Why ViewPart can't implements an interface from another Project?

Question

Currently, I created a plug-in using available Templates (Plug-in with a view) in eclipse. The plug-in is still OK until I tried implementing an interface FROM ANOTHER PROJECT (It's still ok if this interface was created in Plug-in project). The first thing I thought is a class maybe can't implement an interface from another project but When I tried creating a normal class then implement above interface it's ok. So why ViewPart can't implement above interface? Thank you.

public class SampleView extends ViewPart implements Iface {

    @Override
    public void updateFace(){

    }
    ...
}

Answer 1

You had it right there in the question: "It's still ok if this interface was created in Plug-in project". Everything at runtime has to come from a plug-in you declare as a dependency somehow, including other classes and interfaces you want to refer to. Otherwise the runtime literally doesn't know where to look for those class files. You can make the other project a plug-in itself, or package it as a jar and include it in your existing plug-in by adding it to the Classpath section of the Runtime tab in the Plug-in Manifest Editor.