The question might sound absurd. Every where when I see the difference between Contract First and Contract Last, I see the word WSDL.
How does it fit in the current world of annotation and JSON Objects? Does it map like
Contract-First --> Use definitive Java class as Request Object Contract-last -> Use generic hashmap as Request Object
Instead of Contract First look for the synonym API First. I use Swagger/ Open API Specification for that approach. As you tagged the question with CXF the Swagger2Feature might be interesting for you: http://cxf.apache.org/docs/swagger2feature.html
What you named Contract Last is Code First that means you write your Java classes first, annotate them and the Contract (WSDL, WADL, Swagger) will be generated afterwards.