I took the Sun certification for Java 2 about 20 years or so ago. Actually I took two: the programmer certification, which was a multiple choice exam, and the developer certification that required you to develop an app, turn it in, and then answer some questions about some of your design decisions (I assume to prove that you really wrote it yourself). My overall experience was similar: I spent about a month studying on and off, learned a lot, and felt like I was a better Java developer for it.