public class Object
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 and Description |
|---|
Object() |
| Modifier and Type | Method and Description |
|---|---|
boolean |
equals(Object obj)
Indicates whether some other object is "equal to" this one.
|
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. |
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.Literals, section 15.8.2 of
The Java™ Language Specification.public int hashCode()
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.
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),
System.identityHashCode(java.lang.Object)public 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(),
HashMappublic 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:
notify method for this
object and thread T happens to be arbitrarily chosen as
the thread to be awakened.
notifyAll method for this
object.
timeout is zero, however, then real time is not taken into
consideration and the thread simply waits until notified.
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 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()