usage of abstract class instead of implementation on interfaces

example.java

// In Java 8 we have
public interface List<E> extends Collection<E> {
  default void sort(Comparator<? super E> c)
  {
    Collections.sort(this, c);
  }
}

public class ArrayList<E> extends AbstractList<E>
          implements List<E>, RandomAccess, Cloneable, java.io.Serializable{}          

// What I would do in general.
// Generally you just add an abstract level, right?

public interface List<E> extends Collection<E> {
  void sort(Comparator<? super E> c);
}

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
  public void sort(Comparator<? super E> c){
    Collections.sort(this, c);
  }
}
          
public class ArrayList<E> extends AbstractList<E>
          implements List<E>, RandomAccess, Cloneable, java.io.Serializable{} 
          
// Basically you have two responsibilites associated to the interface:
// Declaring a contract and implementing a part of it.