But is there a way to just use the default value specified in the constructor of the Data class when the values are null? ... Kotlin does not infer return types for functions with block bodies because such functions may have complex control flow in the body, and the return type will be non-obvious to the reader (and sometimes even for the compiler). A default value is defined using the = after the type. A constructor is a special kind of function and it is called as soon as the Class object is created. What the challenge of using Kotlin and JSON boils down to is: We want to use Kotlin data classes for concise code, non-nullable types for null-safety and default arguments for the data class constructor to work when a field is missing in a given JSON. Kotlin functions and constructors can define default arguments, allowing calls to them to skip any argument that has a default value. There are two types of constructors in Kotlin. Default Values in the Constructor. sayHello(last = "Doe") //>Hello Mr Doe Using default parameters in constructors. We can also provide default values for the properties of the class right inside the constructor - ... Kotlin internally calls the default (implicit) getters/setters of the properties. Here, we have one class called User with two default parameters for the constructor : name and age.By default, name is an empty string and age is equal to 0. You can use one another constructor along with the primary constructor in Kotlin. Fortunately, Kotlin unlike Java, provide built-in support for this concept! For this purpose, it’ll instantiate an object with default values. I could not find anything in the documentation, but I was hoping something like this would work: val data = Data(newName()?, newNumber()?) class Dog(var breed:String="labrador") : Animal() { } Now if we don’t pass any value for breed it will get “labrador” by default. This way, Kotlin can work well with such libraries as Jackson or JPA, which use the no-args constructor to create class instances: class Person(val name: String = "") A constructor is the most sophisticated way to initialize the properties of a class. Using default parameters in a constructor is no different. This way Kotlin knows that the value should be assigned to the second parameter last and not to the first parameter as it would have been if we didn't provide the parameter name. //all the 3 values are passed val person1 = Person("Pubg", 25, 100000) //update name and take the default value of age and salary val person2 = Person("John") //take all default value val person3 = Person() Secondary Constructor. So the line person.lastName = "Jobs" in Kotlin is … Yeah, that's a cool feature of Kotlin. As JPA requires, @Entity classes should have a default (non-arg) constructor to instantiate the objects when retrieving them from the database. But that does not compile. 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