Many things in Qi4j are derived from the lessons learned from the Domain Driven Design book and approach. Rickard Öberg covers these mechanics in his latest blog entry.
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Domain Driven Design has a special status in Qi4j, and its tactical bits are embraced in Qi4j itself. These includes Entities, Services and Values. In Rickard's latest blog entry, he covers these topics in a relatively easy to understand and pedagogic format. I highly recommended read for people who want to get a quick overview of what Qi4j is really about.
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