This is could be due to a few things:
- As noted by syrion, Java's virtual machine is allowed to perform optimizations on your code as it is running. Your first block is likely taking longer because Java hasn't yet optimized your code fully. As the first block runs, the JVM is applying changes which can then be utilized in the other blocks.
- Your processor could be caching the results of your code, speeding up future blocks. This is similar to the previous point, but can vary even between identical JVM implementations.
- While your program is running, your computer is also performing other tasks. These include handling the OS's UI, checking for program updates, etc. For this reason, some blocks of code can be slower than others, because your computer isn't concentrating as much resources towards its execution.
- Java's virtual machine is garbage collected. That is to say, at unspecified points during your program's execution, the JVM takes some time to clean up any objects that are no longer used.
Points 1 and 2 are likely the cause for the large difference in the first block's execution time. Point 3 could be the reason for the smaller fluctuations, and point 4, as noted by Stephen, probably caused the large stall in block 3.
Another thing that I didn't notice is your use of both long and Long. The object form contains a larger memory overhead, and both are subject to different optimizations.
