You are declaring your main method in PerkusjaTester
which is an inner class of Perkusja
. That is forbidden.
You should declare your test class outside of class Perkusja
Note : PerkusjaTester
is an inner class, not a static nested class. That is why PerkusjaTester
is not a static type. As mentionned by Jon Skeet, you could also add the keyword static
on class PerkusjaTester
.