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 |
---|---|
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.
|
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. |
protected Object clone() throws CloneNotSupportedException
x
, 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.Cloneable
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()
,
HashMap
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 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 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 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()
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.