// note: this version is in a single file; an actual implementation
//       would have these in separate files

public class Person {
    private String name;
    public Person(String name) {
        this.name = name;
    }
    public getName() { return name; }
}

interface HasCrew {
    Person firstInCommand();
    Person secondInCommand();
}
    
public class Engine extends Car implements HasCrew {
    private Person engineer, conductor;
    //...
    Person firstInCommand() { return engineer; }
    Person secondInCommand() { return conductor; }
}

public class Ship implements HasCrew {
    private Person captain, firstMate;
    private Person[] sailors;
    //...
    Person firstInCommand() { return captain; }
    Person secondInCommand() { return firstMate; }
}

public class PassengerPlane implements HasCrew {
    private Person pilot, copilot, navigator;
    //...
    Person firstInCommand() { return pilot; }
    Person secondInCommand() { return copilot; }
}