Linux – Sorted tree view in htop or alternative

The scale used by top is 100% when a core is fully used. Or when one core is 20% and a second one is 80%. This lead to strange results on multicore computers because it easely can exceed 100%.
If you have 8 cores, then top can display from 0% (idle system) to 800% (full power).

Your program is just using your 4 cores with hyperthreading (so 8 virtual cores) at maximum capacity. So top gives you nearly 8 x 100% = 800%.

You can't explicitly tell the JVM to use a single thread, it will use multiple threads to do its work, even if you only use a single thread in your application.

You can use the htop utility as a better top alternative. You'll see your program using all of the 8 cores.