How exactly does String.split() method in Java work when regex is provided?

Question

I'm preparing for OCPJP exam and I ran into the following example:

class Test {
   public static void main(String args[]) {
      String test = "I am preparing for OCPJP";
      String[] tokens = test.split("\\S");
      System.out.println(tokens.length);
   }
}

This code prints 16. I was expecting something like no_of_characters + 1. Can someone explain me, what does the split() method actually do in this case? I just don't get it...

Answer 1

It splits on every "\\S" which in regex engine represents \S non-whitespace character.

So lets try to split "x x" on non-whitespace (\S). Since this regex can be matched by one character lets iterate over them to mark places of split (we will use pipe | for that).

• is 'x' non-whitespace? YES, so lets mark it | x
• is ' ' non-whitespace? NO, so we leave it as is
• is last 'x' non-whitespace? YES, so lets mark it | |

So as result we need to split our string at start and at end which initially gives us result array

["", " ", ""]
   ^    ^ - here we split

But since trailing empty strings are removed, result would be

[""," "]     <- result
        ,""] <- removed trailing empty string

so split returns array ["", " "] which contains only two elements.

BTW. To turn off removing last empty strings you need to use split(regex,limit) with negative value of limit like split("\\S",-1).

---

Now lets get back to your example. In case of your data you are splitting on each of

I am preparing for OCPJP
| || ||||||||| ||| |||||

which means

 ""|" "|""|" "|""|""|""|""|""|""|""|""|" "|""|""|" "|""|""|""|""|""

So this represents this array

[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]  

but since trailing empty strings "" are removed (if their existence was caused by split - more info at: Confusing output from String.split)

[""," ",""," ","","","","","","","",""," ","",""," ","","","","",""]  
                                                     ^^ ^^ ^^ ^^ ^^

you are getting as result array which contains only this part:

[""," ",""," ","","","","","","","",""," ","",""," "]  

which are exactly 16 elements.