persistent.java

数据结构及算法 对于参加ACM的学生有一定帮助 里面有详细的解答

//
//  Unified Library Application
//  Case study in Unified Modeling Language Toolkit
//
//  Persistent.java: super class for class that wants to be
//  persistent. Subclass must implement read and write operations
//  that serializes the object to disk. This class reads files
//  sequentially and could be improved in performance.
//
//  Copyright (c) 1998 John Wiley & Sons, Inc. All rights reserved.
//  Reproduction or translations of this work beyond that permitted
//  in Section 117 of the 1976 United States Copyright Act without
//  the express written permission of the copyright owner is unlawful.
//  Requests for further information should be addressed to Permissions
//  Department, John Wiley & Sons, Inc. The purchaser may make back-up
//  copies for his/her own use only and not for distribution or resale.
//  The Publisher assumes no responsibility for errors, omissions, or
//  damages, caused by the use of these programs of from the use of the
//  information contained herein.


package db;
import util.*;
import java.io.*;
import java.net.*;

public abstract class Persistent
{
    private int objid;
    private static RandomAccessFile iterfile;
    public Persistent()
    {
        String cname = getClass().getName();
        try {
            RandomAccessFile file = new RandomAccessFile(cname + ".dat","rw");

            try
            {   // Update object id counter
                objid = file.readInt();
                file.seek(0);
                objid++;
                file.writeInt(objid);
            }
            catch (IOException e)
            {
                // File is new, write first object id
                objid = 1;
                try
                {
                    file.writeInt(objid);
                }
                catch (IOException e2)
                {
                    System.err.println(e2.getMessage());
                }

            }
            if (file != null)
                file.close();
        }
        catch (IOException ioe)
        {
            System.err.println(ioe.getMessage());
        }
    }
    public ObjId getObjId()
    {
        return new ObjId(getClass().getName(),objid);
    }
    protected static final Persistent getObject(ObjId id)
    {
        Persistent res = null;
        String classname = id.getName();
        int no = id.getId();
        try
        {
            RandomAccessFile in =
                new RandomAccessFile(classname + ".dat","r");
            Class cl = Class.forName(classname);
            res = (Persistent) cl.newInstance();
            in.readInt();
            while (in.getFilePointer() < in.length())
            {
                res.objid = in.readInt();
                res.read(in);
                if (res.objid == no)
                {
                    in.close();
                    return res;
                }
            }
            in.close();
        }
        catch (ClassNotFoundException e)
        {
            System.err.println(e.getMessage());
        }
        catch (InstantiationException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IllegalAccessException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IOException e)
        {
            System.err.println(e.getMessage());
        }
        return null;
    }

    public final void store()
    {
        try
        {
            RandomAccessFile out =
                new RandomAccessFile(getClass().getName() + ".dat","rw");
            // Position at end of file
            out.seek(out.length());
            out.writeInt(objid);
            write(out);
            out.close();
        }
        catch (IOException e)
        {
            System.err.println(e.getMessage());
        }
    }
    public boolean delete()
    {
       Persistent res = null;
       try
       {
            RandomAccessFile in =
                new RandomAccessFile(getClass().getName() + ".dat","rw");
            Class cl = Class.forName(getClass().getName());
            res = (Persistent) cl.newInstance();
            in.readInt();
            long filepointer = in.getFilePointer();
            res.objid = in.readInt();
            res.read(in);
            do
            {
                if (res.objid == objid)
                {
                    in.seek(filepointer);
                    in.writeInt(0);
                    in.close();
                    return true;
                }
                else
                {
                    filepointer = in.getFilePointer();
                    res.objid = in.readInt();
                    res.read(in);
                }
            }
            while (true);
        }
        catch (ClassNotFoundException e)
        {
            System.err.println(e.getMessage());
        }
        catch (InstantiationException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IllegalAccessException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IOException e)
        {
            System.err.println(e.getMessage());
        }
        return false;
    }
    public boolean update()
    {
        delete();
        store();
        return true;
    }
    public static Persistent iterate(String classname, boolean start)
    {
        Persistent res = null;
        try
        {
            if (start)
            {
                iterfile =
                    new RandomAccessFile(classname + ".dat","r");
                iterfile.readInt();
            }
            Class cl = Class.forName(classname);
            do
            {
                res = (Persistent) cl.newInstance();
                res.objid = iterfile.readInt();
                res.read(iterfile);
            } while (res.objid == 0);
            return res;
        }
        catch (ClassNotFoundException e)
        {
            System.err.println(e.getMessage());
        }
        catch (InstantiationException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IllegalAccessException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IOException e)
        {
            // This is a "expected" exception,
            // means that all objects have been iterated
            try {
                 if (iterfile != null)
                    iterfile.close();
            }
            catch (IOException e2)
            {
                System.err.println(e2.getMessage());
            }
        }
        return null;

    }
    // This method can be used to "clean up" a file filled with
    // deleted objects.
    public void clean()
    {
        Persistent ref = null;
        String classname = getClass().getName();

        try
        {
            RandomAccessFile in =
                    new RandomAccessFile(classname + ".dat","rw");
            RandomAccessFile out =
                    new RandomAccessFile(classname + ".cln","rw");
            Class cl = Class.forName(classname);
            ref = (Persistent) cl.newInstance();
            int high = in.readInt();
            out.writeInt(high);
            while (in.getFilePointer() < in.length())
            {
                ref.objid = in.readInt();
                ref.read(in);
                if (ref.objid != 0)
                {
                    out.writeInt(ref.objid);
                    ref.write(out);
                }
            }
            out.seek(0);
            in.close();
            int ch;
            // Copy the outfile back to the original file
            FileOutputStream dat = new FileOutputStream(classname + ".dat");
            while ((ch = out.read()) != -1)
                dat.write(ch);
            dat.close();
            out.close();
            // Delete the cln file
            File newf = new File(classname + ".cln");
            boolean ok = newf.delete();
        }
        catch (ClassNotFoundException e)
        {
            System.err.println(e.getMessage());
        }
        catch (InstantiationException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IllegalAccessException e)
        {
            System.err.println(e.getMessage());
        }
        catch (IOException e)
        {
            System.err.println(e.getMessage());
        }

    }

    public abstract void write(RandomAccessFile out) throws IOException;
    public abstract void read(RandomAccessFile in) throws IOException;
}