Why does "split" on an empty string return a non-empty array?

Split on an empty string returns an array of size 1 :

scala> "".split(',')
res1: Array[String] = Array("")

Consider that this returns empty array:

scala> ",,,,".split(',')
res2: Array[String] = Array()

Please explain :)

Solution 1:

If you split an orange zero times, you have exactly one piece - the orange.

Solution 2:

The Java and Scala split methods operate in two steps like this:

First, split the string by delimiter. The natural consequence is that if the string does not contain the delimiter, a singleton array containing just the input string is returned,
Second, remove all the rightmost empty strings. This is the reason ",,,".split(",") returns empty array.

According to this, the result of "".split(",") should be an empty array because of the second step, right?

It should. Unfortunately, this is an artificially introduced corner case. And that is bad, but at least it is documented in java.util.regex.Pattern, if you remember to take a look at the documentation:

For n == 0, the result is as for n < 0, except trailing empty strings will not be returned. (Note that the case where the input is itself an empty string is special, as described above, and the limit parameter does not apply there.)

Solution 1: Always pass -1 as the second parameter

So, I advise you to always pass n == -1 as the second parameter (this will skip step two above), unless you specifically know what you want to achieve / you are sure that the empty string is not something that your program would get as an input.

Solution 2: Use Guava Splitter class

If you are already using Guava in your project, you can try the Splitter (documentation) class. It has a very rich API, and makes your code very easy to understand.

Splitter.on(".").split(".a.b.c.") // "", "a", "b", "c", ""
Splitter.on(",").omitEmptyStrings().split("a,,b,,c") // "a", "b", "c"
Splitter.on(CharMatcher.anyOf(",.")).split("a,b.c") // "a", "b", "c"
Splitter.onPattern("=>?").split("a=b=>c") // "a", "b", "c"
Splitter.on(",").limit(2).split("a,b,c") // "a", "b,c"

Solution 3:

Splitting an empty string returns the empty string as the first element. If no delimiter is found in the target string, you will get an array of size 1 that is holding the original string, even if it is empty.

Solution 4:

For the same reason that

",test" split ','

and

",test," split ','

will return an array of size 2. Everything before the first match is returned as the first element.