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As norm pointed out, using the toString() method of array will use the default toString() method - see Object (Java Platform SE 7 )
Loop over the array and write each value as a String using the...

The data is being saved as binary - try writing in text form (eg convert the byte to a String). As suggested in the API for FileOutputStream:

It is unclear, at least to me, how you 'see' the file contents. Do you display the file contents elsewhere? How are you reading the file? How do you know this is a writing problem rather than a...