Position literals first in all String comparisons, if the second argument is null then NullPointerExceptions can be avoided, they will just return false. Note that switching literal positions for compareTo and compareToIgnoreCase may change the result, see examples. Note that compile-time constant strings are treated like literals. This is because they are inlined into the class file, are necessarily non-null, and therefore cannot cause an NPE at runtime. LiteralsFirstInComparisons (Priority: 1, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#literalsfirstincomparisons

Excessive coupling to implementation types (e.g., `HashSet`) limits your ability to use alternate implementations in the future as requirements change. Whenever available, declare variables and parameters using a more general type (e.g, `Set`). This rule reports uses of concrete collection types. User-defined types that should be treated the same as interfaces can be configured with the property `allowedTypes`. LooseCoupling (Priority: 1, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#loosecoupling

Excessive coupling to implementation types (e.g., `HashSet`) limits your ability to use alternate implementations in the future as requirements change. Whenever available, declare variables and parameters using a more general type (e.g, `Set`). This rule reports uses of concrete collection types. User-defined types that should be treated the same as interfaces can be configured with the property `allowedTypes`. LooseCoupling (Priority: 1, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#loosecoupling