How can I extend spinner to implement a new functi

问题:

I'm trying to extend Spinner Android class in my code to implement a new function, but for some reason this didn't work.

This is the extended class:

import android.content.Context;
import android.widget.AdapterView;
import android.widget.Spinner;

public class CustomSpinner extends Spinner {

    public CustomSpinner(Context context) {
        super(context);
    }

    public void setSelectionByItemId(AdapterView<?> parent, long id){
        for (int i = 0; i < parent.getCount(); i++) {              
            long itemIdAtPosition = parent.getItemIdAtPosition(i);
            if (itemIdAtPosition == id) {
                parent.setSelection(i);
                break;
            }
        }
    }
}

And this is the way I'm instantiating this class:

CustomSpinner spinner = (CustomSpinner) findViewById(R.id.sphofentries);

This give me an error at runtime.

All this is if R.id.sphofentries is declared in my layout as an spinner.

But now, if I declare sphofentries as a CustomSpinner I get a runtime error just in the moment I set the Layout to the Activity:

setContentView(R.layout.settings);

Also I am pretty sure that the problem is that I need to declare sphofentries as a CustomSpinner because if I do this:

CustomSpinner spinner = new CustomSpinner(this);
spinner = (CustomSpinner) findViewById(R.id.sphofentries);

This goes without problem trough the first line but gives a runtime error in the second then the problem isn't creating a new CustomSpinner but setting the sphofentries in this CustomSpinner (This with sphofentries declared like a Spinner not a CustomSpinner).

Maybe I am doing something wrong in the layout, this is the way I am declaring sphofentries as a CustomSpinner:

<CustomSpinner 
    android:id="@+id/sphofentries"
    android:layout_below="@+id/tvhofentries"
    android:layout_width="300dip"
    android:layout_height="wrap_content"
    android:gravity="center_horizontal"
/>

回答1:

Finally there was two reasons for this to not work properly, the two previous answers are right:

1. It's necessary to define also the second constructor with the AttributeSet parameter.

   public CustomSpinner(Context context, AttributeSet attrs) {
       super(context, attrs);
   

2. In the layout the Spinner must be defined as a CustomSpinner and needs to be declared with all the fully qualified name:

   <net.domain.package.CustomSpinner  
       android:id="@+id/sphofentries" 
       android:layout_below="@+id/tvhofentries"
       android:layout_width="300dip"       
       android:layout_height="wrap_content"
       android:gravity="center_horizontal"
   />
   

回答2:

It's a common mistake. Just add this constructor, which is the one called by the layout inflater:

public CustomSpinner(Context context, AttributeSet attrs) {
    super(context, attrs);
}

回答3:

Is sphofentries a Spinner or a CustomSpinner?

It would need to be declared as a CustomSpinner in your layout for you to cast it as a CustomSpinner in your code.

The other way around would work. You would be able to cast a CustomSpinner as a Spinner, because it is a subclass.