Is it possible to extends a class that has a non empty constructor with a companion object

user_5752410 0
Viewed 164

I have a code in Java that i want to change to Kotlin syntax. The jave code is:

  public class CountryDataItem (String countryNane,String countryUrl)
{
    public static RecyclerView.ViewHolder onCreateViewHolder (ViewGroup parent)
    {
        new ViewHolder (parent);
    }

    public static class ViewHolder extends RecyclerView.ViewHolder
    {
        private TextView countryTextView;
        private ImageView countryImageView;
     
        public ViewHolder(@NonNull View view)
        {
            super(view);
            view.findViewById...
            ...
        }
    } 
}

Code is related to RecyclerView. I want to be able to create as many as ViewHolder's as i want from the static nested class type. I wrote the following code, but it feels me like a bad code, unreadable (I prefer not to write anonymous class but didn't know how to write the "static" ViewHolder Class and also always return the same field.

Code I have wrote:

   class CountryDataItem (val countryName :String, var countryFlagUrl )
{
    companion object
    {

         fun onCreateViewHolder(parent: ViewGroup): RecyclerView.ViewHolder {
             return object : RecyclerView.ViewHolder(parent) {
                 val countryNameTextView: TextView = parent.findViewById(R.id.country_name_tv)
                 val countryFlagUrl: ImageView = parent.findViewById(R.id.country_iv)
             }
         }
    }

I prefer to write a code with a companion object that extends RecyclerView.ViewHolder class buy since writing:

object ViewHolder: RecyclewView.ViewHolder enforce me the provide () and argument of type View to RecyclewView.ViewHolder

i can't do it

kotlin-companion kotlin static android java
2 Answers

Nested classes are static by default in Kotlin. You have to mark them inner to make them not static. So your Java class could be like this in Kotlin:

class CountryDataItem (val countryName: String, var countryFlagUrl: String) {

    companion object {
        fun onCreateViewHolder(parent: ViewGroup) = ViewHolder(parent)
    }

    class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val countryTextView: TextView = view.findViewById...
        val countryImageView: ImageView = view.findViewById...
    } 
}
user_5067960

You should have the companion object within the ViewHolder class. Like so:

class CountryDataItem(countryName: String, countryUrl) {

    class ViewHolder private constructor(view: View): RecyclerView.ViewHolder(view) {
        private val textView: TextView = view.findViewById(R.id.textView)
        private val imageView: ImageView = view.findViewById(R.id.imageView)

        fun bind(model: Model) {
            textView.text = ...
        }

        companion object {
            fun from(parent: ViewGroup): ViewHolder {
                val layoutInflater = LayoutInflater.from(parent.context)
                val view = layoutInflater.inflate(R.layout.list_item, parent, false)
                
                return ViewHolder(view)
            }
        }
    }
}

Now to create the ViewHolder you can just call the from method. Like so:

CountryDataItem.ViewHolder.from(parent)
user_123131570
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