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

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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.

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