jdk/src/share/classes/java/lang/Object.java

/*
 * Copyright 1994-2006 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */

package java.lang;

/**
 * Class <code>Object</code> is the root of the class hierarchy.
 * Every class has <code>Object</code> as a superclass. All objects,
 * including arrays, implement the methods of this class.
 *
 * @author  unascribed
 * @see     java.lang.Class
 * @since   JDK1.0
 */
public class Object {

    private static native void registerNatives();
    static {
        registerNatives();
    }

    /**
     * Returns the runtime class of this {@code Object}. The returned
     * {@code Class} object is the object that is locked by {@code
     * static synchronized} methods of the represented class.
     *
     * <p><b>The actual result type is {@code Class<? extends |X|>}
     * where {@code |X|} is the erasure of the static type of the
     * expression on which {@code getClass} is called.</b> For
     * example, no cast is required in this code fragment:</p>
     *
     * <p>
     * {@code Number n = 0;                             }<br>
     * {@code Class<? extends Number> c = n.getClass(); }
     * </p>
     *
     * @return The {@code Class} object that represents the runtime
     *         class of this object.
     * @see    <a href="http://java.sun.com/docs/books/jls/">The Java
     *         Language Specification, Third Edition (15.8.2 Class
     *         Literals)</a>
     */
    public final native Class<?> getClass();

    /**
     * Returns a hash code value for the object. This method is
     * supported for the benefit of hashtables such as those provided by
     * <code>java.util.Hashtable</code>.
     * <p>
     * The general contract of <code>hashCode</code> is:
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during
     *     an execution of a Java application, the <tt>hashCode</tt> method
     *     must consistently return the same integer, provided no information
     *     used in <tt>equals</tt> 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.
     * <li>If two objects are equal according to the <tt>equals(Object)</tt>
     *     method, then calling the <code>hashCode</code> method on each of
     *     the two objects must produce the same integer result.
     * <li>It is <em>not</em> required that if two objects are unequal
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
     *     method, then calling the <tt>hashCode</tt> 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 hashtables.
     * </ul>
     * <p>
     * As much as is reasonably practical, the hashCode method defined by
     * class <tt>Object</tt> 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<font size="-2"><sup>TM</sup></font> programming language.)
     *
     * @return  a hash code value for this object.
     * @see     java.lang.Object#equals(java.lang.Object)
     * @see     java.util.Hashtable
     */
    public native int hashCode();

    /**
     * Indicates whether some other object is "equal to" this one.
     * <p>
     * The <code>equals</code> method implements an equivalence relation
     * on non-null object references:
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     *     <code>x</code>, <code>x.equals(x)</code> should return
     *     <code>true</code>.
     * <li>It is <i>symmetric</i>: for any non-null reference values
     *     <code>x</code> and <code>y</code>, <code>x.equals(y)</code>
     *     should return <code>true</code> if and only if
     *     <code>y.equals(x)</code> returns <code>true</code>.
     * <li>It is <i>transitive</i>: for any non-null reference values
     *     <code>x</code>, <code>y</code>, and <code>z</code>, if
     *     <code>x.equals(y)</code> returns <code>true</code> and
     *     <code>y.equals(z)</code> returns <code>true</code>, then
     *     <code>x.equals(z)</code> should return <code>true</code>.
     * <li>It is <i>consistent</i>: for any non-null reference values
     *     <code>x</code> and <code>y</code>, multiple invocations of
     *     <tt>x.equals(y)</tt> consistently return <code>true</code>
     *     or consistently return <code>false</code>, provided no
     *     information used in <code>equals</code> comparisons on the
     *     objects is modified.
     * <li>For any non-null reference value <code>x</code>,
     *     <code>x.equals(null)</code> should return <code>false</code>.
     * </ul>
     * <p>
     * The <tt>equals</tt> method for class <code>Object</code> implements
     * the most discriminating possible equivalence relation on objects;
     * that is, for any non-null reference values <code>x</code> and
     * <code>y</code>, this method returns <code>true</code> if and only
     * if <code>x</code> and <code>y</code> refer to the same object
     * (<code>x == y</code> has the value <code>true</code>).
     * <p>
     * Note that it is generally necessary to override the <tt>hashCode</tt>
     * method whenever this method is overridden, so as to maintain the
     * general contract for the <tt>hashCode</tt> method, which states
     * that equal objects must have equal hash codes.
     *
     * @param   obj   the reference object with which to compare.
     * @return  <code>true</code> if this object is the same as the obj
     *          argument; <code>false</code> otherwise.
     * @see     #hashCode()
     * @see     java.util.Hashtable
     */
    public boolean equals(Object obj) {
        return (this == obj);
    }

    /**
     * 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 object <tt>x</tt>, the expression:
     * <blockquote>
     * <pre>
     * x.clone() != x</pre></blockquote>
     * will be true, and that the expression:
     * <blockquote>
     * <pre>
     * x.clone().getClass() == x.getClass()</pre></blockquote>
     * will be <tt>true</tt>, but these are not absolute requirements.
     * While it is typically the case that:
     * <blockquote>
     * <pre>
     * x.clone().equals(x)</pre></blockquote>
     * will be <tt>true</tt>, this is not an absolute requirement.
     * <p>
     * By convention, the returned object should be obtained by calling
     * <tt>super.clone</tt>.  If a class and all of its superclasses (except
     * <tt>Object</tt>) obey this convention, it will be the case that
     * <tt>x.clone().getClass() == x.getClass()</tt>.
     * <p>
     * By convention, the object returned by this method should be independent
     * of this object (which is being cloned).  To achieve this independence,
     * it may be necessary to modify one or more fields of the object returned
     * by <tt>super.clone</tt> before returning it.  Typically, this means
     * copying any mutable objects that comprise the internal "deep structure"
     * of the object being cloned and replacing the references to these
     * objects with references to the copies.  If a class contains only
     * primitive fields or references to immutable objects, then it is usually
     * the case that no fields in the object returned by <tt>super.clone</tt>
     * need to be modified.
     * <p>
     * The method <tt>clone</tt> for class <tt>Object</tt> performs a
     * specific cloning operation. First, if the class of this object does
     * not implement the interface <tt>Cloneable</tt>, then a
     * <tt>CloneNotSupportedException</tt> is thrown. Note that all arrays
     * are considered to implement the interface <tt>Cloneable</tt>.
     * 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.
     * <p>
     * The class <tt>Object</tt> does not itself implement the interface
     * <tt>Cloneable</tt>, so calling the <tt>clone</tt> method on an object
     * whose class is <tt>Object</tt> will result in throwing an
     * exception at run time.
     *
     * @return     a clone of this instance.
     * @exception  CloneNotSupportedException  if the object's class does not
     *               support the <code>Cloneable</code> interface. Subclasses
     *               that override the <code>clone</code> method can also
     *               throw this exception to indicate that an instance cannot
     *               be cloned.
     * @see java.lang.Cloneable
     */
    protected native Object clone() throws CloneNotSupportedException;

    /**
     * Returns a string representation of the object. In general, the
     * <code>toString</code> 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.
     * <p>
     * The <code>toString</code> method for class <code>Object</code>
     * returns a string consisting of the name of the class of which the
     * object is an instance, the at-sign character `<code>@</code>', 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:
     * <blockquote>
     * <pre>
     * getClass().getName() + '@' + Integer.toHexString(hashCode())
     * </pre></blockquote>
     *
     * @return  a string representation of the object.
     */
    public String toString() {
        return getClass().getName() + "@" + Integer.toHexString(hashCode());
    }

    /**
     * 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 the <code>wait</code> methods.
     * <p>
     * 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.
     * <p>
     * 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:
     * <ul>
     * <li>By executing a synchronized instance method of that object.
     * <li>By executing the body of a <code>synchronized</code> statement
     *     that synchronizes on the object.
     * <li>For objects of type <code>Class,</code> by executing a
     *     synchronized static method of that class.
     * </ul>
     * <p>
     * Only one thread at a time can own an object's monitor.
     *
     * @exception  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.
     * @see        java.lang.Object#notifyAll()
     * @see        java.lang.Object#wait()
     */
    public final native void notify();

    /**
     * Wakes up all threads that are waiting on this object's monitor. A
     * thread waits on an object's monitor by calling one of the
     * <code>wait</code> methods.
     * <p>
     * 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.
     * <p>
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the <code>notify</code> method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @exception  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#wait()
     */
    public final native void notifyAll();

    /**
     * Causes the current thread to wait until either another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object, or a
     * specified amount of time has elapsed.
     * <p>
     * The current thread must own this object's monitor.
     * <p>
     * This method causes the current thread (call it <var>T</var>) to
     * place itself in the wait set for this object and then to relinquish
     * any and all synchronization claims on this object. Thread <var>T</var>
     * becomes disabled for thread scheduling purposes and lies dormant
     * until one of four things happens:
     * <ul>
     * <li>Some other thread invokes the <tt>notify</tt> method for this
     * object and thread <var>T</var> happens to be arbitrarily chosen as
     * the thread to be awakened.
     * <li>Some other thread invokes the <tt>notifyAll</tt> method for this
     * object.
     * <li>Some other thread {@linkplain Thread#interrupt() interrupts}
     * thread <var>T</var>.
     * <li>The specified amount of real time has elapsed, more or less.  If
     * <tt>timeout</tt> is zero, however, then real time is not taken into
     * consideration and the thread simply waits until notified.
     * </ul>
     * The thread <var>T</var> is then removed from the wait set for this
     * object and re-enabled for thread scheduling. It then competes in the
     * usual manner with other threads for the right to synchronize on the
     * object; once it has gained 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 the <tt>wait</tt>
     * method was invoked. Thread <var>T</var> then returns from the
     * invocation of the <tt>wait</tt> method. Thus, on return from the
     * <tt>wait</tt> method, the synchronization state of the object and of
     * thread <tt>T</tt> is exactly as it was when the <tt>wait</tt> method
     * was invoked.
     * <p>
     * A thread can also wake up without being notified, interrupted, or
     * timing out, a so-called <i>spurious wakeup</i>.  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:
     * <pre>
     *     synchronized (obj) {
     *         while (&lt;condition does not hold&gt;)
     *             obj.wait(timeout);
     *         ... // Perform action appropriate to condition
     *     }
     * </pre>
     * (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).
     *
     * <p>If the current thread is {@linkplain java.lang.Thread#interrupt()
     * interrupted} by any thread before or while it is waiting, then an
     * <tt>InterruptedException</tt> is thrown.  This exception is not
     * thrown until the lock status of this object has been restored as
     * described above.
     *
     * <p>
     * Note that the <tt>wait</tt> 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.
     * <p>
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the <code>notify</code> method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @param      timeout   the maximum time to wait in milliseconds.
     * @exception  IllegalArgumentException      if the value of timeout is
     *               negative.
     * @exception  IllegalMonitorStateException  if the current thread is not
     *               the owner of the object's monitor.
     * @exception  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The <i>interrupted
     *             status</i> of the current thread is cleared when
     *             this exception is thrown.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#notifyAll()
     */
    public final native void wait(long timeout) throws InterruptedException;

    /**
     * Causes the current thread to wait until another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object, or
     * some other thread interrupts the current thread, or a certain
     * amount of real time has elapsed.
     * <p>
     * This method is similar to the <code>wait</code> 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:
     * <blockquote>
     * <pre>
     * 1000000*timeout+nanos</pre></blockquote>
     * <p>
     * In all other respects, this method does the same thing as the
     * method {@link #wait(long)} of one argument. In particular,
     * <tt>wait(0, 0)</tt> means the same thing as <tt>wait(0)</tt>.
     * <p>
     * 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:
     * <ul>
     * <li>Another thread notifies threads waiting on this object's monitor
     *     to wake up either through a call to the <code>notify</code> method
     *     or the <code>notifyAll</code> method.
     * <li>The timeout period, specified by <code>timeout</code>
     *     milliseconds plus <code>nanos</code> nanoseconds arguments, has
     *     elapsed.
     * </ul>
     * <p>
     * The thread then waits until it can re-obtain ownership of the
     * monitor and resumes execution.
     * <p>
     * As in the one argument version, interrupts and spurious wakeups are
     * possible, and this method should always be used in a loop:
     * <pre>
     *     synchronized (obj) {
     *         while (&lt;condition does not hold&gt;)
     *             obj.wait(timeout, nanos);
     *         ... // Perform action appropriate to condition
     *     }
     * </pre>
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the <code>notify</code> method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @param      timeout   the maximum time to wait in milliseconds.
     * @param      nanos      additional time, in nanoseconds range
     *                       0-999999.
     * @exception  IllegalArgumentException      if the value of timeout is
     *                      negative or the value of nanos is
     *                      not in the range 0-999999.
     * @exception  IllegalMonitorStateException  if the current thread is not
     *               the owner of this object's monitor.
     * @exception  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The <i>interrupted
     *             status</i> of the current thread is cleared when
     *             this exception is thrown.
     */
    public final void wait(long timeout, int nanos) throws InterruptedException {
        if (timeout < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos >= 500000 || (nanos != 0 && timeout == 0)) {
            timeout++;
        }

        wait(timeout);
    }

    /**
     * Causes the current thread to wait until another thread invokes the
     * {@link java.lang.Object#notify()} method or the
     * {@link java.lang.Object#notifyAll()} method for this object.
     * In other words, this method behaves exactly as if it simply
     * performs the call <tt>wait(0)</tt>.
     * <p>
     * The current thread must own this object's monitor. The thread
     * releases ownership of this monitor and waits until another thread
     * notifies threads waiting on this object's monitor to wake up
     * either through a call to the <code>notify</code> method or the
     * <code>notifyAll</code> method. The thread then waits until it can
     * re-obtain ownership of the monitor and resumes execution.
     * <p>
     * As in the one argument version, interrupts and spurious wakeups are
     * possible, and this method should always be used in a loop:
     * <pre>
     *     synchronized (obj) {
     *         while (&lt;condition does not hold&gt;)
     *             obj.wait();
     *         ... // Perform action appropriate to condition
     *     }
     * </pre>
     * This method should only be called by a thread that is the owner
     * of this object's monitor. See the <code>notify</code> method for a
     * description of the ways in which a thread can become the owner of
     * a monitor.
     *
     * @exception  IllegalMonitorStateException  if the current thread is not
     *               the owner of the object's monitor.
     * @exception  InterruptedException if any thread interrupted the
     *             current thread before or while the current thread
     *             was waiting for a notification.  The <i>interrupted
     *             status</i> of the current thread is cleared when
     *             this exception is thrown.
     * @see        java.lang.Object#notify()
     * @see        java.lang.Object#notifyAll()
     */
    public final void wait() throws InterruptedException {
        wait(0);
    }

    /**
     * Called by the garbage collector on an object when garbage collection
     * determines that there are no more references to the object.
     * A subclass overrides the <code>finalize</code> method to dispose of
     * system resources or to perform other cleanup.
     * <p>
     * The general contract of <tt>finalize</tt> is that it is invoked
     * if and when the Java<font size="-2"><sup>TM</sup></font> virtual
     * machine has determined that there is no longer any
     * means by which this object can be accessed by any thread that has
     * not yet died, except as a result of an action taken by the
     * finalization of some other object or class which is ready to be
     * finalized. The <tt>finalize</tt> method may take any action, including
     * making this object available again to other threads; the usual purpose
     * of <tt>finalize</tt>, 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.
     * <p>
     * The <tt>finalize</tt> method of class <tt>Object</tt> performs no
     * special action; it simply returns normally. Subclasses of
     * <tt>Object</tt> may override this definition.
     * <p>
     * The Java programming language does not guarantee which thread will
     * invoke the <tt>finalize</tt> 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.
     * <p>
     * After the <tt>finalize</tt> 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.
     * <p>
     * The <tt>finalize</tt> method is never invoked more than once by a Java
     * virtual machine for any given object.
     * <p>
     * Any exception thrown by the <code>finalize</code> method causes
     * the finalization of this object to be halted, but is otherwise
     * ignored.
     *
     * @throws Throwable the <code>Exception</code> raised by this method
     */
    protected void finalize() throws Throwable { }
}