java.lang Object

java.lang
Object

Declaration
public class Object
java.lang.Object
Description
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.
Since: JDK1.0, CLDC 1.0


Member Summary
Constructors
Object()
Methods
boolean equals(Object obj)
Class getClass()
int hashCode()
void notify()
void notifyAll()
String toString()
void wait()
void wait(long timeout)
void wait(long timeout, int nanos)


Constructors
Object()
Declaration:
public Object()
Methods
getClass()
Declaration:
public final java.lang.Class getClass()


Description:
Returns the runtime class of an object. That Class object is the object that is locked by static
synchronized methods of the represented class.
Returns: the object of type Class that represents the runtime class of the object.
hashCode()
Declaration:
public int hashCode()
Description:
Returns a hash code value for the object. This method is supported for the benefit of hashtables such as
those provided by java.util.Hashtable.
The general contract of hashCode is:
• Whenever it is invoked on the same object more than once during an execution of a Java application,
the 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.
• If two objects are equal according to the equals(Object) method, then calling the hashCode
method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(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.)
Returns: a hash code value for this object.
See Also: equals(Object), java.util.Hashtable
equals(Object)
Declaration:
public boolean equals(java.lang.Object obj)
Description:
Indicates whether some other object is “equal to” this one.
The equals method implements an equivalence relation:
• It is reflexive: for any reference value x, x.equals(x) should return true.
• It is symmetric: for any reference values x and y, x.equals(y) should return true if and only if
y.equals(x) returns true.
• It is transitive: for any reference values x, y, and z, if x.equals(y) returns true and
y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any reference values 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 object is modified.
• For any non-null reference value x, x.equals(null) should return false.


toString()
184
The equals method for class Object implements the most discriminating possible equivalence relation
on objects; that is, for any 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).
Parameters:
obj - the reference object with which to compare.
Returns: true if this object is the same as the obj argument; false otherwise.
See Also: Boolean.hashCode(), java.util.Hashtable
toString()
Declaration:
public java.lang.String toString()
Description:
Returns a string representation of the object. In general, the 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())
Returns: a string representation of the object.
notify()
Declaration:
public final void notify()
Description:
Wakes up a single thread that is waiting on this object’s monitor. If any threads are waiting on this object,
one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the
implementation. A thread waits on an object’s monitor by calling one of the 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:
• By executing a synchronized instance method of that object.
• By executing the body of a synchronized statement that synchronizes on the object.
• For objects of type Class, by executing a synchronized static method of that class.
Only one thread at a time can own an object’s monitor.
Throws:
IllegalMonitorStateException - if the current thread is not the owner of this object’s
monitor.

notifyAll()
Declaration:
public final void notifyAll()
Description:
Wakes up all threads that are waiting on this object’s monitor. A thread waits on an object’s monitor by
calling one of the 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.
Throws:
IllegalMonitorStateException - if the current thread is not the owner of this object’s
monitor.
See Also: notify(), wait()
wait(long)
Declaration:
public final void wait(long timeout)
throws InterruptedException
Description:
Causes 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.
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:
• Some other thread invokes the notify method for this object and thread T happens to be arbitrarily
chosen as the thread to be awakened.
• Some other thread invokes the notifyAll method for this object.
• Some other thread interrupts thread T.
• The specified amount of real time has elapsed, more or less. If timeout is zero, however, then real
time is not taken into consideration and the thread simply waits until notified.
The thread T is then removed from the wait set for this object and re-enabled for thread scheduling. It then
competes in the usual manner with other threads for the right to synchronize on the object; once it has
gained 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.
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.


wait(long, int)
186
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.
Parameters:
timeout - the maximum time to wait in milliseconds.
Throws:
IllegalArgumentException - if the value of timeout is negative.
IllegalMonitorStateException - if the current thread is not the owner of the object’s
monitor.
InterruptedException - if another thread has interrupted the current thread. The interrupted
status of the current thread is cleared when this exception is thrown.
See Also: notify(), notifyAll()
wait(long, int)
Declaration:
public final void wait(long timeout, int nanos)
throws InterruptedException
Description:
Causes 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.
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:
• 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 timeout period, specified by timeout milliseconds plus nanos nanoseconds arguments, has
elapsed.
The thread then waits until it can re-obtain ownership of the monitor and resumes execution
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.
Parameters:
timeout - the maximum time to wait in milliseconds.
nanos - additional time, in nanoseconds range 0-999999.



Throws:
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 another thread has interrupted the current thread. The interrupted
status of the current thread is cleared when this exception is thrown.
wait()
Declaration:
public final void wait()
throws InterruptedException
Description:
Causes current thread to wait until another thread invokes the notify() method or the notifyAll()
method for this object. In other word’s 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.
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.
Throws:
IllegalMonitorStateException - if the current thread is not the owner of the object’s
monitor.
InterruptedException - if another thread has interrupted the current thread. The interrupted
status of the current thread is cleared when this exception is thrown.
See Also: notify(), notifyAll()