Make generated Java enum fields final (#2899)

* Make generated Java enum fields final

The enum should be immutable, so the fields should be final.

* Fix tests

* Update CHANGELOG.md

* Update generator_tools.dart

* Update pubspec.yaml

packages/pigeon/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 4.2.10
+
+* Changes generated Java enum field to be final.
+
 ## 4.2.9
 
 * [kotlin] Fixes a bug with some methods that return `void`.

packages/pigeon/lib/generator_tools.dart

@@ -9,7 +9,7 @@ import 'dart:mirrors';
 import 'ast.dart';
 
 /// The current version of pigeon. This must match the version in pubspec.yaml.
-const String pigeonVersion = '4.2.9';
+const String pigeonVersion = '4.2.10';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {

packages/pigeon/lib/java_generator.dart

@@ -578,7 +578,7 @@ void generateJava(JavaOptions options, Root root, StringSink sink) {
       // discouraged. The toMap and fromMap API matches class API to allow
       // the same code to work with enums and classes, but this
       // can also be done directly in the host and flutter APIs.
-      indent.writeln('private int index;');
+      indent.writeln('private final int index;');
       indent.write('private ${anEnum.name}(final int index) ');
       indent.scoped('{', '}', () {
         indent.writeln('this.index = index;');

packages/pigeon/pubspec.yaml

@@ -2,7 +2,7 @@ name: pigeon
 description: Code generator tool to make communication between Flutter and the host platform type-safe and easier.
 repository: https://github.com/flutter/packages/tree/main/packages/pigeon
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3Apigeon
-version: 4.2.9 # This must match the version in lib/generator_tools.dart
+version: 4.2.10 # This must match the version in lib/generator_tools.dart
 
 environment:
   sdk: ">=2.12.0 <3.0.0"

packages/pigeon/test/java_generator_test.dart

@@ -61,7 +61,7 @@ void main() {
     expect(code, contains('    ONE(0),'));
     expect(code, contains('    TWO_THREE_FOUR(1),'));
     expect(code, contains('    REMOTE_DB(2);'));
-    expect(code, contains('private int index;'));
+    expect(code, contains('private final int index;'));
     expect(code, contains('private Foobar(final int index) {'));
     expect(code, contains('      this.index = index;'));
   });
@@ -587,7 +587,7 @@ void main() {
     expect(code, contains('    ONE(0),'));
     expect(code, contains('    TWO_THREE_FOUR(1),'));
     expect(code, contains('    REMOTE_DB(2);'));
-    expect(code, contains('private int index;'));
+    expect(code, contains('private final int index;'));
     expect(code, contains('private Enum1(final int index) {'));
     expect(code, contains('      this.index = index;'));