[Pigeon] Make the generated Java Builder class final (#1247)

* make the builder class final

* add test

* upgrade version

packages/pigeon/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.0.3
+
+* Makes the generated Java Builder class final.
+
 ## 2.0.2
 
 * Fixes Java crash for nullable nested type.

packages/pigeon/lib/generator_tools.dart

@@ -8,7 +8,7 @@ import 'dart:mirrors';
 import 'ast.dart';
 
 /// The current version of pigeon. This must match the version in pubspec.yaml.
-const String pigeonVersion = '2.0.2';
+const String pigeonVersion = '2.0.3';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {

packages/pigeon/lib/java_generator.dart

@@ -567,7 +567,7 @@ void generateJava(JavaOptions options, Root root, StringSink sink) {
     }
 
     void writeBuilder() {
-      indent.write('public static class Builder ');
+      indent.write('public static final class Builder ');
       indent.scoped('{', '}', () {
         for (final NamedType field in klass.fields) {
           final HostDatatype hostDatatype = getHostDatatype(field, root.classes,

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: 2.0.2 # This must match the version in lib/generator_tools.dart
+version: 2.0.3 # 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

@@ -32,6 +32,7 @@ void main() {
     final String code = sink.toString();
     expect(code, contains('public class Messages'));
     expect(code, contains('public static class Foobar'));
+    expect(code, contains('public static final class Builder'));
     expect(code, contains('private @Nullable Long field1;'));
   });