Java Cookbook

Seeking

Problem

You need to read from or write to a particular location in a file, such as an indexed file.

Solution

Use a RandomAccessFile.

Discussion

The class java.io.RandomAccessFile allows you to move the read/write position to any location within a file, or past the end when writing. This allows you to create or access “files with holes” on some platforms and lets you read/write indexed or other database-like files in Java. The primary methods of interest are void(long where), which moves the position for the next read/write to where; int skipBytes(int howmany), which moves the position forward by howmany bytes; and long getFilePointer( ) , which returns the position.

RandomAccessFile class also implements the DataInput and DataOutput interfaces, so everything I said about DataStreams in Section 9.14 also applies here. This example reads a binary integer from the beginning of the file, treats that as the position to read from, finds that position, and reads a string from that location within the file.

import java.io.*; /** * Read a file containing an offset, and a String at that offset. */ public class ReadRandom { final static String FILENAME = "random.dat"; protected String fileName; protected RandomAccessFile seeker; public static void main(String argv[]) throws IOException { ReadRandom r = new ReadRandom(FILENAME); System.out.println("Offset is " + r.readOffset( )); System.out.println("Message is \"" + r.readMessage( ) + "\"."); } /** Constructor: save filename, construct ...