[java] New Rule: Use Explicit Types

Fixes #2847

docs/pages/release_notes.md

@@ -182,6 +182,7 @@ Contributors: [Wener](https://github.com/wener-tiobe) (@wener-tiobe)
 * {% rule apex/design/UnusedMethod %} finds unused methods in your code.
 
 **Java**
+* {% rule java/bestpractices/UseExplicitTypes %} reports usages of `var` keyword, which was introduced with Java 10.
 * {% rule java/codestyle/UnnecessaryBoxing %} reports boxing and unboxing conversions that may be made implicit.
 
 **Kotlin**
@@ -453,6 +454,7 @@ Language specific fixes:
     * [#2806](https://github.com/pmd/pmd/issues/2806): \[java] SwitchStmtsShouldHaveDefault false-positive with Java 14 switch non-fallthrough branches
     * [#2822](https://github.com/pmd/pmd/issues/2822): \[java] LooseCoupling rule: Extend to cover user defined implementations and interfaces
     * [#2843](https://github.com/pmd/pmd/pull/2843):   \[java] Fix UnusedAssignment FP with field accesses
+    * [#2847](https://github.com/pmd/pmd/issues/2847): \[java] New Rule: Use Explicit Types
     * [#2882](https://github.com/pmd/pmd/issues/2882): \[java] UseTryWithResources - false negative for explicit close
     * [#2883](https://github.com/pmd/pmd/issues/2883): \[java] JUnitAssertionsShouldIncludeMessage false positive with method call
     * [#2890](https://github.com/pmd/pmd/issues/2890): \[java] UnusedPrivateMethod false positive with generics

docs/pages/release_notes_pmd7.md

@@ -537,6 +537,7 @@ Related issue: [[core] Explicitly name all language versions (#4120)](https://gi
 * {% rule apex/design/UnusedMethod %} finds unused methods in your code.
 
 **Java**
+* {% rule java/bestpractices/UseExplicitTypes %} reports usages of `var` keyword, which was introduced with Java 10.
 * {% rule java/codestyle/UnnecessaryBoxing %} reports boxing and unboxing conversions that may be made implicit.
 
 **Kotlin**

pmd-core/src/main/resources/rulesets/releases/700.xml

@@ -10,6 +10,7 @@ This ruleset contains links to rules that are new in PMD v7.0.0
 
     <rule ref="category/apex/design.xml/UnusedMethod"/>
 
+    <rule ref="category/java/bestpractices.xml/UseExplicitTypes"/>
     <rule ref="category/java/codestyle.xml/UnnecessaryBoxing"/>
 
     <rule ref="category/kotlin/bestpractices.xml/FunctionNameTooShort"/>

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

@@ -1688,6 +1688,34 @@ public class Foo {
         </example>
     </rule>
 
+    <rule name="UseExplicitTypes"
+          language="java"
+          minimumLanguageVersion="10"
+          since="7.0.0"
+          message="Use Explicit Types"
+          class="net.sourceforge.pmd.lang.rule.XPathRule"
+          externalInfoUrl="${pmd.website.baseurl}/pmd_rules_java_bestpractices.html#useexplicittypes">
+        <description>
+Java 10 introduced the `var` keyword. This reduces the amount of typing but decreases the reading comprehension of the
+code.
+        </description>
+        <priority>3</priority>
+        <properties>
+            <property name="allowLiterals" type="Boolean" value="false" description="Allow when variables are directly initialized with literals"/>
+            <property name="allowCtors" type="Boolean" value="false" description="Allow when variables are directly initialized with a constructor call"/>
+            <property name="version" value="3.1"/>
+            <property name="xpath">
+                <value>
+                    <![CDATA[
+//LocalVariableDeclaration[@TypeInferred = true()]
+    [not(VariableDeclarator[*[pmd-java:nodeIs("Literal")]]) or $allowLiterals = false()]
+    [not(VariableDeclarator[ConstructorCall]) or $allowCtors = false()]
+]]>
+                </value>
+            </property>
+        </properties>
+    </rule>
+
     <rule name="UseStandardCharsets"
           language="java"
           since="6.34.0"

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/rule/bestpractices/UseExplicitTypesTest.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;
+
+class UseExplicitTypesTest extends PmdRuleTst {
+    // no additional unit tests
+}

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

@@ -0,0 +1,48 @@
+<?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">
+
+    <code-fragment id="basic-sample"><![CDATA[
+import java.util.function.Function;
+
+public class Foo {
+    public void bar() {
+        var s = "a"; // line 5 - violation - literal
+        var i = 1;   // line 6 - violation - literal
+        String s2 = "a"; // no violation - explicit type
+        var f = getFoo(); // line 8 - violation
+        var f = new Foo(); // line 9 - violation - ctor call
+        Function<Integer, Integer> quadrat = (var x) -> x*x;
+    }
+
+    private String getFoo() {
+        return "a";
+    }
+}
+]]></code-fragment>
+
+    <test-code>
+        <description>No vars anywhere</description>
+        <expected-problems>4</expected-problems>
+        <expected-linenumbers>5,6,8,9</expected-linenumbers>
+        <code-ref id="basic-sample"/>
+    </test-code>
+
+    <test-code>
+        <description>Allow literals</description>
+        <rule-property name="allowLiterals">true</rule-property>
+        <expected-problems>2</expected-problems>
+        <expected-linenumbers>8,9</expected-linenumbers>
+        <code-ref id="basic-sample"/>
+    </test-code>
+
+    <test-code>
+        <description>Allow constructor calls</description>
+        <rule-property name="allowCtors">true</rule-property>
+        <expected-problems>3</expected-problems>
+        <expected-linenumbers>5,6,8</expected-linenumbers>
+        <code-ref id="basic-sample"/>
+    </test-code>
+</test-data>