code: @SuppressWarnings("unchecked")
    public <ContainingType extends MessageLite>
        GeneratedMessageLite.GeneratedExtension<ContainingType, ?>
          findLiteExtensionByNumber(
            final ContainingType containingTypeDefaultInstance,
            final int fieldNumber) {
      return (GeneratedMessageLite.GeneratedExtension<ContainingType, ?>)
        extensionsByNumber.get(
          new ObjectIntPair(containingTypeDefaultInstance, fieldNumber));
    }
docs: Find an extension by containing type and field number.
  
  @return Information about the extension if found, or {@code null}
  otherwise.