why use \0 to include highEndPoint as part of the sublist

I saw the code below from java tutorial oracle. In order to count the number of words between doorbell (inclusive) and pickle(inclusive), the author added \0 after the word pickle. I understand that the effect of adding \0 after pickle, is that the word pickle is now included as part of the subset. But my question is, why use \0? Could someone please help me out? Thanks in advance for any help!

SortedSet<String> dictionary = new TreeSet<>(entire collection of words from a dictionary);
int count = dictionary.subSet("doorbell", "pickle\0").size();
System.out.println(count);

Edit:

Also, what happen if the variable dictionary is a reference to SortedSet? what should I do now if I want to include the highEndPoint?