Modify rule S2694: Add code example for local classes (#3748)

rules/S2694/java/rule.adoc

@@ -11,10 +11,21 @@ However, while a nested/``++static++`` class would be more efficient, it's worth
 * an inner class can only be instantiated within the context of an instance of the outer class.
 * a nested (``++static++``) class can be instantiated independently of the outer class.
 
+== How to fix it
 
-=== Noncompliant code example
+There are two scenarios in which this rule will raise an issue:
 
-[source,java]
+1. On an _inner class_: make it `static`.
+
+2. On a _local class_: extract it as a `static` _inner class_.
+
+=== Code examples
+
+==== Noncompliant code example
+
+Inner classes that don't use the outer class reference should be static.
+
+[source,java,diff-id=1,diff-type=noncompliant]
 ----
 public class Fruit {
   // ...
@@ -31,10 +42,9 @@ public class Fruit {
 }
 ----
 
+==== Compliant solution
 
-=== Compliant solution
+[source,java,diff-id=1,diff-type=compliant]
-
-[source,java]
 ----
 public class Fruit {
   // ...
@@ -51,6 +61,70 @@ public class Fruit {
 }
 ----
 
+Local classes that don't use the outer class reference should be extracted as a static inner classes.
+
+==== Noncompliant code example
+
+[source,java,diff-id=2,diff-type=noncompliant]
+----
+public class Foo {
+  public Foo() {
+    class Bar { // Noncompliant
+      void doSomething() {
+        // ...
+      }
+    }
+    new Bar().doSomething();
+  }
+
+  public void method() {
+    class Baz { // Noncompliant
+      void doSomething() {
+        // ...
+      }
+    }
+    new Baz().doSomething();
+  }
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=2,diff-type=compliant]
+----
+public class Foo {
+  public Foo() {
+    new Bar().doSomething();
+  }
+
+  public void method()  {
+    new Baz().doSomething();
+  }
+
+  private static class Bar { // Compliant
+    void doSomething() {
+      // ...
+    }
+  }
+
+  private static class Baz { // Compliant
+    void doSomething() {
+      // ...
+    }
+  }
+}
+----
+
+== Resources
+
+=== Documentation
+
+* https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html[Oracle Java SE - Nested Classes]
+* https://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html[Oracle Java SE - Local Classes]
+
+=== Articles & blog posts
+
+* https://www.geeksforgeeks.org/difference-between-static-and-non-static-nested-class-in-java/[GeeksforGeeks - Difference between static and non-static nested class in Java]
 
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