Question

New to inheritance and generics so bear with me if my question is stupid:

I have the following:

public abstract class car {
    private long id;
    private String name;

    public car (long id, String name) {
        this.id = id;
        this.name = name;
    }

    public final long getID(){
        return id;
    }

    public final String getName(){
        return name;
    }
}

public class sedan extends car {
    public sedan(long id, String name) {
        super (id, name);
    }
    ...
}

public class jeep extends car {
    public jeep(long id, String name) {
        super (id, name);
    }
    ...
}

I’m trying to extend the ArrayAdapter for a spinner as follows:

public class DropdownAdapter extends ArrayAdapter<car> {
    private Context context;
    private List<car> values;

    public DropdownAdapter(Context context, int textViewResourceId, List<car> values) {
        super(context, textViewResourceId, values);
        this.context = context;
        this.values = values;
    }
    ...
}

The problem comes when I try to use it:

adapter = new DropdownAdapter (this, android.R.layout.simple_spinner_item, lstSedan);

will give me the error: The constructor DropdownAdapter (ScreenItemList, int, List< sedan >) is undefined

adapter = new DropdownAdapter (this, android.R.layout.simple_spinner_item, lstJeep);

will give me the error: The constructor DropdownAdapter (ScreenItemList, int, List< jeep >) is undefined

What is the issue here? Why can’t I use lstSedan or lstJeep as parameters when they are derived from car? How do I resolve this? Thanks!

Answer 1

You defined a method that expects a parameter of type List<a>

  public SpinnerAdapter(Context context, int textViewResourceId, List<a> values)

That is, it expects a list of items of type a.
But then you pass a parameter of type b that inherited a, instead of a parameter of type List<b>
Try to use this way:

  List<b> lstB= new ArrayList<b>();
  adapter = new SpinnerAdapter (this, android.R.layout.simple_spinner_item, lstB);

As JoxTraex said, you should read java name convention

Update:
Sorry, I made a mistake. Because java doesn’t allow passing List<ChildClass> parameter to List<ParentClass>, so you shouldn’t use inheritance in this case. Try using generic

public class DropdownAdapter extends ArrayAdapter<T> {
private Context context;
private List<T> values;

public DropdownAdapter(Context context, int textViewResourceId, List<T> values) {
    super(context, textViewResourceId, values);
    this.context = context;
    this.values = values;
}
...
}

And this function call should work:

 adapter = new SpinnerAdapter (this, android.R.layout.simple_spinner_item, lstB);