Fix issues as noted by SonarLint

Declarations should use Java collection interfaces such as "List" rather than specific implementation classes such as "LinkedList"
Empty statements should be removed
JUnit assertTrue/assertFalse should be simplified to the corresponding dedicated assertion
JUnit5 test classes and methods should have default package visibility
Methods and field names should not be the same or differ only by capitalization
Nested "enum"s should not be declared static