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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-7.9.0/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-7.9.0/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-7.9.0/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-7.9.0/pmd_rules_java_performance.html#redundantfieldinitializer

The isEmpty() method on java.util.Collection is provided to determine if a collection has any elements. Comparing the value of size() to 0 does not convey intent as well as the isEmpty() method. UseCollectionIsEmpty (Priority: 3, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.9.0/pmd_rules_java_bestpractices.html#usecollectionisempty

The isEmpty() method on java.util.Collection is provided to determine if a collection has any elements. Comparing the value of size() to 0 does not convey intent as well as the isEmpty() method. UseCollectionIsEmpty (Priority: 3, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.9.0/pmd_rules_java_bestpractices.html#usecollectionisempty

The conversion of literals to strings by concatenating them with empty strings is inefficient. It is much better to use one of the type-specific `toString()` methods instead or `String.valueOf()`. AddEmptyString (Priority: 3, Ruleset: Performance) https://docs.pmd-code.org/pmd-doc-7.9.0/pmd_rules_java_performance.html#addemptystring

The conversion of literals to strings by concatenating them with empty strings is inefficient. It is much better to use one of the type-specific `toString()` methods instead or `String.valueOf()`. AddEmptyString (Priority: 3, Ruleset: Performance) https://docs.pmd-code.org/pmd-doc-7.9.0/pmd_rules_java_performance.html#addemptystring

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-7.9.0/pmd_rules_java_bestpractices.html#unusedprivatefield