Simple inheritance example in Kotlin

Question

In Java I have these two classes :

public class Base {
    protected Long id;
    // getter setter
}
public User extends Base {
    private String name;
    private Integer age;
    public User(Long id) { this.id = id; }
    // getter setter
}

I created these two classes in Kotlin :

open class Base(var id: Long? = null)
class User(
        var id: Long? = null,
        var name: String? = null,
        var age: Int? = null
) : Base()

Now in Java I want to call the User() constructor with only the 'id' parameter :

new User(5);

This seems wrong to me, because by doing this I re-declared the "id" field in the User class.

How can I set the id field of the base class in Kotlin (like I did in Java with "this.id = id;" ?

Answer 1

First, your Kotlin code would not compile, because you would get the following error: Error:'id' hides member of supertype 'Base' and needs 'override' modifier.

Basically, the compiler would also complain that you redeclared id from Base, in User.

If you want to redeclare it, you need to first make id open in Base, and then in User, you need to use the override keyword, when redeclarding id. (also see this)

Furthermore, for the Kotlin compiler to generate overloads of the User constructor, you need to annotate your primary constructor with @JvmOverloads. Without @JvmOverloads, you wouldn't be able to use the User(id) constructor (just id as param) in Java, and would have to specify all 3 parameters.

So your code would become:

open class Base(open var id: Long? = null)

class User @JvmOverloads constructor(override var id: Long? = null,
                                     var name: String? = null,
                                     var age: Int? = null
) : Base(id)

Now, if you don't want to redeclare id in User, you can simply not use var in front of it, and instead pass it directly to Base, when calling its constructor. So basically this:

open class Base(var id: Long? = null)

class User @JvmOverloads constructor(id: Long? = null,
                                     var name: String? = null,
                                     var age: Int? = null
) : Base(id)