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

When doing `String::toLowerCase()/toUpperCase()` conversions, use an explicit locale argument to specify the case transformation rules. Using `String::toLowerCase()` without arguments implicitly uses `Locale::getDefault()`. The problem is that the default locale depends on the current JVM setup (and usually on the system in which it is running). Using the system default may be exactly what you want (e.g. if you are manipulating strings you got through standard input), but it may as well not be the case (e.g. if you are getting the string over the network or a file, and the encoding is well-defined and independent of the environment). In the latter case, using the default locale makes the case transformation brittle, as it may yield unexpected results on a machine whose locale has other case translation rules. For example, in Turkish, the uppercase form of `i` is `İ` (U+0130, not ASCII) and not `I` (U+0049) as in English. The rule is intended to *force* developers to think about locales when dealing with strings. By taking a conscious decision about the choice of locale at the time of writing, you reduce the risk of surprising behaviour down the line, and communicate your intent to future readers. UseLocaleWithCaseConversions (Priority: 1, Ruleset: Error Prone) https://docs.pmd-code.org/snapshot/pmd_rules_java_errorprone.html#uselocalewithcaseconversions

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/snapshot/pmd_rules_java_design.html#classwithonlyprivateconstructorsshouldbefinal

Reports fields which may be converted to a local variable. This is so because in every method where the field is used, it is assigned before it is first read. Hence, the value that the field had before the method call may not be observed, so it might as well not be stored in the enclosing object. Limitations: * We can only check private fields for now. * The rule is not aware of threading, so it may cause false positives in concurrent code. Such FPs are best handled by suppression (see also the `ignoredAnnotations` property). SingularField (Priority: 1, Ruleset: Design) https://docs.pmd-code.org/snapshot/pmd_rules_java_design.html#singularfield

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/snapshot/pmd_rules_java_design.html#classwithonlyprivateconstructorsshouldbefinal

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