001    /* Externalizable.java -- Interface for saving and restoring object data
002       Copyright (C) 1998 Free Software Foundation, Inc.
003    
004    This file is part of GNU Classpath.
005    
006    GNU Classpath is free software; you can redistribute it and/or modify
007    it under the terms of the GNU General Public License as published by
008    the Free Software Foundation; either version 2, or (at your option)
009    any later version.
010     
011    GNU Classpath is distributed in the hope that it will be useful, but
012    WITHOUT ANY WARRANTY; without even the implied warranty of
013    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014    General Public License for more details.
015    
016    You should have received a copy of the GNU General Public License
017    along with GNU Classpath; see the file COPYING.  If not, write to the
018    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019    02110-1301 USA.
020    
021    Linking this library statically or dynamically with other modules is
022    making a combined work based on this library.  Thus, the terms and
023    conditions of the GNU General Public License cover the whole
024    combination.
025    
026    As a special exception, the copyright holders of this library give you
027    permission to link this library with independent modules to produce an
028    executable, regardless of the license terms of these independent
029    modules, and to copy and distribute the resulting executable under
030    terms of your choice, provided that you also meet, for each linked
031    independent module, the terms and conditions of the license of that
032    module.  An independent module is a module which is not derived from
033    or based on this library.  If you modify this library, you may extend
034    this exception to your version of the library, but you are not
035    obligated to do so.  If you do not wish to do so, delete this
036    exception statement from your version. */
037    
038    
039    package java.io;
040    
041    /**
042     * This interface provides a way that classes can completely control how
043     * the data of their object instances  are written and read to and from 
044     * streams.  It has two methods which are used to write the data to a stream 
045     * and to read the data from a stream.  The read method must read the data 
046     * in exactly the way it was written by the write method. 
047     * <p>
048     * Note that classes which implement this interface must take into account
049     * that all superclass data must also be written to the stream as well.  
050     * The class implementing this interface must figure out how to make that
051     * happen.
052     * <p>
053     * This interface can be used to provide object persistence.  When an 
054     * object is to be stored externally, the <code>writeExternal</code> method is
055     * called to save state.  When the object is restored, an instance is
056     * created using the default no-argument constructor and the 
057     * <code>readExternal</code> method is used to restore the state.
058     *
059     * @author Aaron M. Renn (arenn@urbanophile.com)
060     */
061    public interface Externalizable extends Serializable
062    {
063      /**
064       * This method restores an object's state by reading in the instance data
065       * for the object from the passed in stream.  Note that this stream is not
066       * a subclass of <code>InputStream</code>, but rather is a class that 
067       * implements
068       * the <code>ObjectInput</code> interface.  That interface provides a 
069       * mechanism for
070       * reading in Java data types from a stream.
071       * <p>
072       * Note that this method must be compatible with <code>writeExternal</code>.
073       * It must read back the exact same types that were written by that
074       * method in the exact order they were written.
075       * <p>
076       * If this method needs to read back an object instance, then the class
077       * for that object must be found and loaded.  If that operation fails,
078       * then this method throws a <code>ClassNotFoundException</code>
079       *
080       * @param in An <code>ObjectInput</code> instance for reading in the object 
081       * state
082       *
083       * @exception ClassNotFoundException If the class of an object being 
084       * restored cannot be found
085       * @exception IOException If any other error occurs
086       */
087      void readExternal(ObjectInput in) 
088        throws ClassNotFoundException, IOException;
089    
090      /**
091       * This method is responsible for writing the instance data of an object
092       * to the passed in stream.  Note that this stream is not a subclass of
093       * <code>OutputStream</code>, but rather is a class that implements the
094       * <code>ObjectOutput</code> interface.  That interface provides a 
095       * number of methods
096       * for writing Java data values to a stream.
097       * <p>
098       * Not that the implementation of this method must be coordinated with
099       * the implementation of <code>readExternal</code>.
100       *
101       * @param out An <code>ObjectOutput</code> instance for writing the 
102       * object state
103       *
104       * @exception IOException If an error occurs
105       */
106      void writeExternal(ObjectOutput out) throws IOException;
107    }