Working with Abstract Classes and Interfaces

public abstract class Vehicle {
    protected String id;
    public Vehicle(String id) { this.id = id; }
    public abstract double fuelCost();   // no body
    public String summary() { return id + ": $" + fuelCost(); }
}

public interface Repository<T, ID> {
    T findById(ID id);
    List<T> findAll();
    void save(T entity);
    default boolean exists(ID id) { return findById(id) != null; }
    static Repository<?, ?> empty() { return new EmptyRepo(); }
}

interface A { default String hi() { return "A"; } }
interface B { default String hi() { return "B"; } }
class C implements A, B {
    @Override public String hi() { return A.super.hi() + B.super.hi(); }
}

@FunctionalInterface
interface Validator<T> {
    boolean isValid(T value);
}
Validator<String> nonEmpty = s -> s != null && !s.isEmpty();