Constructor Summary

Method Summary

Constructor Details

• Object

  public Object()

  Constructs a new object.

Method Details

• getClass

  public final Class<?> getClass()

  Returns the runtime class of thisObject. The returnedClass object is the object that is locked by static 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();

  Returns:

  TheClass object that represents the runtime class of this object.

  SeeJava Language Specification:

  15.8.2 Class Literals
  

• hashCode

  public int hashCode()

  Returns a hash code value for this object. This method is supported for the benefit of hash tables such as those provided byHashMap.

  The general contract ofhashCode is:

  • Whenever it is invoked on the same object more than once during an execution of a Java application, thehashCode 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.
  • If two objects are equal according to theequals method, then calling the hashCode method on each of the two objects must produce the same integer result.
  • It isnot required that if two objects are unequal according to theequals 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.

  API Note:

  Thehash andhashCode methods ofObjects can be used to help construct simple hash codes.

  Implementation Requirements:

  As far as is reasonably practical, thehashCode method defined by classObject returns distinct integers for distinct objects.

  Returns:

  a hash code value for this object

  See Also:

  • equals(java.lang.Object)
  • System.identityHashCode(java.lang.Object)

• equals

  public boolean equals(Object obj)

  Indicates whether some other object is "equal to" this one.

  Theequals method implements an equivalence relation on non-null object references:

  • It isreflexive: for any non-null reference valuex,x.equals(x) should returntrue.
  • It issymmetric: for any non-null reference valuesx andy,x.equals(y) should returntrue if and only ify.equals(x) returnstrue.
  • It istransitive: for any non-null reference valuesx,y, andz, ifx.equals(y) returnstrue andy.equals(z) returnstrue, thenx.equals(z) should returntrue.
  • It isconsistent: for any non-null reference valuesx andy, multiple invocations ofx.equals(y) consistently returntrue or consistently returnfalse, provided no information used inequals comparisons on the objects is modified.
  • For any non-null reference valuex,x.equals(null) should returnfalse.

  An equivalence relation partitions the elements it operates on intoequivalence classes; all the members of an equivalence class are equal to each other. Members of an equivalence class are substitutable for each other, at least for some purposes.

  API Note:

  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.

  The two-argumentObjects.equals method implements an equivalence relation on two possibly-null object references.

  Implementation Requirements:

  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). In other words, under the reference equality equivalence relation, each equivalence class only has a single element.

  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:

  • hashCode()
  • HashMap

• clone

  protected Object clone() throwsCloneNotSupportedException

  Creates and returns a copy of this object. The precise meaning of "copy" may depend on the class of the object. The general intent is that, for any objectx, the expression:

   x.clone() != x

  will be true, and that the expression:

   x.clone().getClass() == x.getClass()

  will betrue, but these are not absolute requirements. While it is typically the case that:

   x.clone().equals(x)

  will betrue, 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.

  Implementation Requirements:

  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.

  Returns:

  a clone of this instance.

  Throws:

  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.

  See Also:

  • Cloneable

• toString

  public String toString()

  Returns a string representation of the object.

  API Note:

  In general, thetoString 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 string output is not necessarily stable over time or across JVM invocations.

  Implementation Requirements:

  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:

  Copy

  getClass().getName() + '@' + Integer.toHexString(hashCode())

  TheObjects.toIdentityString method returns the string for an object equal to the string that would be returned if neither thetoString norhashCode methods were overridden by the object's class.

  Returns:

  a string representation of the object

• notify

  public final void notify()

  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 thewait 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 asynchronized statement that synchronizes on the object.
  • For objects of typeClass, by executing a static synchronized 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.

  See Also:

  • notifyAll()
  • wait()

• notifyAll

  public final void notifyAll()

  Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one of thewait 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.

  Throws:

  IllegalMonitorStateException - if the current thread is not the owner of this object's monitor.

  See Also:

  • notify()
  • wait()

• wait

  public final void wait() throwsInterruptedException

  Causes the current thread to wait until it is awakened, typically by beingnotified orinterrupted.

  In all respects, this method behaves as ifwait(0L, 0) had been called. See the specification of thewait(long, int) method for details.

  Throws:

  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. Theinterrupted status of the current thread is cleared when this exception is thrown.

  See Also:

  • notify()
  • notifyAll()
  • wait(long)
  • wait(long, int)

• wait

  public final void wait(long timeoutMillis) throwsInterruptedException

  Causes the current thread to wait until it is awakened, typically by beingnotified orinterrupted, or until a certain amount of real time has elapsed.

  In all respects, this method behaves as ifwait(timeoutMillis, 0) had been called. See the specification of thewait(long, int) method for details.

  Parameters:

  timeoutMillis - the maximum time to wait, in milliseconds

  Throws:

  IllegalArgumentException - iftimeoutMillis 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. Theinterrupted status of the current thread is cleared when this exception is thrown.

  See Also:

  • notify()
  • notifyAll()
  • wait()
  • wait(long, int)

• wait

  public final void wait(long timeoutMillis, int nanos) throwsInterruptedException

  Causes the current thread to wait until it is awakened, typically by beingnotified orinterrupted, or until a certain amount of real time has elapsed.

  The current thread must own this object's monitor lock. See thenotify method for a description of the ways in which a thread can become the owner of a monitor lock.

  This method causes the current thread (referred to here asT) to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. Note that only the locks on this object are relinquished; any other objects on which the current thread may be synchronized remain locked while the thread waits.

  ThreadT then becomes disabled for thread scheduling purposes and lies dormant until one of the following occurs:

  • Some other thread invokes thenotify method for this object and threadT happens to be arbitrarily chosen as the thread to be awakened.
  • Some other thread invokes thenotifyAll method for this object.
  • Some other threadinterrupts threadT.
  • The specified amount of real time has elapsed, more or less. The amount of real time, in nanoseconds, is given by the expression1000000 * timeoutMillis + nanos. IftimeoutMillis andnanos are both zero, then real time is not taken into consideration and the thread waits until awakened by one of the other causes.
  • ThreadT is awakened spuriously. (See below.)

  The threadT is then removed from the wait set for this object and re-enabled for thread scheduling. It competes in the usual manner with other threads for the right to synchronize on the object; once it has regained 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 thewait 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 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. See the example below.

  For more information on this topic, see section 14.2, "Condition Queues," in Brian Goetz and others'Java Concurrency in Practice (Addison-Wesley, 2006) or Item 81 in Joshua Bloch'sEffective Java, Third Edition (Addison-Wesley, 2018).

  If the current thread isinterrupted by any thread before or while it is waiting, then anInterruptedException is thrown. Theinterrupted status of the current thread is cleared when this exception is thrown. This exception is not thrown until the lock status of this object has been restored as described above.

  API Note:

  The recommended approach to waiting is to check the condition being awaited in awhile loop around the call towait, as shown in the example below. Among other things, this approach avoids problems that can be caused by spurious wakeups.

  Copy

      synchronized (obj) {        while ( <condition does not hold and timeout not exceeded> ) {            long timeoutMillis = ... ; // recompute timeout values            int nanos = ... ;            obj.wait(timeoutMillis, nanos);        }        ... // Perform action appropriate to condition or timeout    }

  Parameters:

  timeoutMillis - the maximum time to wait, in milliseconds

  nanos - additional time, in nanoseconds, in the range 0-999999 inclusive

  Throws:

  IllegalArgumentException - iftimeoutMillis is negative, or if the value ofnanos is out of range

  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. Theinterrupted status of the current thread is cleared when this exception is thrown.

  See Also:

  • notify()
  • notifyAll()
  • wait()
  • wait(long)

• finalize

  @Deprecated(since="9",forRemoval=true)protected void finalize() throwsThrowable

  Deprecated, for removal: This API element is subject to removal in a future version.

  Finalization is deprecated and subject to removal in a future release. The use of finalization can lead to problems with security, performance, and reliability. SeeJEP 421 for discussion and alternatives.

  Subclasses that overridefinalize to perform cleanup should use alternative cleanup mechanisms and remove thefinalize method. UseCleaner andPhantomReference as safer ways to release resources when an object becomes unreachable. Alternatively, add aclose method to explicitly release resources, and implementAutoCloseable to enable use of thetry-with-resources statement.

  This method will remain in place until finalizers have been removed from most existing code.

  Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides thefinalize method to dispose of system resources or to perform other cleanup.

  When running in a Java virtual machine in which finalization has been disabled or removed, the garbage collector will never callfinalize(). In a Java virtual machine in which finalization is enabled, the garbage collector might callfinalize only after an indefinite delay.

  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.

  API Note:

  Classes that embed non-heap resources have many options for cleanup of those resources. The class must ensure that the lifetime of each instance is longer than that of any resource it embeds.Reference.reachabilityFence(java.lang.Object) can be used to ensure that objects remain reachable while resources embedded in the object are in use.

  A subclass should avoid overriding thefinalize method unless the subclass embeds non-heap resources that must be cleaned up before the instance is collected. Finalizer invocations are not automatically chained, unlike constructors. If a subclass overridesfinalize it must invoke the superclass finalizer explicitly. To guard against exceptions prematurely terminating the finalize chain, the subclass should use atry-finally block to ensuresuper.finalize() is always invoked. For example,

  Copy

      @Override    protected void finalize() throws Throwable {        try {            ... // cleanup subclass state        } finally {            super.finalize();        }    }

  Throws:

  Throwable - theException raised by this method

  SeeJava Language Specification:

  12.6 Finalization of Class Instances
  

  See Also:

  • WeakReference
  • PhantomReference