No Markers

The bytes on the disk are just raw bytes. You (the programmer) have to know what you wrote in order to make sense of it. Here is another program:

import java.io.*;
class TwoShorts
{
  public static void main ( String[] args ) throws IOException
  {
    String fileName = "mixedTypes.dat" ;

    DataOutputStream dataOut = new DataOutputStream(
        new BufferedOutputStream(
        new FileOutputStream( fileName  ) ) );

    dataOut.writeShort( 0 );
    dataOut.writeShort( 0 );

    dataOut.writeDouble( 12.45 );

    dataOut.close();
  }
}

Two 16-bit shorts are written, each containing a zero. The previous program wrote a 32-bit zero. The double is the same as before.