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/snapshot/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

Configurable naming conventions for field declarations. This rule reports variable declarations which do not match the regex that applies to their specific kind ---e.g. constants (static final), enum constant, final field. Each regex can be configured through properties. By default this rule uses the standard Java naming convention (Camel case), and uses the ALL_UPPER convention for constants and enum constants. FieldNamingConventions (Priority: 1, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#fieldnamingconventions

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/snapshot/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

Whenever using a log level, one should check if it is actually enabled, or otherwise skip the associate String creation and manipulation, as well as any method calls. An alternative to checking the log level are substituting parameters, formatters or lazy logging with lambdas. The available alternatives depend on the actual logging framework. GuardLogStatement (Priority: 2, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#guardlogstatement

Whenever using a log level, one should check if it is actually enabled, or otherwise skip the associate String creation and manipulation, as well as any method calls. An alternative to checking the log level are substituting parameters, formatters or lazy logging with lambdas. The available alternatives depend on the actual logging framework. GuardLogStatement (Priority: 2, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#guardlogstatement

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/snapshot/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

Whenever using a log level, one should check if it is actually enabled, or otherwise skip the associate String creation and manipulation, as well as any method calls. An alternative to checking the log level are substituting parameters, formatters or lazy logging with lambdas. The available alternatives depend on the actual logging framework. GuardLogStatement (Priority: 2, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#guardlogstatement

Whenever using a log level, one should check if it is actually enabled, or otherwise skip the associate String creation and manipulation, as well as any method calls. An alternative to checking the log level are substituting parameters, formatters or lazy logging with lambdas. The available alternatives depend on the actual logging framework. GuardLogStatement (Priority: 2, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#guardlogstatement

Reports functional interfaces that were not explicitly declared as such with the annotation `@FunctionalInterface`. If an interface is accidentally a functional interface, then it should bear a `@SuppressWarnings("PMD.ImplicitFunctionalInterface")` annotation to make this clear. ImplicitFunctionalInterface (Priority: 2, Ruleset: Best Practices) https://docs.pmd-code.org/snapshot/pmd_rules_java_bestpractices.html#implicitfunctionalinterface

Reports method and constructor parameters that can be made final because they are never reassigned within the body of the method. This rule ignores unused parameters so as not to overlap with the rule {% rule java/bestpractices/UnusedFormalParameter %}. It will also ignore the parameters of abstract methods. MethodArgumentCouldBeFinal (Priority: 3, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#methodargumentcouldbefinal

Reports method and constructor parameters that can be made final because they are never reassigned within the body of the method. This rule ignores unused parameters so as not to overlap with the rule {% rule java/bestpractices/UnusedFormalParameter %}. It will also ignore the parameters of abstract methods. MethodArgumentCouldBeFinal (Priority: 3, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#methodargumentcouldbefinal

Reports import statements that can be removed. They are either unused, duplicated, or the members they import are already implicitly in scope, because they're in java.lang, or the current package. If some imports cannot be resolved, for instance because you run PMD with an incomplete auxiliary classpath, some imports may be conservatively marked as used even if they're not to avoid false positives. UnnecessaryImport (Priority: 4, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#unnecessaryimport

Reports import statements that can be removed. They are either unused, duplicated, or the members they import are already implicitly in scope, because they're in java.lang, or the current package. If some imports cannot be resolved, for instance because you run PMD with an incomplete auxiliary classpath, some imports may be conservatively marked as used even if they're not to avoid false positives. UnnecessaryImport (Priority: 4, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#unnecessaryimport

Reports import statements that can be removed. They are either unused, duplicated, or the members they import are already implicitly in scope, because they're in java.lang, or the current package. If some imports cannot be resolved, for instance because you run PMD with an incomplete auxiliary classpath, some imports may be conservatively marked as used even if they're not to avoid false positives. UnnecessaryImport (Priority: 4, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#unnecessaryimport

Short Classnames with fewer than e.g. five characters are not recommended. ShortClassName (Priority: 4, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#shortclassname

If a class is marked as `Serializable`, then all fields need to be serializable as well. In order to exclude a field, it can be marked as transient. Static fields are not considered. This rule reports all fields, that are not serializable. If a class implements the methods to perform manual serialization (`writeObject`, `readObject`) or uses a replacement object (`writeReplace`, `readResolve`) then this class is ignored. Note: This rule has been revamped with PMD 6.52.0. It was previously called "BeanMembersShouldSerialize". The property `prefix` has been deprecated, since in a serializable class all fields have to be serializable regardless of the name. NonSerializableClass (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/snapshot/pmd_rules_java_errorprone.html#nonserializableclass

Fields, local variables, enum constant names or parameter names that are very short are not helpful to the reader. ShortVariable (Priority: 3, Ruleset: Code Style) https://docs.pmd-code.org/snapshot/pmd_rules_java_codestyle.html#shortvariable

Serializable classes should provide a serialVersionUID field. The serialVersionUID field is also needed for abstract base classes. Each individual class in the inheritance chain needs an own serialVersionUID field. See also [Should an abstract class have a serialVersionUID](https://stackoverflow.com/questions/893259/should-an-abstract-class-have-a-serialversionuid). MissingSerialVersionUID (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/snapshot/pmd_rules_java_errorprone.html#missingserialversionuid