[java] New rule: FinalInterfaceMethodParameterIsUnclear

Add the rule to check that public method in interface do not use final qualifier which is unclear

pmd-java/src/main/resources/category/java/bestpractices.xml

@@ -541,6 +541,41 @@ return a;
         </example>
     </rule>
 
+    <rule name="FinalInterfaceMethodParameterIsUnclear"
+          language="java"
+          since="6.42.0"
+          message="Using final in public (non default) interface method is unclear"
+          class="net.sourceforge.pmd.lang.rule.XPathRule"
+          typeResolution="true"
+          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_bestpractices.html#finalinterfacemethodparameterisunclear">
+        <description>
+            Declaring a method parameter as final for an interface method is useless because the implementation may choose to not respect it.
+        </description>
+        <priority>1</priority>
+        <properties>
+            <property name="version" value="2.0"/>
+            <property name="xpath">
+                <value>
+<![CDATA[
+//ClassOrInterfaceDeclaration[@Interface = true()]
+    /ClassOrInterfaceBody
+    /ClassOrInterfaceBodyDeclaration
+    /MethodDeclaration[@Public = true() and @Default = false() and @Static = false()]
+    /MethodDeclarator
+    /FormalParameters
+    /FormalParameter[@Final = true()]
+]]>
+                </value>
+            </property>
+        </properties>
+        <example>
+<![CDATA[
+public interface MyInterface {
+  void process(final Object arg); // Avoid using final here
+}
+]]>
+        </example>
+    </rule>
 
     <rule name="ForLoopCanBeForeach"
           language="java"

pmd-java/src/main/resources/rulesets/java/quickstart.xml

@@ -20,6 +20,7 @@
     <rule ref="category/java/bestpractices.xml/ConstantsInInterface"/>
     <rule ref="category/java/bestpractices.xml/DefaultLabelNotLastInSwitchStmt"/>
     <rule ref="category/java/bestpractices.xml/DoubleBraceInitialization"/>
+    <rule ref="category/java/bestpractices.xml/FinalInterfaceMethodParameterIsUnclear"/>
     <rule ref="category/java/bestpractices.xml/ForLoopCanBeForeach"/>
     <!-- <rule ref="category/java/bestpractices.xml/ForLoopVariableCount" /> -->
     <rule ref="category/java/bestpractices.xml/GuardLogStatement"/>

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/rule/bestpractices/FinalInterfaceMethodParameterIsUnclearTest.java

@@ -0,0 +1,11 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.lang.java.rule.bestpractices;
+
+import net.sourceforge.pmd.testframework.PmdRuleTst;
+
+public class FinalInterfaceMethodParameterIsUnclearTest extends PmdRuleTst {
+    // no additional unit tests
+}

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/bestpractices/xml/FinalInterfaceMethodParameterIsUnclear.xml

@@ -0,0 +1,62 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<test-data
+        xmlns="http://pmd.sourceforge.net/rule-tests"
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:schemaLocation="http://pmd.sourceforge.net/rule-tests http://pmd.sourceforge.net/rule-tests_1_0_0.xsd">
+
+    <test-code>
+        <description>Final is not allowed in interface public method</description>
+        <expected-problems>1</expected-problems>
+        <expected-messages>
+            <message>Using final in public (non default) interface method is unclear</message>
+        </expected-messages>
+        <code><![CDATA[
+public interface Foo {
+    void bar(final int a);
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>Final is allowed in interface static method</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public interface Foo {
+    static void bar(final int a) {}
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>Final is allowed in interface private method</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public interface Foo {
+    private void bar(final int a) {}
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>Final is allowed in interface default method</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public interface Foo {
+    default void bar(final int a) {}
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>Final is allowed in interface inner class method</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public interface Foo {
+  class Inner {
+    public void bar(final int a) {}
+  }
+}
+        ]]></code>
+    </test-code>
+
+</test-data>