Unable To Prevent Data From Overwritten

I am trying to prevent data from being overwritten and appended instead when I save it to a file. I already set the append boolean to true on the FileOutputStream, but the data is still being overwritten. My code is below:

public class Main {
public static void main(String[] args) {
LinkedHashMultimap<String, Integer> testMap = LinkedHashMultimap.create();
//ListMultimap<String, Integer> test = ArrayListMultimap.create();
OutputStreamWriter outputStream;
testMap.put("NumberA",1);
testMap.put("NumberA", 33);

[Code]...

I performed a print statement to verify that the LinkedHashMultiMap is not replacing values with the same key (which it was not) and realized that the overwriting was occurring when I wrote the data to a file.