public classObjectObject is the root of the class hierarchy. Every class hasObject as a superclass. All objects, including arrays, implement the methods of this class.Class| Constructor | Description |
|---|---|
Object() |
| Modifier and Type | Method | Description |
|---|---|---|
protectedObject | 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 returnedClass object is the object that is locked bystatic synchronized methods of the represented class.The actual result type isClass<? extends |X|> where|X| is the erasure of the static type of the expression on whichgetClass 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 ofhashCode is:
hashCode method must consistently return the same integer, provided no information used inequals 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 thehashCode method on each of the two objects must produce the same integer result.equals(java.lang.Object) method, then calling thehashCode 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 classObject 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)
Theequals method implements an equivalence relation on non-null object references:
x,x.equals(x) should returntrue.x andy,x.equals(y) should returntrue if and only ify.equals(x) returnstrue.x,y, andz, ifx.equals(y) returnstrue andy.equals(z) returnstrue, thenx.equals(z) should returntrue.x andy, multiple invocations ofx.equals(y) consistently returntrue or consistently returnfalse, provided no information used inequals comparisons on the objects is modified.x,x.equals(null) should returnfalse. Theequals method for classObject implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference valuesx andy, this method returnstrue if and only ifx andy refer to the same object (x == y has the valuetrue).
Note that it is generally necessary to override thehashCode method whenever this method is overridden, so as to maintain the general contract for thehashCode 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() throwsCloneNotSupportedException
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 callingsuper.clone. If a class and all of its superclasses (exceptObject) obey this convention, it will be the case thatx.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 bysuper.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 bysuper.clone need to be modified.
The methodclone for classObject performs a specific cloning operation. First, if the class of this object does not implement the interfaceCloneable, then aCloneNotSupportedException is thrown. Note that all arrays are considered to implement the interfaceCloneable and that the return type of theclone method of an array typeT[] isT[] 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 classObject does not itself implement the interfaceCloneable, so calling theclone method on an object whose class isObject will result in throwing an exception at run time.
CloneNotSupportedException - if the object's class does not support theCloneable interface. Subclasses that override theclone 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. ThetoString method for classObject 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 thenotify 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) throwsInterruptedException
notify() method or thenotifyAll() 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 itT) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. ThreadT becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:
notify method for this object and threadT 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. ThreadT then returns from the invocation of thewait method. Thus, on return from thewait method, the synchronization state of the object and of threadT is exactly as it was when thewait method was invoked.A thread can also wake up without being notified, interrupted, or timing out, a so-calledspurious 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 isinterrupted by any thread before or while it is waiting, then anInterruptedException is thrown. This exception is not thrown until the lock status of this object has been restored as described above.
Note that thewait 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 thenotify 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. Theinterrupted status of the current thread is cleared when this exception is thrown.notify(),notifyAll()public final void wait(long timeout, int nanos) throwsInterruptedException
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. This method is similar to thewait 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 methodwait(long) of one argument. In particular,wait(0, 0) means the same thing aswait(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 thenotifyAll method.timeout milliseconds plusnanos 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 thenotify 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. Theinterrupted status of the current thread is cleared when this exception is thrown.public final void wait() throwsInterruptedException
notify() method or thenotifyAll() method for this object. In other words, this method behaves exactly as if it simply performs the callwait(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 thenotify method or thenotifyAll 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 thenotify 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. Theinterrupted status of the current thread is cleared when this exception is thrown.notify(),notifyAll()protected void finalize() throwsThrowable
finalize method to dispose of system resources or to perform other cleanup. The general contract offinalize 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. Thefinalize method may take any action, including making this object available again to other threads; the usual purpose offinalize, 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.
Thefinalize method of classObject performs no special action; it simply returns normally. Subclasses ofObject may override this definition.
The Java programming language does not guarantee which thread will invoke thefinalize 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 thefinalize 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.
Thefinalize method is never invoked more than once by a Java virtual machine for any given object.
Any exception thrown by thefinalize method causes the finalization of this object to be halted, but is otherwise ignored.
Throwable - theException raised by this methodWeakReference,PhantomReference