specification for null handling in String, StringBuilder, etc.

David Holmes david.holmes at oracle.com
Tue Jun 12 05:40:48 UTC 2012

Hi Jim,

On 12/06/2012 7:59 AM, Jim Gish wrote:
> While triaging outstanding String bugs, I was looking at 4247235, "(spec
> str) StringBuffer.insert(int, char[]) specification is inconsistent"
> Although the description is out of date w.r.t. the current code, I did
> find what I would consider weaknesses (maybe even holes) in the specs
> relative to this issue.
> It appears that the common practice is to spec checked exceptions but
> not unchecked exceptions (which I understand). For example, in the case
> mentioned the spec indicates that StringIndexOutOfBoundsException is
> thrown if the offset is invalid, and is silent about the consequences of
> the char[] being null. In the latter case, it throws
> NullPointerException, but the str == null is not checked, rather it
> simply tries to access str.length and fails if str is null.
> My personal feeling is that pre-conditions should be explicitly checked
> for and be spec'd.

This is very, very common in the core libraries. Rather than document 
every single method where a null parameter triggers NPE they are often 
covered (directly or indirectly) by blanket statements of the form 
"unless otherwise stated ...".

I'm strongly of the opinion that people should be reading the complete 
specification for any type they use, not just individual methods. So I 
don't have a problem with not documenting this on each method - there 
are likely to be far more significant misunderstandings of behaviour if 
you don't read all the docs. But I understand others will see this from 
the other side.

Also note that the handling of unchecked exceptions by JavaDoc 
complicates things because overriding implementations need to re-state 
that they throw the NPE (or whatever it may be), using @inheritDoc, even 
if there is no change from the superclass or interface specification of 
the method.


> One might argue that the spec is complete, because it says that "The
> overall effect is exactly as if the second argument were converted to a
> string by String.valueOf( char[] ),..." If you look at the class javadoc
> for String there is a statement that "Unless otherwise noted, passing a
> null argument to a constructor or method in this class will cause a ||
> <http://docs.oracle.com/javase/7/docs/api/java/lang/NullPointerException.html>|NullPointerException|
> to be thrown." However, if the user simply looks at the javadoc for
> String.valueOf( char[] ) nothing is said about null handling there. The
> user would have to have read the class javadoc to catch the reference to
> NullPointerException. Seems like an unreasonably indirect chain to have
> to follow, when all we'd have to say is that the original insert method
> throws NPE if the char[] is null.
> I suggest we improve the readability here (and in related places) and
> tell the user straight off the effect of passing null.
> Cheers
> Jim Gish

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