Why can't I define interface for overlay type lightweight collections?

I want to define interface for lightweight collections as blow,but it
can't be compiled successfully.Any mistake here?
//...........................Interface...........................................
@SingleJsoImpl(JsArrayExtImpl.class)
public interface JsArrayExt<M>{


public void add(int addBefore, M model) ;

public void add(M model) ;

public void addAll(JsArrayExt<M> models);
}

//................................Implementation
public class JsArrayExtImpl<M extends JavaScriptObject> extends
JsArray<M> implements JsArrayExt<M>
{
protected JsArrayExtImpl()
{
}

@Override
public final native void add(int addBefore, M model) /*-{
this.splice(addBefore, 0, model);
}-*/;

@Override
public final void add(M model)
{
...
}
}

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