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. | 
| 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 thenotifyAll()method for this object. | 
| void | wait(long timeout)Causes the current thread to wait until either another thread invokes the
  notify()method or thenotifyAll()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 thenotifyAll()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.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
 Java™ 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(), 
HashMapprotected 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.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:
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()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 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.
Throwable - the Exception raised by this methodWeakReference, 
PhantomReference Submit a bug or feature 
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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