Empty constructor for kotlin object to use Firebas

问题:

I'm trying to save an object inside Firebase database from Kotlin, I dont feel right providing a default empty constructor and put values as nullable, having to change all my code bc of this.

My class:

class Video(var id: String, var url: String, var owner: User) :  {
    constructor() : this("", "", User("", "", ""))
}

Firebase push:

FirebaseDatabase.getInstance().reference.child("public").push().setValue(video)

Error:

is missing a constructor with no arguments

Is there a better solution for this?

回答1:

Use default arguments:

class Video(var id: String = "", var url: String = "",
    var owner: User = User("", "", ""))

(also, consider using val).

回答2:

I have not tested if it works with Firebase, but according to the kotlin documentation,

NOTE: On the JVM, if all of the parameters of the primary constructor have default values, the compiler will generate an additional parameterless constructor which will use the default values. This makes it easier to use Kotlin with libraries such as Jackson or JPA that create class instances through parameterless constructors.

So I suppose, by applying default values for all the constructor parameters will generate the parameterless constructor for you.

回答3:

You need to add an empty constructor to the Video class. This is required to be able to use classes with Firebase.