I have come across an interesting post from here. Its an from MIT.

It says:

constant_pool[0] may be used by the implementation for whatever purposes it wishes.

One possible use is suggested on the same MIT website, which can be found here link description here. In the example in the link, the in-memory constant_pool[0] is used as an optimization when used to check which class fields have been resolved, in other words if other constant_pool entries in the in-memory constant_pool have had constant_pool[x] read in from the class file, where x > 1. The class file could possibly be mmaped. This would be an example of lazy symbol resolution, otherwise all the class files would have to be parsed and stored in an in-memory representation of the class file when the JVM starts. This would cause slow JVM starts.

There are other possible uses as explained in another stackoverflow question that can be found here.