Reports classes that may be made final because they cannot be extended from outside their compilation unit anyway. This is because all their constructors are private, so a subclass could not call the super constructor. ClassWithOnlyPrivateConstructorsShouldBeFinal (Priority: 1, Ruleset: Design) https://docs.pmd-code.org/pmd-doc-7.0.0-rc3/pmd_rules_java_design.html#classwithonlyprivateconstructorsshouldbefinal

Reports classes that may be made final because they cannot be extended from outside their compilation unit anyway. This is because all their constructors are private, so a subclass could not call the super constructor. ClassWithOnlyPrivateConstructorsShouldBeFinal (Priority: 1, Ruleset: Design) https://docs.pmd-code.org/pmd-doc-7.0.0-rc3/pmd_rules_java_design.html#classwithonlyprivateconstructorsshouldbefinal

Reports classes that may be made final because they cannot be extended from outside their compilation unit anyway. This is because all their constructors are private, so a subclass could not call the super constructor. ClassWithOnlyPrivateConstructorsShouldBeFinal (Priority: 1, Ruleset: Design) https://docs.pmd-code.org/pmd-doc-7.0.0-rc3/pmd_rules_java_design.html#classwithonlyprivateconstructorsshouldbefinal

For any method that returns an collection (such as an array, Collection or Map), it is better to return an empty one rather than a null reference. This removes the need for null checking all results and avoids inadvertent NullPointerExceptions. See Effective Java, 3rd Edition, Item 54: Return empty collections or arrays instead of null ReturnEmptyCollectionRatherThanNull (Priority: 1, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.0.0-rc3/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

Empty Catch Block finds instances where an exception is caught, but nothing is done. In most circumstances, this swallows an exception which should either be acted on or reported. EmptyCatchBlock (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.0.0-rc3/pmd_rules_java_errorprone.html#emptycatchblock

Empty Catch Block finds instances where an exception is caught, but nothing is done. In most circumstances, this swallows an exception which should either be acted on or reported. EmptyCatchBlock (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.0.0-rc3/pmd_rules_java_errorprone.html#emptycatchblock

Empty Catch Block finds instances where an exception is caught, but nothing is done. In most circumstances, this swallows an exception which should either be acted on or reported. EmptyCatchBlock (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.0.0-rc3/pmd_rules_java_errorprone.html#emptycatchblock