Assembling a computer is an easy task. Many developers do it at some point in their careers. But it’s really only easy once you understand what each part does and how it fits into the larger system. If I gave a bag of computer parts to someone that didn’t know what a computer did and told them to put it together, things may not go so well.
In the enterprise Java world, there are many domains that transfer well. The MVC pattern common in most web frameworks is an example. Once you know one MVC framework, picking up another is just a matter of understanding the syntax ...