Why defining Interface for singleton with getInstance() doesnt work?

Question

Lets say I want to do the following:
package adapters
    public interface IAdapter
        function init():void         
        function getInstance():IAdapter
package adapters
    public class DAAdapter extends EventDispatcher implements IAdapter
        private static var _instance:DAAdapter;
        public static function getInstance():IAdapter {
            if(!_instance)
                _instance = new DAAdapter();
            return _instance;
        function DAAdapter(target:IEventDispatcher=null)
            super(target);
     public function init() {}
I getting:
1044: Interface method getInstance in namespace adapters:IAdapter not implemented by class adapters:DAAdapter.
Why?

Answer 1

In AS3 you cannot use interface definitions for static methods.
http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/statements.html#static