writeDouble()

Here is a program that writes four doubles to a file.

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

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

    out.writeDouble( 0.0 );
    out.writeDouble( 1.0 );
    out.writeDouble( 255.0 );
    out.writeDouble( -1.0 );

    out.close();
  }
}

Primitive type double uses 64 bits, or eight bytes per value. The hex dump of doubleData.dat shows four groups of eight bytes, highlighted with red braces (which I added). The first group is recognizable as zero, but the others are mysterious. Floating point data are represented with a different style of bit patterns than integer data.

The right side of each line in the dump attempts to interpret the bytes as ASCII characters. The "." means that the byte does not contain a bit pattern that represents a character. Some bytes happen to contain a pattern that could be a character. One byte, part of the bytes that represent -1, contains a pattern that could represent the character "@". The byte after it contains a pattern that could represent "o".