Calls to `System.gc()`, `Runtime.getRuntime().gc()`, and `System.runFinalization()` are not advised. Code should have the same behavior whether the garbage collection is disabled using the option `-Xdisableexplicitgc` or not. Moreover, "modern" JVMs do a very good job handling garbage collections. If memory usage issues unrelated to memory leaks develop within an application, it should be dealt with JVM options rather than within the code itself. DoNotCallGarbageCollectionExplicitly (Priority: 2, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_errorprone.html#donotcallgarbagecollectionexplicitly

Process completed with exit code 1.

Process completed with exit code 1.

Detects when a private field is declared and/or assigned a value, but not used. Since PMD 6.50.0 private fields are ignored, if the fields are annotated with any annotation or the enclosing class has any annotation. Annotations often enable a framework (such as dependency injection, mocking or e.g. Lombok) which use the fields by reflection or other means. This usage can't be detected by static code analysis. Previously these frameworks where explicitly allowed by listing their annotations in the property "ignoredAnnotations", but that turned out to be prone of false positive for any not explicitly considered framework. UnusedPrivateField (Priority: 3, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_bestpractices.html#unusedprivatefield

Reports parameters of methods and constructors that are not referenced them in the method body. Parameters whose name starts with `ignored` or `unused` are filtered out. Removing unused formal parameters from public methods could cause a ripple effect through the code base. Hence, by default, this rule only considers private methods. To include non-private methods, set the `checkAll` property to `true`. UnusedFormalParameter (Priority: 3, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_bestpractices.html#unusedformalparameter

Switch statements should be exhaustive, to make their control flow easier to follow. This can be achieved by adding a `default` case, or, if the switch is on an enum type, by ensuring there is one switch branch for each enum constant. This rule doesn't consider Switch Statements, that use Pattern Matching, since for these the compiler already ensures that all cases are covered. The same is true for Switch Expressions, which are also not considered by this rule. NonExhaustiveSwitch (Priority: 3, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_bestpractices.html#nonexhaustiveswitch

Java allows the use of several variables declaration of the same type on one line. However, it can lead to quite messy code. This rule looks for several declarations on the same line. OneDeclarationPerLine (Priority: 4, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_bestpractices.html#onedeclarationperline

Java allows the use of several variables declaration of the same type on one line. However, it can lead to quite messy code. This rule looks for several declarations on the same line. OneDeclarationPerLine (Priority: 4, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_bestpractices.html#onedeclarationperline

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: 3, Ruleset: Design) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_design.html#singularfield

Java will initialize fields with known default values so any explicit initialization of those same defaults is redundant and results in a larger class file (approximately three additional bytecode instructions per field). RedundantFieldInitializer (Priority: 3, Ruleset: Performance) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_performance.html#redundantfieldinitializer

Reports non-final fields whose value never changes once object initialization ends, and hence may be marked final. Note that this rule does not enforce that the field value be deeply immutable itself. An object can still have mutable state, even if all its member fields are declared final. This is referred to as shallow immutability. For more information on mutability, see *Effective Java, 3rd Edition, Item 17: Minimize mutability*. Limitations: We can only check private fields for now. ImmutableField (Priority: 3, Ruleset: Design) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_design.html#immutablefield

Java will initialize fields with known default values so any explicit initialization of those same defaults is redundant and results in a larger class file (approximately three additional bytecode instructions per field). RedundantFieldInitializer (Priority: 3, Ruleset: Performance) https://docs.pmd-code.org/pmd-doc-7.7.0/pmd_rules_java_performance.html#redundantfieldinitializer