The dot "." is a special character in java regex engine, so you have to use "\\." to escape this character:

final String extensionRemoved = filename.split("\\.")[0];

^{I see only solutions here but no full explanation of the problem so I decided to post this answer}

Problem

You need to know few things about text.split(delim). split method:

1. accepts as argument regular expression (regex) which describes delimiter on which we want to split,
2. if delim exists at end of text like in a,b,c,, (where delimiter is ,) split at first will create array like ["a" "b" "c" "" ""] but since in most cases we don't really need these trailing empty strings it also removes them automatically for us. So it creates another array without these trailing empty strings and returns it.

You also need to know that dot . is special character in regex. It represents any character (except line separators but this can be changed with Pattern.DOTALL flag).

So for string like "abc" if we split on "." split method will

1. create array like ["" "" "" ""],
2. but since this array contains only empty strings and they all are trailing they will be removed (like shown in previous second point)

which means we will get as result empty array [] (with no elements, not even empty string), so we can't use fn[0] because there is no index 0.

Solution

To solve this problem you simply need to create regex which will represents dot. To do so we need to escape that .. There are few ways to do it, but simplest is probably by using \ (which in String needs to be written as "\\" because \ is also special there and requires another \ to be escaped).

So solution to your problem may look like

String[] fn = filename.split("\\.");

Bonus

You can also use other ways to escape that dot like

• using character class split("[.]")
• wrapping it in quote split("\\Q.\\E")
• using proper Pattern instance with Pattern.LITERAL flag
• or simply use split(Pattern.quote(".")) and let regex do escaping for you.

When you say split you are using delim as a regex pattern. It is treated differently. Please have a look to this regular expression.

But when you are using delim in sysout you are using it as string. the difference is obviuos

When you use split with dot split(".") it split in any character which mean all the characters in the string considers as a delimiter, for that you get 0.

here is how split in this case work :

aader
^------------------------split here, so the result is 
ader
^------------------------split here, so the result is 
der
^------------------------split here, so the result is 
der
^------------------------split here, so the result is 
er
^------------------------split here, so the result is 
r
^------------------------split here, so the result is 

^------------------------this is the final result (empty, 0 result)

to split in each character you can split with empty :

String[] result = a.split("");// this return [a, a, d, e, r]
int max = split.length;// the result is 5

The regex "." would match any character as you state. However an escaped dot "\." would match literal dot characters. Thus 192.168.1.1 split on "\." would result in {"192", "168", "1", "1"}.

Your wording isn't completely clear, but I think this is what you're asking.