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Is it possible for split to return a null String[]? I am curious as I want to try to be as defensive as possible in my code without having unnecessary checks. The code is as follows: 

String[] parts = myString.split("\\w");

do I need to perform a null check before I use parts post splitting?

Woot4Moo
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4 Answers4

58

It never returns null. You should always check the javadoc of the method if you are not sure. For example String#split(String) says

This method works as if by invoking the two-argument split method

...and String#split(String,int) says:

If the expression does not match any part of the input then the resulting array has just one element, namely this string.

From Javadoc you can also find out what kind of exceptions can happen and more importantly why were those exceptions thrown. Also one very important thing to check is if the classes are thread safe or not.

T.J. Crowder
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palto
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  • I did check the javadoc, returning null != throwing NPE. I saw the PatternException that it can throw. – Woot4Moo Aug 01 '11 at 17:33
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    The Javadoc for `split(String regex)` does not indicate that null cannot be returned. – Paul Aug 01 '11 at 17:35
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    @Paul: No, but if `null` could be returned, it (or the `Pattern#split` docs) would say so. And the two-argument version, which the single argument says it calls, says *"If the expression does not match any part of the input then the resulting array has just one element, namely this string."* – T.J. Crowder Aug 01 '11 at 17:37
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I recommend you download the Java source code for those times when the API is unclear. All the answers here just tell you the answer but you should really see for yourself. You can download the Java source code from here (look near the bottom of the page).

By following the source code you'll end up at String[] Pattern.split(CharSequence input, int limit). The return value comes from a non-null ArrayList on which toArray is called. toArray does not return null: in the event of an empty list you'll get an empty array back.

So in the end, no, you don't have to check for null.

Paul
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No, you don't need to check for null on the parts.

mkro
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Split method calls the Patter.split method which does this in the beginning of the method:

ArrayList<String> matchList = new ArrayList<String>();

And at the end does matchList.toArray() to return an Array.

So no need to test of nulls.

Varun Achar
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    Never rely on looking at the implementation. The implementation can change. The documentation is the contract you're working to. – T.J. Crowder Aug 01 '11 at 17:40
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    True.. but the implementation of the this method unlikely to change, or the entire java world who've used this method without a null check will be in disarray. – Varun Achar Aug 01 '11 at 17:46
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    In this case, it's clear from the docs that if it changes, it must not return `null`. – T.J. Crowder Aug 01 '11 at 17:49