If your goal is to just get certified fast, the 1Z0-817 Upgrade Exam is the shortest path. That said, if your goal is to learn the material, then I recommend the 1Z0-819 exam. It has a lot of topics not on the Java 8 exam (nor the upgrade exam) that are important for learning Java such as annotations and security. It's a harder exam, but you'll feel a lot more confident after you pass it.
Ayoub Rossi wrote:. . . I feel like if I should take the exact same exam I wouldn't pass. . . .
Most people wouldn't pass. It is usually necessary to revise exam topics in order to get anywhere near a pass mark.
When I was at school we sat the “mock” A level maths exam, followed one week later by another internal school exam. Only one person revised anything second time round, and he got good marks in both exams; the rest of us got about 30% less in the second exam than the first. One place I worked a long time ago, somebody sat an exam analogous to the cert, and thought they would pass because the questions covered what they did every day; they failed repeatedly.
Good night. Drive safely. Here's a tiny ad for the road: