Code 34m0

I just came across this Groovy gotcha today while accessing a list within a chain of attributes. I had a statement like this in a gsp:
${invoice?.invoiceItems[0]?.name}
This is the (erroneous) recommended style when data binding many ended associations in Grails, but it could easily arise in other scenarios. If I try this "safe" statement where invoiceItems is null, I'll see a null pointer exception. In fact I'll even see it when invoice is null. The reason for the exception is a combination of how lists and the safe navigation work in Groovy.

Lists are much more elegant in Groovy than Java, as they share the square brackets that primitive arrays use. Creating a list is as easy as writing:
def x = [1,2,3]
Just like an array you can access the first element of this list like so:
println x[0]
However, unlike a primitive array the [] operator on a list is mapped (by some Groovy magic) behind the scenes to the getAt method of the particular List object. We can expand our exampl…