Enable FinalClass in CheckStyle

Author: eregon

src/main/.checkstyle_checks.xml

@@ -62,7 +62,7 @@
       <property name="ignoreConstructorParameter" value="true"/>
       <metadata name="net.sf.eclipsecs.core.lastEnabledSeverity" value="inherit"/>
     </module>
-    <!--<module name="FinalClass"/>-->
+    <module name="FinalClass"/>
     <module name="HideUtilityClassConstructor">
       <property name="severity" value="ignore"/>
       <metadata name="net.sf.eclipsecs.core.lastEnabledSeverity" value="inherit"/>

src/main/java/org/truffleruby/core/array/ArrayBuilderNode.java

@@ -34,7 +34,7 @@
  * by another usage (e.g. recursive) of this ArrayBuilderNode. */
 public abstract class ArrayBuilderNode extends RubyBaseNode {
 
-    public static class BuilderState {
+    public static final class BuilderState {
         protected int capacity;
         protected int nextIndex = 0;
         protected Object store;

src/main/java/org/truffleruby/core/array/ArrayNodes.java

@@ -2367,7 +2367,7 @@ protected Object markNativeStore(RubyArray array) {
     @Primitive(name = "array_flatten_helper", lowerFixnum = 2)
     public abstract static class FlattenHelperNode extends PrimitiveArrayArgumentsNode {
 
-        static class Entry {
+        static final class Entry {
             final RubyArray array;
             final int index;
 

src/main/java/org/truffleruby/core/array/library/ZeroLengthArrayStore.java

@@ -27,7 +27,7 @@
 
 @ExportLibrary(ArrayStoreLibrary.class)
 @GenerateUncached
-public class ZeroLengthArrayStore {
+public final class ZeroLengthArrayStore {
 
     private ZeroLengthArrayStore() {
     }

src/main/java/org/truffleruby/core/basicobject/BasicObjectNodes.java

@@ -523,7 +523,7 @@ protected Object methodMissing(Object self, Object name, Object[] args, Object b
             throw new RaiseException(getContext(), buildMethodMissingException(self, name, args, block));
         }
 
-        private static class FrameAndCallNode {
+        private static final class FrameAndCallNode {
             final Frame frame;
             final Node callNode;
 

src/main/java/org/truffleruby/core/fiber/FiberManager.java

@@ -467,7 +467,7 @@ public DescriptorAndArgs getDescriptorAndArgs() {
         }
     }
 
-    private static class FiberSafepointMessage implements FiberMessage {
+    private static final class FiberSafepointMessage implements FiberMessage {
         private final RubyFiber sendingFiber;
         private final SafepointAction action;
 

src/main/java/org/truffleruby/core/format/MissingValue.java

@@ -9,7 +9,7 @@
  */
 package org.truffleruby.core.format;
 
-public class MissingValue {
+public final class MissingValue {
 
     public static final MissingValue INSTANCE = new MissingValue();
 

src/main/java/org/truffleruby/core/hash/HashNodes.java

@@ -447,7 +447,7 @@ public abstract static class MapNode extends PrimitiveArrayArgumentsNode impleme
         @Child HashStoreLibrary.YieldPairNode yieldPair = HashStoreLibrary.YieldPairNode.create();
         @Child ArrayBuilderNode arrayBuilder = ArrayBuilderNode.create();
 
-        private static class MapState {
+        private static final class MapState {
             final BuilderState builderState;
             final RubyProc block;
 

src/main/java/org/truffleruby/core/hash/library/EmptyHashStore.java

@@ -29,7 +29,7 @@
 
 @ExportLibrary(value = HashStoreLibrary.class)
 @GenerateUncached
-public class EmptyHashStore {
+public final class EmptyHashStore {
 
     public static final EmptyHashStore NULL_HASH_STORE = new EmptyHashStore();
 

src/main/java/org/truffleruby/core/thread/ThreadManager.java

@@ -548,7 +548,7 @@ public BlockingCallInterruptible(
         }
 
         @ValueType
-        private static class State {
+        private static final class State {
             final RubyThread thread;
             final Object executable;
             final Object[] args;