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copied!<p>java.io.RandomAccessFile is the class you're looking for. Here's an example implementation (you'll probably want to write some unit tests, as I haven't :)</p> <pre><code>package test; import java.io.IOException; import java.io.RandomAccessFile; public class Raf { private static class Record{ private final double price; private final int id; private final int stock; public Record(int id, int stock, double price){ this.id = id; this.stock = stock; this.price = price; } public void pack(int n, int offset, byte[] array){ array[offset + 0] = (byte)(n & 0xff); array[offset + 1] = (byte)((n >> 8) & 0xff); array[offset + 2] = (byte)((n >> 16) & 0xff); array[offset + 3] = (byte)((n >> 24) & 0xff); } public void pack(double n, int offset, byte[] array){ long bytes = Double.doubleToRawLongBits(n); pack((int) (bytes & 0xffffffff), offset, array); pack((int) ((bytes >> 32) & 0xffffffff), offset + 4, array); } public byte[] getBytes() { byte[] record = new byte[16]; pack(id, 0, record); pack(stock, 4, record); pack(price, 8, record); return record; } } private static final int RECORD_SIZE = 16; private static final int N_RECORDS = 1024; /** * @param args * @throws IOException */ public static void main(String[] args) throws IOException { RandomAccessFile raf = new RandomAccessFile(args[0], "rw"); try{ raf.seek(RECORD_SIZE * N_RECORDS); raf.seek(0); raf.write(new Record(1001, 476, 28.35).getBytes()); raf.write(new Record(1002, 240, 32.56).getBytes()); } finally { raf.close(); } } } </code></pre>

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