Add @Override annotation to generated Java code where appropriate.

Patch from Brian Atkinson <nairb774@gmail.com>.

CONTRIBUTORS.txt

@@ -47,3 +47,5 @@ Non-Google patch contributors:
   Dilip Joseph <dilip.antony.joseph@gmail.com>
     * Heuristic detection of sub-messages when printing unknown fields in
       text format.
+  Brian Atkinson <nairb774@gmail.com>
+    * Added @Override annotation to generated Java code where appropriate.

src/google/protobuf/compiler/java/java_message.cc

@@ -241,6 +241,7 @@ void MessageGenerator::Generate(io::Printer* printer) {
     "  return $fileclass$.internal_$identifier$_descriptor;\n"
     "}\n"
     "\n"
+    "@Override\n"
     "protected com.google.protobuf.GeneratedMessage.FieldAccessorTable\n"
     "    internalGetFieldAccessorTable() {\n"
     "  return $fileclass$.internal_$identifier$_fieldAccessorTable;\n"
@@ -293,6 +294,7 @@ GenerateMessageSerializationMethods(io::Printer* printer) {
        ExtensionRangeOrdering());
 
   printer->Print(
+    "@Override\n"
     "public void writeTo(com.google.protobuf.CodedOutputStream output)\n"
     "                    throws java.io.IOException {\n");
   printer->Indent();
@@ -331,6 +333,7 @@ GenerateMessageSerializationMethods(io::Printer* printer) {
     "}\n"
     "\n"
     "private int memoizedSerializedSize = -1;\n"
+    "@Override\n"
     "public int getSerializedSize() {\n"
     "  int size = memoizedSerializedSize;\n"
     "  if (size != -1) return size;\n"
@@ -485,19 +488,23 @@ void MessageGenerator::GenerateCommonBuilderMethods(io::Printer* printer) {
     "\n"
     "$classname$ result = new $classname$();\n"
     "\n"
+    "@Override\n"
     "protected $classname$ internalGetResult() {\n"
     "  return result;\n"
     "}\n"
     "\n"
+    "@Override\n"
     "public Builder clear() {\n"
     "  result = new $classname$();\n"
     "  return this;\n"
     "}\n"
     "\n"
+    "@Override\n"
     "public Builder clone() {\n"
     "  return new Builder().mergeFrom(result);\n"
     "}\n"
     "\n"
+    "@Override\n"
     "public com.google.protobuf.Descriptors.Descriptor\n"
     "    getDescriptorForType() {\n"
     "  return $classname$.getDescriptor();\n"
@@ -550,6 +557,7 @@ void MessageGenerator::GenerateCommonBuilderMethods(io::Printer* printer) {
 
   if (descriptor_->file()->options().optimize_for() == FileOptions::SPEED) {
     printer->Print(
+      "@Override\n"
       "public Builder mergeFrom(com.google.protobuf.Message other) {\n"
       "  if (other instanceof $classname$) {\n"
       "    return mergeFrom(($classname$)other);\n"
@@ -586,6 +594,7 @@ void MessageGenerator::GenerateBuilderParsingMethods(io::Printer* printer) {
     SortFieldsByNumber(descriptor_));
 
   printer->Print(
+    "@Override\n"
     "public Builder mergeFrom(\n"
     "    com.google.protobuf.CodedInputStream input)\n"
     "    throws java.io.IOException {\n"
@@ -593,6 +602,7 @@ void MessageGenerator::GenerateBuilderParsingMethods(io::Printer* printer) {
     "    com.google.protobuf.ExtensionRegistry.getEmptyRegistry());\n"
     "}\n"
     "\n"
+    "@Override\n"
     "public Builder mergeFrom(\n"
     "    com.google.protobuf.CodedInputStream input,\n"
     "    com.google.protobuf.ExtensionRegistry extensionRegistry)\n"
@@ -656,6 +666,7 @@ void MessageGenerator::GenerateBuilderParsingMethods(io::Printer* printer) {
 
 void MessageGenerator::GenerateIsInitialized(io::Printer* printer) {
   printer->Print(
+    "@Override\n"
     "public final boolean isInitialized() {\n");
   printer->Indent();