src/share/classes/java/lang/Object.java
author darcy
Thu Jan 29 09:04:41 2009 -0800 (9 months ago)
changeset 845 7241bd422542
parent 037a05a11f281
permissions -rw-r--r--
6239194: Object.hashCode() should reference System.identityHashCode()
Reviewed-by: emcmanus
        1 /*
        2  * Copyright 1994-2006 Sun Microsystems, Inc.  All Rights Reserved.
        3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
        4  *
        5  * This code is free software; you can redistribute it and/or modify it
        6  * under the terms of the GNU General Public License version 2 only, as
        7  * published by the Free Software Foundation.  Sun designates this
        8  * particular file as subject to the "Classpath" exception as provided
        9  * by Sun in the LICENSE file that accompanied this code.
       10  *
       11  * This code is distributed in the hope that it will be useful, but WITHOUT
       12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
       13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
       14  * version 2 for more details (a copy is included in the LICENSE file that
       15  * accompanied this code).
       16  *
       17  * You should have received a copy of the GNU General Public License version
       18  * 2 along with this work; if not, write to the Free Software Foundation,
       19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
       20  *
       21  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
       22  * CA 95054 USA or visit www.sun.com if you need additional information or
       23  * have any questions.
       24  */
       25 
       26 package java.lang;
       27 
       28 /**
       29  * Class {@code Object} is the root of the class hierarchy.
       30  * Every class has {@code Object} as a superclass. All objects,
       31  * including arrays, implement the methods of this class.
       32  *
       33  * @author  unascribed
       34  * @see     java.lang.Class
       35  * @since   JDK1.0
       36  */
       37 public class Object {
       38 
       39     private static native void registerNatives();
       40     static {
       41         registerNatives();
       42     }
       43 
       44     /**
       45      * Returns the runtime class of this {@code Object}. The returned
       46      * {@code Class} object is the object that is locked by {@code
       47      * static synchronized} methods of the represented class.
       48      *
       49      * <p><b>The actual result type is {@code Class<? extends |X|>}
       50      * where {@code |X|} is the erasure of the static type of the
       51      * expression on which {@code getClass} is called.</b> For
       52      * example, no cast is required in this code fragment:</p>
       53      *
       54      * <p>
       55      * {@code Number n = 0;                             }<br>
       56      * {@code Class<? extends Number> c = n.getClass(); }
       57      * </p>
       58      *
       59      * @return The {@code Class} object that represents the runtime
       60      *         class of this object.
       61      * @see    <a href="http://java.sun.com/docs/books/jls/">The Java
       62      *         Language Specification, Third Edition (15.8.2 Class
       63      *         Literals)</a>
       64      */
       65     public final native Class<?> getClass();
       66 
       67     /**
       68      * Returns a hash code value for the object. This method is
       69      * supported for the benefit of hash tables such as those provided by
       70      * {@link java.util.HashMap}.
       71      * <p>
       72      * The general contract of {@code hashCode} is:
       73      * <ul>
       74      * <li>Whenever it is invoked on the same object more than once during
       75      *     an execution of a Java application, the {@code hashCode} method
       76      *     must consistently return the same integer, provided no information
       77      *     used in {@code equals} comparisons on the object is modified.
       78      *     This integer need not remain consistent from one execution of an
       79      *     application to another execution of the same application.
       80      * <li>If two objects are equal according to the {@code equals(Object)}
       81      *     method, then calling the {@code hashCode} method on each of
       82      *     the two objects must produce the same integer result.
       83      * <li>It is <em>not</em> required that if two objects are unequal
       84      *     according to the {@link java.lang.Object#equals(java.lang.Object)}
       85      *     method, then calling the {@code hashCode} method on each of the
       86      *     two objects must produce distinct integer results.  However, the
       87      *     programmer should be aware that producing distinct integer results
       88      *     for unequal objects may improve the performance of hash tables.
       89      * </ul>
       90      * <p>
       91      * As much as is reasonably practical, the hashCode method defined by
       92      * class {@code Object} does return distinct integers for distinct
       93      * objects. (This is typically implemented by converting the internal
       94      * address of the object into an integer, but this implementation
       95      * technique is not required by the
       96      * Java<font size="-2"><sup>TM</sup></font> programming language.)
       97      *
       98      * @return  a hash code value for this object.
       99      * @see     java.lang.Object#equals(java.lang.Object)
      100      * @see     java.lang.System#identityHashCode
      101      */
      102     public native int hashCode();
      103 
      104     /**
      105      * Indicates whether some other object is "equal to" this one.
      106      * <p>
      107      * The {@code equals} method implements an equivalence relation
      108      * on non-null object references:
      109      * <ul>
      110      * <li>It is <i>reflexive</i>: for any non-null reference value
      111      *     {@code x}, {@code x.equals(x)} should return
      112      *     {@code true}.
      113      * <li>It is <i>symmetric</i>: for any non-null reference values
      114      *     {@code x} and {@code y}, {@code x.equals(y)}
      115      *     should return {@code true} if and only if
      116      *     {@code y.equals(x)} returns {@code true}.
      117      * <li>It is <i>transitive</i>: for any non-null reference values
      118      *     {@code x}, {@code y}, and {@code z}, if
      119      *     {@code x.equals(y)} returns {@code true} and
      120      *     {@code y.equals(z)} returns {@code true}, then
      121      *     {@code x.equals(z)} should return {@code true}.
      122      * <li>It is <i>consistent</i>: for any non-null reference values
      123      *     {@code x} and {@code y}, multiple invocations of
      124      *     {@code x.equals(y)} consistently return {@code true}
      125      *     or consistently return {@code false}, provided no
      126      *     information used in {@code equals} comparisons on the
      127      *     objects is modified.
      128      * <li>For any non-null reference value {@code x},
      129      *     {@code x.equals(null)} should return {@code false}.
      130      * </ul>
      131      * <p>
      132      * The {@code equals} method for class {@code Object} implements
      133      * the most discriminating possible equivalence relation on objects;
      134      * that is, for any non-null reference values {@code x} and
      135      * {@code y}, this method returns {@code true} if and only
      136      * if {@code x} and {@code y} refer to the same object
      137      * ({@code x == y} has the value {@code true}).
      138      * <p>
      139      * Note that it is generally necessary to override the {@code hashCode}
      140      * method whenever this method is overridden, so as to maintain the
      141      * general contract for the {@code hashCode} method, which states
      142      * that equal objects must have equal hash codes.
      143      *
      144      * @param   obj   the reference object with which to compare.
      145      * @return  {@code true} if this object is the same as the obj
      146      *          argument; {@code false} otherwise.
      147      * @see     #hashCode()
      148      * @see     java.util.HashMap
      149      */
      150     public boolean equals(Object obj) {
      151         return (this == obj);
      152     }
      153 
      154     /**
      155      * Creates and returns a copy of this object.  The precise meaning
      156      * of "copy" may depend on the class of the object. The general
      157      * intent is that, for any object {@code x}, the expression:
      158      * <blockquote>
      159      * <pre>
      160      * x.clone() != x</pre></blockquote>
      161      * will be true, and that the expression:
      162      * <blockquote>
      163      * <pre>
      164      * x.clone().getClass() == x.getClass()</pre></blockquote>
      165      * will be {@code true}, but these are not absolute requirements.
      166      * While it is typically the case that:
      167      * <blockquote>
      168      * <pre>
      169      * x.clone().equals(x)</pre></blockquote>
      170      * will be {@code true}, this is not an absolute requirement.
      171      * <p>
      172      * By convention, the returned object should be obtained by calling
      173      * {@code super.clone}.  If a class and all of its superclasses (except
      174      * {@code Object}) obey this convention, it will be the case that
      175      * {@code x.clone().getClass() == x.getClass()}.
      176      * <p>
      177      * By convention, the object returned by this method should be independent
      178      * of this object (which is being cloned).  To achieve this independence,
      179      * it may be necessary to modify one or more fields of the object returned
      180      * by {@code super.clone} before returning it.  Typically, this means
      181      * copying any mutable objects that comprise the internal "deep structure"
      182      * of the object being cloned and replacing the references to these
      183      * objects with references to the copies.  If a class contains only
      184      * primitive fields or references to immutable objects, then it is usually
      185      * the case that no fields in the object returned by {@code super.clone}
      186      * need to be modified.
      187      * <p>
      188      * The method {@code clone} for class {@code Object} performs a
      189      * specific cloning operation. First, if the class of this object does
      190      * not implement the interface {@code Cloneable}, then a
      191      * {@code CloneNotSupportedException} is thrown. Note that all arrays
      192      * are considered to implement the interface {@code Cloneable}.
      193      * Otherwise, this method creates a new instance of the class of this
      194      * object and initializes all its fields with exactly the contents of
      195      * the corresponding fields of this object, as if by assignment; the
      196      * contents of the fields are not themselves cloned. Thus, this method
      197      * performs a "shallow copy" of this object, not a "deep copy" operation.
      198      * <p>
      199      * The class {@code Object} does not itself implement the interface
      200      * {@code Cloneable}, so calling the {@code clone} method on an object
      201      * whose class is {@code Object} will result in throwing an
      202      * exception at run time.
      203      *
      204      * @return     a clone of this instance.
      205      * @exception  CloneNotSupportedException  if the object's class does not
      206      *               support the {@code Cloneable} interface. Subclasses
      207      *               that override the {@code clone} method can also
      208      *               throw this exception to indicate that an instance cannot
      209      *               be cloned.
      210      * @see java.lang.Cloneable
      211      */
      212     protected native Object clone() throws CloneNotSupportedException;
      213 
      214     /**
      215      * Returns a string representation of the object. In general, the
      216      * {@code toString} method returns a string that
      217      * "textually represents" this object. The result should
      218      * be a concise but informative representation that is easy for a
      219      * person to read.
      220      * It is recommended that all subclasses override this method.
      221      * <p>
      222      * The {@code toString} method for class {@code Object}
      223      * returns a string consisting of the name of the class of which the
      224      * object is an instance, the at-sign character `{@code @}', and
      225      * the unsigned hexadecimal representation of the hash code of the
      226      * object. In other words, this method returns a string equal to the
      227      * value of:
      228      * <blockquote>
      229      * <pre>
      230      * getClass().getName() + '@' + Integer.toHexString(hashCode())
      231      * </pre></blockquote>
      232      *
      233      * @return  a string representation of the object.
      234      */
      235     public String toString() {
      236         return getClass().getName() + "@" + Integer.toHexString(hashCode());
      237     }
      238 
      239     /**
      240      * Wakes up a single thread that is waiting on this object's
      241      * monitor. If any threads are waiting on this object, one of them
      242      * is chosen to be awakened. The choice is arbitrary and occurs at
      243      * the discretion of the implementation. A thread waits on an object's
      244      * monitor by calling one of the {@code wait} methods.
      245      * <p>
      246      * The awakened thread will not be able to proceed until the current
      247      * thread relinquishes the lock on this object. The awakened thread will
      248      * compete in the usual manner with any other threads that might be
      249      * actively competing to synchronize on this object; for example, the
      250      * awakened thread enjoys no reliable privilege or disadvantage in being
      251      * the next thread to lock this object.
      252      * <p>
      253      * This method should only be called by a thread that is the owner
      254      * of this object's monitor. A thread becomes the owner of the
      255      * object's monitor in one of three ways:
      256      * <ul>
      257      * <li>By executing a synchronized instance method of that object.
      258      * <li>By executing the body of a {@code synchronized} statement
      259      *     that synchronizes on the object.
      260      * <li>For objects of type {@code Class,} by executing a
      261      *     synchronized static method of that class.
      262      * </ul>
      263      * <p>
      264      * Only one thread at a time can own an object's monitor.
      265      *
      266      * @exception  IllegalMonitorStateException  if the current thread is not
      267      *               the owner of this object's monitor.
      268      * @see        java.lang.Object#notifyAll()
      269      * @see        java.lang.Object#wait()
      270      */
      271     public final native void notify();
      272 
      273     /**
      274      * Wakes up all threads that are waiting on this object's monitor. A
      275      * thread waits on an object's monitor by calling one of the
      276      * {@code wait} methods.
      277      * <p>
      278      * The awakened threads will not be able to proceed until the current
      279      * thread relinquishes the lock on this object. The awakened threads
      280      * will compete in the usual manner with any other threads that might
      281      * be actively competing to synchronize on this object; for example,
      282      * the awakened threads enjoy no reliable privilege or disadvantage in
      283      * being the next thread to lock this object.
      284      * <p>
      285      * This method should only be called by a thread that is the owner
      286      * of this object's monitor. See the {@code notify} method for a
      287      * description of the ways in which a thread can become the owner of
      288      * a monitor.
      289      *
      290      * @exception  IllegalMonitorStateException  if the current thread is not
      291      *               the owner of this object's monitor.
      292      * @see        java.lang.Object#notify()
      293      * @see        java.lang.Object#wait()
      294      */
      295     public final native void notifyAll();
      296 
      297     /**
      298      * Causes the current thread to wait until either another thread invokes the
      299      * {@link java.lang.Object#notify()} method or the
      300      * {@link java.lang.Object#notifyAll()} method for this object, or a
      301      * specified amount of time has elapsed.
      302      * <p>
      303      * The current thread must own this object's monitor.
      304      * <p>
      305      * This method causes the current thread (call it <var>T</var>) to
      306      * place itself in the wait set for this object and then to relinquish
      307      * any and all synchronization claims on this object. Thread <var>T</var>
      308      * becomes disabled for thread scheduling purposes and lies dormant
      309      * until one of four things happens:
      310      * <ul>
      311      * <li>Some other thread invokes the {@code notify} method for this
      312      * object and thread <var>T</var> happens to be arbitrarily chosen as
      313      * the thread to be awakened.
      314      * <li>Some other thread invokes the {@code notifyAll} method for this
      315      * object.
      316      * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
      317      * thread <var>T</var>.
      318      * <li>The specified amount of real time has elapsed, more or less.  If
      319      * {@code timeout} is zero, however, then real time is not taken into
      320      * consideration and the thread simply waits until notified.
      321      * </ul>
      322      * The thread <var>T</var> is then removed from the wait set for this
      323      * object and re-enabled for thread scheduling. It then competes in the
      324      * usual manner with other threads for the right to synchronize on the
      325      * object; once it has gained control of the object, all its
      326      * synchronization claims on the object are restored to the status quo
      327      * ante - that is, to the situation as of the time that the {@code wait}
      328      * method was invoked. Thread <var>T</var> then returns from the
      329      * invocation of the {@code wait} method. Thus, on return from the
      330      * {@code wait} method, the synchronization state of the object and of
      331      * thread {@code T} is exactly as it was when the {@code wait} method
      332      * was invoked.
      333      * <p>
      334      * A thread can also wake up without being notified, interrupted, or
      335      * timing out, a so-called <i>spurious wakeup</i>.  While this will rarely
      336      * occur in practice, applications must guard against it by testing for
      337      * the condition that should have caused the thread to be awakened, and
      338      * continuing to wait if the condition is not satisfied.  In other words,
      339      * waits should always occur in loops, like this one:
      340      * <pre>
      341      *     synchronized (obj) {
      342      *         while (&lt;condition does not hold&gt;)
      343      *             obj.wait(timeout);
      344      *         ... // Perform action appropriate to condition
      345      *     }
      346      * </pre>
      347      * (For more information on this topic, see Section 3.2.3 in Doug Lea's
      348      * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
      349      * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
      350      * Language Guide" (Addison-Wesley, 2001).
      351      *
      352      * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
      353      * interrupted} by any thread before or while it is waiting, then an
      354      * {@code InterruptedException} is thrown.  This exception is not
      355      * thrown until the lock status of this object has been restored as
      356      * described above.
      357      *
      358      * <p>
      359      * Note that the {@code wait} method, as it places the current thread
      360      * into the wait set for this object, unlocks only this object; any
      361      * other objects on which the current thread may be synchronized remain
      362      * locked while the thread waits.
      363      * <p>
      364      * This method should only be called by a thread that is the owner
      365      * of this object's monitor. See the {@code notify} method for a
      366      * description of the ways in which a thread can become the owner of
      367      * a monitor.
      368      *
      369      * @param      timeout   the maximum time to wait in milliseconds.
      370      * @exception  IllegalArgumentException      if the value of timeout is
      371      *               negative.
      372      * @exception  IllegalMonitorStateException  if the current thread is not
      373      *               the owner of the object's monitor.
      374      * @exception  InterruptedException if any thread interrupted the
      375      *             current thread before or while the current thread
      376      *             was waiting for a notification.  The <i>interrupted
      377      *             status</i> of the current thread is cleared when
      378      *             this exception is thrown.
      379      * @see        java.lang.Object#notify()
      380      * @see        java.lang.Object#notifyAll()
      381      */
      382     public final native void wait(long timeout) throws InterruptedException;
      383 
      384     /**
      385      * Causes the current thread to wait until another thread invokes the
      386      * {@link java.lang.Object#notify()} method or the
      387      * {@link java.lang.Object#notifyAll()} method for this object, or
      388      * some other thread interrupts the current thread, or a certain
      389      * amount of real time has elapsed.
      390      * <p>
      391      * This method is similar to the {@code wait} method of one
      392      * argument, but it allows finer control over the amount of time to
      393      * wait for a notification before giving up. The amount of real time,
      394      * measured in nanoseconds, is given by:
      395      * <blockquote>
      396      * <pre>
      397      * 1000000*timeout+nanos</pre></blockquote>
      398      * <p>
      399      * In all other respects, this method does the same thing as the
      400      * method {@link #wait(long)} of one argument. In particular,
      401      * {@code wait(0, 0)} means the same thing as {@code wait(0)}.
      402      * <p>
      403      * The current thread must own this object's monitor. The thread
      404      * releases ownership of this monitor and waits until either of the
      405      * following two conditions has occurred:
      406      * <ul>
      407      * <li>Another thread notifies threads waiting on this object's monitor
      408      *     to wake up either through a call to the {@code notify} method
      409      *     or the {@code notifyAll} method.
      410      * <li>The timeout period, specified by {@code timeout}
      411      *     milliseconds plus {@code nanos} nanoseconds arguments, has
      412      *     elapsed.
      413      * </ul>
      414      * <p>
      415      * The thread then waits until it can re-obtain ownership of the
      416      * monitor and resumes execution.
      417      * <p>
      418      * As in the one argument version, interrupts and spurious wakeups are
      419      * possible, and this method should always be used in a loop:
      420      * <pre>
      421      *     synchronized (obj) {
      422      *         while (&lt;condition does not hold&gt;)
      423      *             obj.wait(timeout, nanos);
      424      *         ... // Perform action appropriate to condition
      425      *     }
      426      * </pre>
      427      * This method should only be called by a thread that is the owner
      428      * of this object's monitor. See the {@code notify} method for a
      429      * description of the ways in which a thread can become the owner of
      430      * a monitor.
      431      *
      432      * @param      timeout   the maximum time to wait in milliseconds.
      433      * @param      nanos      additional time, in nanoseconds range
      434      *                       0-999999.
      435      * @exception  IllegalArgumentException      if the value of timeout is
      436      *                      negative or the value of nanos is
      437      *                      not in the range 0-999999.
      438      * @exception  IllegalMonitorStateException  if the current thread is not
      439      *               the owner of this object's monitor.
      440      * @exception  InterruptedException if any thread interrupted the
      441      *             current thread before or while the current thread
      442      *             was waiting for a notification.  The <i>interrupted
      443      *             status</i> of the current thread is cleared when
      444      *             this exception is thrown.
      445      */
      446     public final void wait(long timeout, int nanos) throws InterruptedException {
      447         if (timeout < 0) {
      448             throw new IllegalArgumentException("timeout value is negative");
      449         }
      450 
      451         if (nanos < 0 || nanos > 999999) {
      452             throw new IllegalArgumentException(
      453                                 "nanosecond timeout value out of range");
      454         }
      455 
      456         if (nanos >= 500000 || (nanos != 0 && timeout == 0)) {
      457             timeout++;
      458         }
      459 
      460         wait(timeout);
      461     }
      462 
      463     /**
      464      * Causes the current thread to wait until another thread invokes the
      465      * {@link java.lang.Object#notify()} method or the
      466      * {@link java.lang.Object#notifyAll()} method for this object.
      467      * In other words, this method behaves exactly as if it simply
      468      * performs the call {@code wait(0)}.
      469      * <p>
      470      * The current thread must own this object's monitor. The thread
      471      * releases ownership of this monitor and waits until another thread
      472      * notifies threads waiting on this object's monitor to wake up
      473      * either through a call to the {@code notify} method or the
      474      * {@code notifyAll} method. The thread then waits until it can
      475      * re-obtain ownership of the monitor and resumes execution.
      476      * <p>
      477      * As in the one argument version, interrupts and spurious wakeups are
      478      * possible, and this method should always be used in a loop:
      479      * <pre>
      480      *     synchronized (obj) {
      481      *         while (&lt;condition does not hold&gt;)
      482      *             obj.wait();
      483      *         ... // Perform action appropriate to condition
      484      *     }
      485      * </pre>
      486      * This method should only be called by a thread that is the owner
      487      * of this object's monitor. See the {@code notify} method for a
      488      * description of the ways in which a thread can become the owner of
      489      * a monitor.
      490      *
      491      * @exception  IllegalMonitorStateException  if the current thread is not
      492      *               the owner of the object's monitor.
      493      * @exception  InterruptedException if any thread interrupted the
      494      *             current thread before or while the current thread
      495      *             was waiting for a notification.  The <i>interrupted
      496      *             status</i> of the current thread is cleared when
      497      *             this exception is thrown.
      498      * @see        java.lang.Object#notify()
      499      * @see        java.lang.Object#notifyAll()
      500      */
      501     public final void wait() throws InterruptedException {
      502         wait(0);
      503     }
      504 
      505     /**
      506      * Called by the garbage collector on an object when garbage collection
      507      * determines that there are no more references to the object.
      508      * A subclass overrides the {@code finalize} method to dispose of
      509      * system resources or to perform other cleanup.
      510      * <p>
      511      * The general contract of {@code finalize} is that it is invoked
      512      * if and when the Java<font size="-2"><sup>TM</sup></font> virtual
      513      * machine has determined that there is no longer any
      514      * means by which this object can be accessed by any thread that has
      515      * not yet died, except as a result of an action taken by the
      516      * finalization of some other object or class which is ready to be
      517      * finalized. The {@code finalize} method may take any action, including
      518      * making this object available again to other threads; the usual purpose
      519      * of {@code finalize}, however, is to perform cleanup actions before
      520      * the object is irrevocably discarded. For example, the finalize method
      521      * for an object that represents an input/output connection might perform
      522      * explicit I/O transactions to break the connection before the object is
      523      * permanently discarded.
      524      * <p>
      525      * The {@code finalize} method of class {@code Object} performs no
      526      * special action; it simply returns normally. Subclasses of
      527      * {@code Object} may override this definition.
      528      * <p>
      529      * The Java programming language does not guarantee which thread will
      530      * invoke the {@code finalize} method for any given object. It is
      531      * guaranteed, however, that the thread that invokes finalize will not
      532      * be holding any user-visible synchronization locks when finalize is
      533      * invoked. If an uncaught exception is thrown by the finalize method,
      534      * the exception is ignored and finalization of that object terminates.
      535      * <p>
      536      * After the {@code finalize} method has been invoked for an object, no
      537      * further action is taken until the Java virtual machine has again
      538      * determined that there is no longer any means by which this object can
      539      * be accessed by any thread that has not yet died, including possible
      540      * actions by other objects or classes which are ready to be finalized,
      541      * at which point the object may be discarded.
      542      * <p>
      543      * The {@code finalize} method is never invoked more than once by a Java
      544      * virtual machine for any given object.
      545      * <p>
      546      * Any exception thrown by the {@code finalize} method causes
      547      * the finalization of this object to be halted, but is otherwise
      548      * ignored.
      549      *
      550      * @throws Throwable the {@code Exception} raised by this method
      551      */
      552     protected void finalize() throws Throwable { }
      553 }