src/share/classes/java/lang/Object.java
author jjh
Wed Apr 13 12:16:13 2011 -0700 (13 months ago)
changeset 4008 2dc552b0ebac
parent 3261a06412e13bf7
permissions -rw-r--r--
7032960: API files in java.awt need to be updated for references to JVM Spec with editions/hyperlinks
7032965: API files in java.io need to updated for references to JVM Spec with editions/hyperlinks
7032958: API files in java.lang need to updated for references to JLS with editions/hyperlinks
7032961: API files in java.lang need to updated for references to JVM with editions/hyperlinks
7032976: API files in javax.lang need to be updated for references to JLS with editions/hyperlinks
7032959: API files in java.util need to updated for references to JLS with editions/hyperlinks
7032962: API files in java.util need to updated for references to JVM Spec with editions/hyperlinks
7032967: API files in java.security need to updated for references to JVM Spec with editions/hyperlinks
7032955: API files in java.math need to updated for references to JLS with editions/hyperlinks
Summary: Removed URLs and 'edition' references
Reviewed-by: darcy
        1 /*
        2  * Copyright (c) 1994, 2010, Oracle and/or its affiliates. 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.  Oracle designates this
        8  * particular file as subject to the "Classpath" exception as provided
        9  * by Oracle 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
       22  * or visit www.oracle.com if you need additional information or have any
       23  * 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    Class Literals, section 15.8.2 of
       62      *         <cite>The Java&trade; Language Specification</cite>.
       63      */
       64     public final native Class<?> getClass();
       65 
       66     /**
       67      * Returns a hash code value for the object. This method is
       68      * supported for the benefit of hash tables such as those provided by
       69      * {@link java.util.HashMap}.
       70      * <p>
       71      * The general contract of {@code hashCode} is:
       72      * <ul>
       73      * <li>Whenever it is invoked on the same object more than once during
       74      *     an execution of a Java application, the {@code hashCode} method
       75      *     must consistently return the same integer, provided no information
       76      *     used in {@code equals} comparisons on the object is modified.
       77      *     This integer need not remain consistent from one execution of an
       78      *     application to another execution of the same application.
       79      * <li>If two objects are equal according to the {@code equals(Object)}
       80      *     method, then calling the {@code hashCode} method on each of
       81      *     the two objects must produce the same integer result.
       82      * <li>It is <em>not</em> required that if two objects are unequal
       83      *     according to the {@link java.lang.Object#equals(java.lang.Object)}
       84      *     method, then calling the {@code hashCode} method on each of the
       85      *     two objects must produce distinct integer results.  However, the
       86      *     programmer should be aware that producing distinct integer results
       87      *     for unequal objects may improve the performance of hash tables.
       88      * </ul>
       89      * <p>
       90      * As much as is reasonably practical, the hashCode method defined by
       91      * class {@code Object} does return distinct integers for distinct
       92      * objects. (This is typically implemented by converting the internal
       93      * address of the object into an integer, but this implementation
       94      * technique is not required by the
       95      * Java<font size="-2"><sup>TM</sup></font> programming language.)
       96      *
       97      * @return  a hash code value for this object.
       98      * @see     java.lang.Object#equals(java.lang.Object)
       99      * @see     java.lang.System#identityHashCode
      100      */
      101     public native int hashCode();
      102 
      103     /**
      104      * Indicates whether some other object is "equal to" this one.
      105      * <p>
      106      * The {@code equals} method implements an equivalence relation
      107      * on non-null object references:
      108      * <ul>
      109      * <li>It is <i>reflexive</i>: for any non-null reference value
      110      *     {@code x}, {@code x.equals(x)} should return
      111      *     {@code true}.
      112      * <li>It is <i>symmetric</i>: for any non-null reference values
      113      *     {@code x} and {@code y}, {@code x.equals(y)}
      114      *     should return {@code true} if and only if
      115      *     {@code y.equals(x)} returns {@code true}.
      116      * <li>It is <i>transitive</i>: for any non-null reference values
      117      *     {@code x}, {@code y}, and {@code z}, if
      118      *     {@code x.equals(y)} returns {@code true} and
      119      *     {@code y.equals(z)} returns {@code true}, then
      120      *     {@code x.equals(z)} should return {@code true}.
      121      * <li>It is <i>consistent</i>: for any non-null reference values
      122      *     {@code x} and {@code y}, multiple invocations of
      123      *     {@code x.equals(y)} consistently return {@code true}
      124      *     or consistently return {@code false}, provided no
      125      *     information used in {@code equals} comparisons on the
      126      *     objects is modified.
      127      * <li>For any non-null reference value {@code x},
      128      *     {@code x.equals(null)} should return {@code false}.
      129      * </ul>
      130      * <p>
      131      * The {@code equals} method for class {@code Object} implements
      132      * the most discriminating possible equivalence relation on objects;
      133      * that is, for any non-null reference values {@code x} and
      134      * {@code y}, this method returns {@code true} if and only
      135      * if {@code x} and {@code y} refer to the same object
      136      * ({@code x == y} has the value {@code true}).
      137      * <p>
      138      * Note that it is generally necessary to override the {@code hashCode}
      139      * method whenever this method is overridden, so as to maintain the
      140      * general contract for the {@code hashCode} method, which states
      141      * that equal objects must have equal hash codes.
      142      *
      143      * @param   obj   the reference object with which to compare.
      144      * @return  {@code true} if this object is the same as the obj
      145      *          argument; {@code false} otherwise.
      146      * @see     #hashCode()
      147      * @see     java.util.HashMap
      148      */
      149     public boolean equals(Object obj) {
      150         return (this == obj);
      151     }
      152 
      153     /**
      154      * Creates and returns a copy of this object.  The precise meaning
      155      * of "copy" may depend on the class of the object. The general
      156      * intent is that, for any object {@code x}, the expression:
      157      * <blockquote>
      158      * <pre>
      159      * x.clone() != x</pre></blockquote>
      160      * will be true, and that the expression:
      161      * <blockquote>
      162      * <pre>
      163      * x.clone().getClass() == x.getClass()</pre></blockquote>
      164      * will be {@code true}, but these are not absolute requirements.
      165      * While it is typically the case that:
      166      * <blockquote>
      167      * <pre>
      168      * x.clone().equals(x)</pre></blockquote>
      169      * will be {@code true}, this is not an absolute requirement.
      170      * <p>
      171      * By convention, the returned object should be obtained by calling
      172      * {@code super.clone}.  If a class and all of its superclasses (except
      173      * {@code Object}) obey this convention, it will be the case that
      174      * {@code x.clone().getClass() == x.getClass()}.
      175      * <p>
      176      * By convention, the object returned by this method should be independent
      177      * of this object (which is being cloned).  To achieve this independence,
      178      * it may be necessary to modify one or more fields of the object returned
      179      * by {@code super.clone} before returning it.  Typically, this means
      180      * copying any mutable objects that comprise the internal "deep structure"
      181      * of the object being cloned and replacing the references to these
      182      * objects with references to the copies.  If a class contains only
      183      * primitive fields or references to immutable objects, then it is usually
      184      * the case that no fields in the object returned by {@code super.clone}
      185      * need to be modified.
      186      * <p>
      187      * The method {@code clone} for class {@code Object} performs a
      188      * specific cloning operation. First, if the class of this object does
      189      * not implement the interface {@code Cloneable}, then a
      190      * {@code CloneNotSupportedException} is thrown. Note that all arrays
      191      * are considered to implement the interface {@code Cloneable} and that
      192      * the return type of the {@code clone} method of an array type {@code T[]}
      193      * is {@code T[]} where T is any reference or primitive type.
      194      * Otherwise, this method creates a new instance of the class of this
      195      * object and initializes all its fields with exactly the contents of
      196      * the corresponding fields of this object, as if by assignment; the
      197      * contents of the fields are not themselves cloned. Thus, this method
      198      * performs a "shallow copy" of this object, not a "deep copy" operation.
      199      * <p>
      200      * The class {@code Object} does not itself implement the interface
      201      * {@code Cloneable}, so calling the {@code clone} method on an object
      202      * whose class is {@code Object} will result in throwing an
      203      * exception at run time.
      204      *
      205      * @return     a clone of this instance.
      206      * @exception  CloneNotSupportedException  if the object's class does not
      207      *               support the {@code Cloneable} interface. Subclasses
      208      *               that override the {@code clone} method can also
      209      *               throw this exception to indicate that an instance cannot
      210      *               be cloned.
      211      * @see java.lang.Cloneable
      212      */
      213     protected native Object clone() throws CloneNotSupportedException;
      214 
      215     /**
      216      * Returns a string representation of the object. In general, the
      217      * {@code toString} method returns a string that
      218      * "textually represents" this object. The result should
      219      * be a concise but informative representation that is easy for a
      220      * person to read.
      221      * It is recommended that all subclasses override this method.
      222      * <p>
      223      * The {@code toString} method for class {@code Object}
      224      * returns a string consisting of the name of the class of which the
      225      * object is an instance, the at-sign character `{@code @}', and
      226      * the unsigned hexadecimal representation of the hash code of the
      227      * object. In other words, this method returns a string equal to the
      228      * value of:
      229      * <blockquote>
      230      * <pre>
      231      * getClass().getName() + '@' + Integer.toHexString(hashCode())
      232      * </pre></blockquote>
      233      *
      234      * @return  a string representation of the object.
      235      */
      236     public String toString() {
      237         return getClass().getName() + "@" + Integer.toHexString(hashCode());
      238     }
      239 
      240     /**
      241      * Wakes up a single thread that is waiting on this object's
      242      * monitor. If any threads are waiting on this object, one of them
      243      * is chosen to be awakened. The choice is arbitrary and occurs at
      244      * the discretion of the implementation. A thread waits on an object's
      245      * monitor by calling one of the {@code wait} methods.
      246      * <p>
      247      * The awakened thread will not be able to proceed until the current
      248      * thread relinquishes the lock on this object. The awakened thread will
      249      * compete in the usual manner with any other threads that might be
      250      * actively competing to synchronize on this object; for example, the
      251      * awakened thread enjoys no reliable privilege or disadvantage in being
      252      * the next thread to lock this object.
      253      * <p>
      254      * This method should only be called by a thread that is the owner
      255      * of this object's monitor. A thread becomes the owner of the
      256      * object's monitor in one of three ways:
      257      * <ul>
      258      * <li>By executing a synchronized instance method of that object.
      259      * <li>By executing the body of a {@code synchronized} statement
      260      *     that synchronizes on the object.
      261      * <li>For objects of type {@code Class,} by executing a
      262      *     synchronized static method of that class.
      263      * </ul>
      264      * <p>
      265      * Only one thread at a time can own an object's monitor.
      266      *
      267      * @exception  IllegalMonitorStateException  if the current thread is not
      268      *               the owner of this object's monitor.
      269      * @see        java.lang.Object#notifyAll()
      270      * @see        java.lang.Object#wait()
      271      */
      272     public final native void notify();
      273 
      274     /**
      275      * Wakes up all threads that are waiting on this object's monitor. A
      276      * thread waits on an object's monitor by calling one of the
      277      * {@code wait} methods.
      278      * <p>
      279      * The awakened threads will not be able to proceed until the current
      280      * thread relinquishes the lock on this object. The awakened threads
      281      * will compete in the usual manner with any other threads that might
      282      * be actively competing to synchronize on this object; for example,
      283      * the awakened threads enjoy no reliable privilege or disadvantage in
      284      * being the next thread to lock this object.
      285      * <p>
      286      * This method should only be called by a thread that is the owner
      287      * of this object's monitor. See the {@code notify} method for a
      288      * description of the ways in which a thread can become the owner of
      289      * a monitor.
      290      *
      291      * @exception  IllegalMonitorStateException  if the current thread is not
      292      *               the owner of this object's monitor.
      293      * @see        java.lang.Object#notify()
      294      * @see        java.lang.Object#wait()
      295      */
      296     public final native void notifyAll();
      297 
      298     /**
      299      * Causes the current thread to wait until either another thread invokes the
      300      * {@link java.lang.Object#notify()} method or the
      301      * {@link java.lang.Object#notifyAll()} method for this object, or a
      302      * specified amount of time has elapsed.
      303      * <p>
      304      * The current thread must own this object's monitor.
      305      * <p>
      306      * This method causes the current thread (call it <var>T</var>) to
      307      * place itself in the wait set for this object and then to relinquish
      308      * any and all synchronization claims on this object. Thread <var>T</var>
      309      * becomes disabled for thread scheduling purposes and lies dormant
      310      * until one of four things happens:
      311      * <ul>
      312      * <li>Some other thread invokes the {@code notify} method for this
      313      * object and thread <var>T</var> happens to be arbitrarily chosen as
      314      * the thread to be awakened.
      315      * <li>Some other thread invokes the {@code notifyAll} method for this
      316      * object.
      317      * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
      318      * thread <var>T</var>.
      319      * <li>The specified amount of real time has elapsed, more or less.  If
      320      * {@code timeout} is zero, however, then real time is not taken into
      321      * consideration and the thread simply waits until notified.
      322      * </ul>
      323      * The thread <var>T</var> is then removed from the wait set for this
      324      * object and re-enabled for thread scheduling. It then competes in the
      325      * usual manner with other threads for the right to synchronize on the
      326      * object; once it has gained control of the object, all its
      327      * synchronization claims on the object are restored to the status quo
      328      * ante - that is, to the situation as of the time that the {@code wait}
      329      * method was invoked. Thread <var>T</var> then returns from the
      330      * invocation of the {@code wait} method. Thus, on return from the
      331      * {@code wait} method, the synchronization state of the object and of
      332      * thread {@code T} is exactly as it was when the {@code wait} method
      333      * was invoked.
      334      * <p>
      335      * A thread can also wake up without being notified, interrupted, or
      336      * timing out, a so-called <i>spurious wakeup</i>.  While this will rarely
      337      * occur in practice, applications must guard against it by testing for
      338      * the condition that should have caused the thread to be awakened, and
      339      * continuing to wait if the condition is not satisfied.  In other words,
      340      * waits should always occur in loops, like this one:
      341      * <pre>
      342      *     synchronized (obj) {
      343      *         while (&lt;condition does not hold&gt;)
      344      *             obj.wait(timeout);
      345      *         ... // Perform action appropriate to condition
      346      *     }
      347      * </pre>
      348      * (For more information on this topic, see Section 3.2.3 in Doug Lea's
      349      * "Concurrent Programming in Java (Second Edition)" (Addison-Wesley,
      350      * 2000), or Item 50 in Joshua Bloch's "Effective Java Programming
      351      * Language Guide" (Addison-Wesley, 2001).
      352      *
      353      * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
      354      * interrupted} by any thread before or while it is waiting, then an
      355      * {@code InterruptedException} is thrown.  This exception is not
      356      * thrown until the lock status of this object has been restored as
      357      * described above.
      358      *
      359      * <p>
      360      * Note that the {@code wait} method, as it places the current thread
      361      * into the wait set for this object, unlocks only this object; any
      362      * other objects on which the current thread may be synchronized remain
      363      * locked while the thread waits.
      364      * <p>
      365      * This method should only be called by a thread that is the owner
      366      * of this object's monitor. See the {@code notify} method for a
      367      * description of the ways in which a thread can become the owner of
      368      * a monitor.
      369      *
      370      * @param      timeout   the maximum time to wait in milliseconds.
      371      * @exception  IllegalArgumentException      if the value of timeout is
      372      *               negative.
      373      * @exception  IllegalMonitorStateException  if the current thread is not
      374      *               the owner of the object's monitor.
      375      * @exception  InterruptedException if any thread interrupted the
      376      *             current thread before or while the current thread
      377      *             was waiting for a notification.  The <i>interrupted
      378      *             status</i> of the current thread is cleared when
      379      *             this exception is thrown.
      380      * @see        java.lang.Object#notify()
      381      * @see        java.lang.Object#notifyAll()
      382      */
      383     public final native void wait(long timeout) throws InterruptedException;
      384 
      385     /**
      386      * Causes the current thread to wait until another thread invokes the
      387      * {@link java.lang.Object#notify()} method or the
      388      * {@link java.lang.Object#notifyAll()} method for this object, or
      389      * some other thread interrupts the current thread, or a certain
      390      * amount of real time has elapsed.
      391      * <p>
      392      * This method is similar to the {@code wait} method of one
      393      * argument, but it allows finer control over the amount of time to
      394      * wait for a notification before giving up. The amount of real time,
      395      * measured in nanoseconds, is given by:
      396      * <blockquote>
      397      * <pre>
      398      * 1000000*timeout+nanos</pre></blockquote>
      399      * <p>
      400      * In all other respects, this method does the same thing as the
      401      * method {@link #wait(long)} of one argument. In particular,
      402      * {@code wait(0, 0)} means the same thing as {@code wait(0)}.
      403      * <p>
      404      * The current thread must own this object's monitor. The thread
      405      * releases ownership of this monitor and waits until either of the
      406      * following two conditions has occurred:
      407      * <ul>
      408      * <li>Another thread notifies threads waiting on this object's monitor
      409      *     to wake up either through a call to the {@code notify} method
      410      *     or the {@code notifyAll} method.
      411      * <li>The timeout period, specified by {@code timeout}
      412      *     milliseconds plus {@code nanos} nanoseconds arguments, has
      413      *     elapsed.
      414      * </ul>
      415      * <p>
      416      * The thread then waits until it can re-obtain ownership of the
      417      * monitor and resumes execution.
      418      * <p>
      419      * As in the one argument version, interrupts and spurious wakeups are
      420      * possible, and this method should always be used in a loop:
      421      * <pre>
      422      *     synchronized (obj) {
      423      *         while (&lt;condition does not hold&gt;)
      424      *             obj.wait(timeout, nanos);
      425      *         ... // Perform action appropriate to condition
      426      *     }
      427      * </pre>
      428      * This method should only be called by a thread that is the owner
      429      * of this object's monitor. See the {@code notify} method for a
      430      * description of the ways in which a thread can become the owner of
      431      * a monitor.
      432      *
      433      * @param      timeout   the maximum time to wait in milliseconds.
      434      * @param      nanos      additional time, in nanoseconds range
      435      *                       0-999999.
      436      * @exception  IllegalArgumentException      if the value of timeout is
      437      *                      negative or the value of nanos is
      438      *                      not in the range 0-999999.
      439      * @exception  IllegalMonitorStateException  if the current thread is not
      440      *               the owner of this object's monitor.
      441      * @exception  InterruptedException if any thread interrupted the
      442      *             current thread before or while the current thread
      443      *             was waiting for a notification.  The <i>interrupted
      444      *             status</i> of the current thread is cleared when
      445      *             this exception is thrown.
      446      */
      447     public final void wait(long timeout, int nanos) throws InterruptedException {
      448         if (timeout < 0) {
      449             throw new IllegalArgumentException("timeout value is negative");
      450         }
      451 
      452         if (nanos < 0 || nanos > 999999) {
      453             throw new IllegalArgumentException(
      454                                 "nanosecond timeout value out of range");
      455         }
      456 
      457         if (nanos >= 500000 || (nanos != 0 && timeout == 0)) {
      458             timeout++;
      459         }
      460 
      461         wait(timeout);
      462     }
      463 
      464     /**
      465      * Causes the current thread to wait until another thread invokes the
      466      * {@link java.lang.Object#notify()} method or the
      467      * {@link java.lang.Object#notifyAll()} method for this object.
      468      * In other words, this method behaves exactly as if it simply
      469      * performs the call {@code wait(0)}.
      470      * <p>
      471      * The current thread must own this object's monitor. The thread
      472      * releases ownership of this monitor and waits until another thread
      473      * notifies threads waiting on this object's monitor to wake up
      474      * either through a call to the {@code notify} method or the
      475      * {@code notifyAll} method. The thread then waits until it can
      476      * re-obtain ownership of the monitor and resumes execution.
      477      * <p>
      478      * As in the one argument version, interrupts and spurious wakeups are
      479      * possible, and this method should always be used in a loop:
      480      * <pre>
      481      *     synchronized (obj) {
      482      *         while (&lt;condition does not hold&gt;)
      483      *             obj.wait();
      484      *         ... // Perform action appropriate to condition
      485      *     }
      486      * </pre>
      487      * This method should only be called by a thread that is the owner
      488      * of this object's monitor. See the {@code notify} method for a
      489      * description of the ways in which a thread can become the owner of
      490      * a monitor.
      491      *
      492      * @exception  IllegalMonitorStateException  if the current thread is not
      493      *               the owner of the object's monitor.
      494      * @exception  InterruptedException if any thread interrupted the
      495      *             current thread before or while the current thread
      496      *             was waiting for a notification.  The <i>interrupted
      497      *             status</i> of the current thread is cleared when
      498      *             this exception is thrown.
      499      * @see        java.lang.Object#notify()
      500      * @see        java.lang.Object#notifyAll()
      501      */
      502     public final void wait() throws InterruptedException {
      503         wait(0);
      504     }
      505 
      506     /**
      507      * Called by the garbage collector on an object when garbage collection
      508      * determines that there are no more references to the object.
      509      * A subclass overrides the {@code finalize} method to dispose of
      510      * system resources or to perform other cleanup.
      511      * <p>
      512      * The general contract of {@code finalize} is that it is invoked
      513      * if and when the Java<font size="-2"><sup>TM</sup></font> virtual
      514      * machine has determined that there is no longer any
      515      * means by which this object can be accessed by any thread that has
      516      * not yet died, except as a result of an action taken by the
      517      * finalization of some other object or class which is ready to be
      518      * finalized. The {@code finalize} method may take any action, including
      519      * making this object available again to other threads; the usual purpose
      520      * of {@code finalize}, however, is to perform cleanup actions before
      521      * the object is irrevocably discarded. For example, the finalize method
      522      * for an object that represents an input/output connection might perform
      523      * explicit I/O transactions to break the connection before the object is
      524      * permanently discarded.
      525      * <p>
      526      * The {@code finalize} method of class {@code Object} performs no
      527      * special action; it simply returns normally. Subclasses of
      528      * {@code Object} may override this definition.
      529      * <p>
      530      * The Java programming language does not guarantee which thread will
      531      * invoke the {@code finalize} method for any given object. It is
      532      * guaranteed, however, that the thread that invokes finalize will not
      533      * be holding any user-visible synchronization locks when finalize is
      534      * invoked. If an uncaught exception is thrown by the finalize method,
      535      * the exception is ignored and finalization of that object terminates.
      536      * <p>
      537      * After the {@code finalize} method has been invoked for an object, no
      538      * further action is taken until the Java virtual machine has again
      539      * determined that there is no longer any means by which this object can
      540      * be accessed by any thread that has not yet died, including possible
      541      * actions by other objects or classes which are ready to be finalized,
      542      * at which point the object may be discarded.
      543      * <p>
      544      * The {@code finalize} method is never invoked more than once by a Java
      545      * virtual machine for any given object.
      546      * <p>
      547      * Any exception thrown by the {@code finalize} method causes
      548      * the finalization of this object to be halted, but is otherwise
      549      * ignored.
      550      *
      551      * @throws Throwable the {@code Exception} raised by this method
      552      */
      553     protected void finalize() throws Throwable { }
      554 }