You can use fileWriter with a true for appending.
try
{
String filename= "MyFile.txt";
FileWriter fw = new FileWriter(filename,true); //the true will append the new data
fw.write("add a line
");//appends the string to the file
fw.close();
}
catch(IOException ioe)
{
System.err.println("IOException: " + ioe.getMessage());
}