Android :: How To Write To File ?

I would like to know how to write to file. I have tried using OutPutStream out = new FileOutPutStream(fileName); out.write(byte [], 0, length);This way always returns Parent directory or File is not writable.I have also used openFileOutPut(fileName, MODE_PRIVATE) method but this method works properly if it is called from the same class which has extended Activity and I am writing to the file in different class and that too in different thread.I have tried to get the Application Context through getApplciationContext and call openFileOutPut method but still theres something wrong that I am doing.So please can someone suggest me a proper way to write to a file.