object.java

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/*
 * @(#)Object.java	1.63 06/10/10
 *
 * Copyright  1990-2008 Sun Microsystems, Inc. All Rights Reserved.  
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER  
 *   
 * This program 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.   
 *   
 * This program 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 at /legal/license.txt).   
 *   
 * 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 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
 * @version 1.55, 05/03/00
 * @see     java.lang.Class
 * @since   JDK1.0
 */
public class Object {

    /* No need to do this if ROMized
    private static native void registerNatives();
    static {
        registerNatives();
    }
    */

    /**
     * Returns the runtime class of an object. That <tt>Class</tt> 
     * object is the object that is locked by <tt>static synchronized</tt> 
     * methods of the represented class.
     *
     * @return  the object of type <code>Class</code> that represents the
     *          runtime class of the object.
     */
    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