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

Question

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?

Answer 1

subSet(a, b) is inclusive of a but exclusive of b. Therefore if you want to find a subset that is inclusive of the upper bound pickle, you have to use the next possible string after pickle as the (exclusive) upper bound.

You can get this by adding the null character \0 to the end of the string.

In Java, a char is "really" just an integer between 0 and 65535, and the compareTo method of String sorts characters by these values. To see that \0 is the smallest possible char value you can print its value like this:

 System.out.println((int) '\0');    // Prints 0

Answer 2

I believe \0 is the null character. The next word in the ASCII dictionary from a lexigraphic point after the word pickle is pickle\0. The JavaDoc for SortedSet.subset() has the following to say:

Returns a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive.

In other words, if your code snippet were the following:

int count = dictionary.subSet("doorbell", "pickle").size();

then the word pickle would not appear in the subset.

A better example might be if you wanted to get all words of the form pickles, but nothing of the form picklet (ending in t), then you would use the following code:

int count = dictionary.subSet("doorbell", "picklet").size();

Here is a link to the ASCII character table.