ClassA.java

package certificacao;

public abstract class ClassA {

	public void run(){
		System.out.println("running...");
	}
}

ClassB.java

package certificacao;

public class ClassB extends ClassA implements Runnable {

}

Main.java

package certificacao;

public class Main {

	public static void main(String[] args) {
		Runnable classB = new ClassB();
		
		new Thread(classB).start();
	}
}