$ javap -c -v ConcatShapes | grep -A3 BootstrapMethods
BootstrapMethods:
  0: #39 REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
    Method arguments:
      #37 user=\u0001 n=\u0001
InnerClasses: