Q:

Implement interface by inherited Class in Java

I have interface Car:
public interface Car {
    void run();
}

Also i have a base class that implements this interface:
public class Base implements Car {
    @Override
    public void run() {
        //some implementation here
    }
}

Also i have one more class that must implement Car interface
So why i can not do something like that?
public class Der extends Base implements Car  {
   //no implementation of interface here because Base class already implements it
}

Why my base class can not implement interface? What is internal reason of this?

A:

When expending from Base, your Der will of course have the run method because it implements the Car. So 
public class Der extends Base {

}

is sufficient
If you need to override run, you can easily do that. The code would look like
public class Der extends Base {

   @Override
   public void run() {
      //do whatever here
   }
}

If you need to use Der wherever Car will be used, you can of course do that.
Finally if you need Car to also implement some other interface the syntax would be 
public class Der extends Base implements SomeInterface{

}

