[java] Support Unnamed Classes and Instance Main Methods for Java 21 Preview

JEP 445

pmd-java/etc/grammar/Java.jjt

@@ -1131,11 +1131,23 @@ ASTCompilationUnit CompilationUnit() :
 {
   [ LOOKAHEAD( ( Annotation() )* "package" ) PackageDeclaration() ( EmptyDeclaration() )* ]
   ( ImportDeclaration() ( EmptyDeclaration() )* )*
+
+  // ModularCompilationUnit:
   // the module decl lookahead needs to be before the type declaration branch,
   // looking for annotations + "open" | "module" will fail faster if it's *not*
   // a module (most common case)
   [ LOOKAHEAD(ModuleDeclLahead()) ModuleDeclaration() ( EmptyDeclaration() )* ]
+
+  // OrdinaryCompilationUnit: -> TopLevelClassOrInterfaceDeclaration
   ( TypeDeclaration() ( EmptyDeclaration() )* )*
+
+  // UnnamedClassCompilationUnit:
+  [
+      ( LOOKAHEAD(3) ClassMemberDeclarationNoMethod() )*
+      ModifierList() MethodDeclaration()
+      ( ClassOrInterfaceBodyDeclaration() )*
+  ]
+
   <EOF>
   {
      jjtThis.setComments(token_source.comments);
@@ -1143,6 +1155,20 @@ ASTCompilationUnit CompilationUnit() :
   }
 }
 
+// see ClassOrInterfaceBodyDeclaration()
+void ClassMemberDeclarationNoMethod() #void:
+{}
+{
+  ModifierList()
+  ( LOOKAHEAD(3) ClassOrInterfaceDeclaration()
+    | LOOKAHEAD({isKeyword("enum")}) EnumDeclaration()
+    | LOOKAHEAD({isKeyword("record")}) RecordDeclaration()
+    | LOOKAHEAD( [ TypeParameters() ] <IDENTIFIER> "(" ) ConstructorDeclaration()
+    | LOOKAHEAD( Type() <IDENTIFIER> (AnnotationList() "[" "]")* ( "," | "=" | ";" ) )  FieldDeclaration()
+    | LOOKAHEAD(2) AnnotationTypeDeclaration()
+  )
+}
+
 private void ModuleDeclLahead() #void:
 {}
 {