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 leads to strange results on multicore computers because it easily can exceed 100%.
If you have 8 cores, then
top can display CPU usage from 0% (idle system) to 800% (full power).
Your program is just using your 4 cores with hyper-threading (so 8 virtual cores) at maximum capacity. So
top gives you nearly 8 x 100% = 800%.
To change this way of calculating CPU usage (it's called Irix mode), press uppercase 'i': Shift+i
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.