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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/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

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/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

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/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

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/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

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/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

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/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#returnemptycollectionratherthannull

Some classes contain overloaded getInstance. The problem with overloaded getInstance methods is that the instance created using the overloaded method is not cached and so, for each call and new objects will be created for every invocation. SingleMethodSingleton (Priority: 2, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#singlemethodsingleton

A singleton class should only ever have one instance. Failure to check whether an instance has already been created may result in multiple instances being created. SingletonClassReturningNewInstance (Priority: 2, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#singletonclassreturningnewinstance

Reports calls to overridable methods on `this` during object initialization. These are invoked on an incompletely constructed object and can be difficult to debug if overridden. This is because the subclass usually assumes that the superclass is completely initialized in all methods. If that is not the case, bugs can appear in the constructor, for instance, some fields that are still null may cause a NullPointerException or be stored somewhere else to blow up later. To avoid this problem, only use methods that are static, private, or final in constructors. Note that those methods also must not call overridable methods transitively to be safe. ConstructorCallsOverridableMethod (Priority: 1, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#constructorcallsoverridablemethod

Reports calls to overridable methods on `this` during object initialization. These are invoked on an incompletely constructed object and can be difficult to debug if overridden. This is because the subclass usually assumes that the superclass is completely initialized in all methods. If that is not the case, bugs can appear in the constructor, for instance, some fields that are still null may cause a NullPointerException or be stored somewhere else to blow up later. To avoid this problem, only use methods that are static, private, or final in constructors. Note that those methods also must not call overridable methods transitively to be safe. ConstructorCallsOverridableMethod (Priority: 1, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#constructorcallsoverridablemethod

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-7.11.0/pmd_rules_java_performance.html#redundantfieldinitializer

Code containing duplicate String literals can usually be improved by declaring the String as a constant field. AvoidDuplicateLiterals (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#avoidduplicateliterals

Code containing duplicate String literals can usually be improved by declaring the String as a constant field. AvoidDuplicateLiterals (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#avoidduplicateliterals

Avoid using hard-coded literals in conditional statements. By declaring them as static variables or private members with descriptive names maintainability is enhanced. By default, the literals "-1" and "0" are ignored. More exceptions can be defined with the property "ignoreMagicNumbers". The rule doesn't consider deeper expressions by default, but this can be enabled via the property `ignoreExpressions`. With this property set to false, if-conditions like `i == 1 + 5` are reported as well. Note that in that case, the property ignoreMagicNumbers is not taken into account, if there are multiple literals involved in such an expression. AvoidLiteralsInIfCondition (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#avoidliteralsinifcondition

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-7.11.0/pmd_rules_java_bestpractices.html#onedeclarationperline

Reports assignments to variables that are never used before the variable is overwritten, or goes out of scope. Unused assignments are those for which 1. The variable is never read after the assignment, or 2. The assigned value is always overwritten by other assignments before the next read of the variable. The rule tracks assignements to fields of `this`, and static fields of the current class. This may cause some false positives in timing-sensitive concurrent code, which the rule cannot detect. The rule may be suppressed with the standard `@SuppressWarnings("unused")` tag. The rule subsumes {% rule "UnusedLocalVariable" %}, and {% rule "UnusedFormalParameter" %}. Those violations are filtered out by default, in case you already have enabled those rules, but may be enabled with the property `reportUnusedVariables`. Variables whose name starts with `ignored` or `unused` are filtered out, as is standard practice for exceptions. Limitations: * The rule currently cannot know which method calls throw exceptions, or which exceptions they throw. In the body of a try block, every method or constructor call is assumed to throw. This may cause false-negatives. The only other language construct that is assumed to throw is the `throw` statement, in particular, things like `assert` statements, or NullPointerExceptions on dereference are ignored. * The rule cannot resolve assignments across constructors, when they're called with the special `this(...)` syntax. This may cause false-negatives. Both of those limitations may be partly relaxed in PMD 7. UnusedAssignment (Priority: 3, Ruleset: Best Practices) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_bestpractices.html#unusedassignment

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-7.11.0/pmd_rules_java_performance.html#redundantfieldinitializer

Code should never throw NullPointerExceptions under normal circumstances. A catch block may hide the original error, causing other, more subtle problems later on. AvoidCatchingNPE (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#avoidcatchingnpe

Assigning a "null" to a variable (outside of its declaration) is usually bad form. Sometimes, this type of assignment is an indication that the programmer doesn't completely understand what is going on in the code. NOTE: This sort of assignment may used in some cases to dereference objects and encourage garbage collection. NullAssignment (Priority: 3, Ruleset: Error Prone) https://docs.pmd-code.org/pmd-doc-7.11.0/pmd_rules_java_errorprone.html#nullassignment