defined
by class Object returns distinct integers for distinct objects.equals(java.lang.Object),
System.identityHashCode(java.lang.Object)
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(),
HashMapx, the expression:
will be true, and that the expression:x.clone() != x
will bex.clone().getClass() == x.getClass()
true, but these are not absolute requirements.
While it is typically the case that:
will bex.clone().equals(x)
true, this is not an absolute requirement.
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 and that
the return type of the clone method of an array type T[]
is T[] where T is any reference or primitive type.
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.CloneabletoString 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())
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()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()
In all respects, this method behaves as if wait(0L, 0)
had been called. See the specification of the wait(long, int) method
for details.
IllegalMonitorStateException - if the current thread is not
the owner of the object's monitorInterruptedException - if any thread interrupted the current thread before or
while the current thread was waiting. The interrupted status of the
current thread is cleared when this exception is thrown.notify(),
notifyAll(),
wait(long),
wait(long, int)
In all respects, this method behaves as if wait(timeoutMillis, 0)
had been called. See the specification of the wait(long, int) method
for details.
timeoutMillis - the maximum time to wait, in millisecondsIllegalArgumentException - if timeoutMillis is negativeIllegalMonitorStateException - if the current thread is not
the owner of the object's monitorInterruptedException - if any thread interrupted the current thread before or
while the current thread was waiting. The interrupted status of the
current thread is cleared when this exception is thrown.notify(),
notifyAll(),
wait(),
wait(long, int)
The current thread must own this object's monitor lock. See the
notify method for a description of the ways in which
a thread can become the owner of a monitor lock.
This method causes the current thread (referred to here as T) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Note that only the locks on this object are relinquished; any other objects on which the current thread may be synchronized remain locked while the thread waits.
Thread T then becomes disabled for thread scheduling purposes and lies dormant until one of the following occurs:
notify method for this
object and thread T happens to be arbitrarily chosen as
the thread to be awakened.
notifyAll method for this
object.
1000000 * timeoutMillis + nanos. If timeoutMillis and nanos
are both zero, then real time is not taken into consideration and the
thread waits until awakened by one of the other causes.
The thread T is then removed from the wait set for this
object and re-enabled for thread scheduling. It competes in the
usual manner with other threads for the right to synchronize on the
object; once it has regained control of the object, all its
synchronization claims on the object are restored to the status quo
ante - that is, to the situation as of the time that the wait
method was invoked. Thread T then returns from the
invocation of the wait method. Thus, on return from the
wait method, the synchronization state of the object and of
thread T is exactly as it was when the wait method
was invoked.
A thread can 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. See the example below.
For more information on this topic, see section 14.2, "Condition Queues," in Brian Goetz and others' Java Concurrency in Practice (Addison-Wesley, 2006) or Item 69 in Joshua Bloch's Effective Java, Second Edition (Addison-Wesley, 2008).
If the current thread is interrupted
by any thread before or while it is waiting, then an InterruptedException
is thrown. The interrupted status of the current thread is cleared when
this exception is thrown. This exception is not thrown until the lock status of
this object has been restored as described above.
while loop around the call to wait, as shown in the example
below. Among other things, this approach avoids problems that can be caused
by spurious wakeups.
synchronized (obj) {
while (<condition does not hold> and <timeout not exceeded>) {
long timeoutMillis = ... ; // recompute timeout values
int nanos = ... ;
obj.wait(timeoutMillis, nanos);
}
... // Perform action appropriate to condition or timeout
}
timeoutMillis - the maximum time to wait, in millisecondsnanos - additional time, in nanoseconds, in the range 0-999999 inclusiveIllegalArgumentException - if timeoutMillis is negative,
or if the value of nanos is out of rangeIllegalMonitorStateException - if the current thread is not
the owner of the object's monitorInterruptedException - if any thread interrupted the current thread before or
while the current thread was waiting. The interrupted status of the
current thread is cleared when this exception is thrown.notify(),
notifyAll(),
wait(),
wait(long)finalize methods of different objects.
Furthermore, there are no guarantees regarding the timing of finalization.
The finalize method might be called on a finalizable object
only after an indefinite delay, if at all.
Classes whose instances hold non-heap resources should provide a method
to enable explicit release of those resources, and they should also
implement AutoCloseable if appropriate.
The Cleaner and PhantomReference
provide more flexible and efficient ways to release resources when an object
becomes unreachable.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 Java 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.
Reference.reachabilityFence(java.lang.Object) can be used to ensure that
objects remain reachable while resources embedded in the object are in use.
A subclass should avoid overriding the finalize method
unless the subclass embeds non-heap resources that must be cleaned up
before the instance is collected.
Finalizer invocations are not automatically chained, unlike constructors.
If a subclass overrides finalize it must invoke the superclass
finalizer explicitly.
To guard against exceptions prematurely terminating the finalize chain,
the subclass should use a try-finally block to ensure
super.finalize() is always invoked. For example,
@Override
protected void finalize() throws Throwable {
try {
... // cleanup subclass state
} finally {
super.finalize();
}
}
Throwable - the Exception raised by this methodWeakReference,
PhantomReferenceReport a bug or suggest an enhancement
For further API reference and developer documentation see the Java SE Documentation, which contains more detailed, developer-targeted descriptions with conceptual overviews, definitions of terms, workarounds, and working code examples.
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