double and string into bytes...

[gbattine]

Hy guys,
i need your help for a question.
It's about a topic i've posted but i've created a new topic because the
arguments are different.
I have a file with this fixed format

string string string string
string double double double
string double double double


with a row(the first) that i have stored in an array of string colled
Intest and the others rows,
the second and the third that i have stored in an arraylist of row
objects(a string field and an array of double).
What i have to do now is converting this file, that is convert these
objects into an array of byte with a fixed format,to rebuild later the
original format from the array of bytes.
The fixed format can be obtained for example adding ; between two
values in two different columns and ç at the end of a row.
How can i do it?
I've seen DataOutputStrem Interface and writeDouble method for double
value.
How can i do with string field?
Can you help me with some idea or code?
Thanks