Elvis operator

Dierk König dierk.koenig at canoo.com
Thu Jul 16 03:13:28 PDT 2009


I made the term "Elvis operator" popular through the  "Groovy in  
Action" and I would like to clarify its
meaning because it is often confused with the "Null-safe dereferencing  

This has led to numerous misquotes in communications, conference  
talks, and podcasts like
in episode 253 of the JavaPosse.

The Elvis operator is a shortcut for the ternary "if" operator when  
the "true" case should
yield the value of the conditional.

The following lines are equivalent(1) in Groovy
   if (a) then a else b // classic if
   a ? a : b                   // ternary if
   a ?: b                       // Elvis

This construction only makes sense in Groovy because "a" can be of any  
arbitrary type -
and is not restricted to be a boolean expression. The boolean value  
inside the
conditional is calculated by the rules of the "Groovy truth", where  
objects can be
casted to boolean such that e.g. null objects are treated as "false"  
just like
empty Strings, Maps, and Lists.
Therefore, an Elvis expression does not necessarily return a boolean  
in Groovy
but can be of any runtime type allowing constructions like
   String s = a ?: "n/a"
to set a reference to a default value if no proper value is available.
The same use comes handy in method calls like
   myMethod( a ?: "n/a" )

The discussion above makes me think that an Elvis operator would be much
less useful in Java...

Elvis is often confused with the null-safe dereferencing of objects  
like in
which does not throw NPE if "a" is null but returns null without  
evaluating foo().

This is __**NOT**__ an Elvis operator unless you assume a one-eyed  
Elvis !!!

I'm glad to see that in recent posts on this list, the distinction has  
much improved.

all the best

(1) "equivalent" does not mean "identical" because in case of the  
Elvis operator, the
"a" expression is only evaluated once.

P.S. Needless to say: there is only one true Elvis!

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