/* Externalizable.java -- Interface for saving and restoring object data
   Copyright (C) 1998 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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package java.io;

/**
 * This interface provides a way that classes can completely control how
 * the data of their object instances  are written and read to and from 
 * streams.  It has two methods which are used to write the data to a stream 
 * and to read the data from a stream.  The read method must read the data 
 * in exactly the way it was written by the write method. 
 * <p>
 * Note that classes which implement this interface must take into account
 * that all superclass data must also be written to the stream as well.  
 * The class implementing this interface must figure out how to make that
 * happen.
 * <p>
 * This interface can be used to provide object persistence.  When an 
 * object is to be stored externally, the <code>writeExternal</code> method is
 * called to save state.  When the object is restored, an instance is
 * created using the default no-argument constructor and the 
 * <code>readExternal</code> method is used to restore the state.
 *
 * @author Aaron M. Renn (arenn@urbanophile.com)
 */
public interface Externalizable extends Serializable
{
  /**
   * This method restores an object's state by reading in the instance data
   * for the object from the passed in stream.  Note that this stream is not
   * a subclass of <code>InputStream</code>, but rather is a class that 
   * implements
   * the <code>ObjectInput</code> interface.  That interface provides a 
   * mechanism for
   * reading in Java data types from a stream.
   * <p>
   * Note that this method must be compatible with <code>writeExternal</code>.
   * It must read back the exact same types that were written by that
   * method in the exact order they were written.
   * <p>
   * If this method needs to read back an object instance, then the class
   * for that object must be found and loaded.  If that operation fails,
   * then this method throws a <code>ClassNotFoundException</code>
   *
   * @param in An <code>ObjectInput</code> instance for reading in the object 
   * state
   *
   * @exception ClassNotFoundException If the class of an object being 
   * restored cannot be found
   * @exception IOException If any other error occurs
   */
  void readExternal(ObjectInput in) 
    throws ClassNotFoundException, IOException;

  /**
   * This method is responsible for writing the instance data of an object
   * to the passed in stream.  Note that this stream is not a subclass of
   * <code>OutputStream</code>, but rather is a class that implements the
   * <code>ObjectOutput</code> interface.  That interface provides a 
   * number of methods
   * for writing Java data values to a stream.
   * <p>
   * Not that the implementation of this method must be coordinated with
   * the implementation of <code>readExternal</code>.
   *
   * @param out An <code>ObjectOutput</code> instance for writing the 
   * object state
   *
   * @exception IOException If an error occurs
   */
  void writeExternal(ObjectOutput out) throws IOException;
}