It seems to be bad to allow this to be fully built. What did you see instead? There is a NPE when toString(), getSerializedSize(), or toByteArray() are called. Exception in thread "main" ...

JavaのMapは、キーと値のペアを格納するデータ構造です。 キーと値は1対1の関係であり、キーを使用して値にアクセスすることができます。

The need to find the length of a Java String is a common programming requirement. Learn how to get the size of a Java String and avoid lengthy errors developers often encounter.