Fix issues as noted by SonarLint + enum comparison

Assignments should not be redundant
Collection sizes and array length comparisons should make sense
Conditionally executed code should be reachable
Empty arrays and collections should be returned instead of null
Local variables should not shadow class fields
Null pointers should not be dereferenced
Pattern Matching for "instanceof" operator should be used instead of simple "instanceof" + cast
Return of boolean expressions should not be wrapped into an "if-then-else" statement
Sections of code should not be commented out
"switch" statements should have at least 3 "case" clauses
Two branches in a conditional structure should not have exactly the same implementation