Questions about the realm mobile database!

Look at realm.io site and study? There's something I don't understand while doing that, so I'm posting questions like this!

-1. I don't understand the second paragraph in [Model] - [Restrictions] as shown in the picture below. It says that the default constructor must be empty, does this mean not to code the default constructor at all? Well, if I remember the licking, I think I learned that if I did extends in Java and created an object without creating a constructor (new EX();), the default constructor (default constructor) is called, and in it is called super(); but... I don't know if it's accurate.

So if this is correct, don't make the default constructor (without parameters) separately, but if you need one, make one with parameters! Is that it?

-2. The top part of the attached picture below is [Relationship] - [Daedaeil] and the bottom part is [Relationship] - [Daedae] part. Here, it says, "If you set the field to null, the reference will be cleaned up, but other objects will not be erased from Realm. Does this mean that the relationship is erased, but what is stored in Realmb is not erased?

This is not a question, but I think it's weird, so I'm telling you

In the [Relationship] - [Connection Query] part, as shown in the picture,

"Think of the model above. If you want to see all the active emails, you can do this." The example below is in different code~~~

The English version document says it is possible to query links or relationships. Consider the model bellow: There was no content about the email example~~ I'm telling you just in case it's edited!

That's it!