Need to add seek() in InputStream classes

Question

I would like to add a seek method into a InputStream, however I have no idea how to do it. The inputstream is created from a File object, so buffering the whole file is out of the question because some of them may get large. I would very much like it to be similar if not the same as the seek function of a random access file. Any suggestions or pointers would be greatly appreciated. Or, if some one nows how to make the skip method skip back bytes as well as forward, that would be very helpful.
Ross

Answer 1

I thought I had written this once, here it is:
* RandomAccessInputStream.java
* Created on 15 October 2007, 14:34
* Copyright (c) Esmond Pitt, 2007. All rights reserved.
package com.verismart.io;
import java.io.DataInput;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
* @author Esmond Pitt
* @version $Revision: $
public class RandomAccessInputStream extends InputStream implements DataInput
     private RandomAccessFile     raf;
     private long     filePointer = -1L;
     /** Creates a new instance of RandomAccessInputStream */
     public RandomAccessInputStream(RandomAccessFile raf)
          this.raf = raf;
     public int available() throws IOException
          return (int)(raf.length()-raf.getFilePointer());
     public void close() throws IOException
          raf.close();
     public void mark(int readlimit)
          try
               this.filePointer = raf.getFilePointer();
          catch (IOException exc)
//               throw new MarkNotSupportedException(exc.getMessage());
     public boolean markSupported()
          return true;
     public int read() throws IOException
          return raf.read();
     public int read(byte[] buffer, int offset, int length) throws IOException
          return raf.read(buffer, offset, length);
     public int read(byte[] buffer) throws IOException
          return raf.read(buffer);
     public boolean readBoolean() throws IOException
          return raf.readBoolean();
     public byte readByte() throws IOException
          return raf.readByte();
     public char readChar() throws IOException
          return raf.readChar();
     public double readDouble() throws IOException
          return raf.readDouble();
     public float readFloat() throws IOException
          return raf.readFloat();
     public void readFully(byte[] b, int off, int len) throws IOException
          raf.readFully(b, off, len);
     public void readFully(byte[] b) throws IOException
          raf.readFully(b);
     public int readInt() throws IOException
          return raf.readInt();
     public String readLine() throws IOException
          return raf.readLine();
     public long readLong() throws IOException
          return raf.readLong();
     public short readShort() throws IOException
          return raf.readShort();
     public String readUTF() throws IOException
          return raf.readUTF();
     public int readUnsignedByte() throws IOException
          return raf.readUnsignedByte();
     public int readUnsignedShort() throws IOException
          return raf.readUnsignedShort();
     public void reset() throws IOException
          if (filePointer == -1L)
               throw new IOException("mark not set");
          raf.seek(filePointer);
     public void     seek(long position) throws IOException
          raf.seek(position);
          filePointer = -1L;
     public long skip(long n) throws IOException
          long     pointer = raf.getFilePointer();
          long     result = Math.min(raf.length()-pointer, n);
          raf.seek(pointer+result);
          return result;
     public int skipBytes(int n) throws IOException
          return raf.skipBytes(n);
}