>>522
522のコードをこうすればＯＫ

import java.awt.Point;
import java.util.Date;

public abstract class Event {

private Date date;
private Passenger passenger;

public Event(Passenger aPassenger){
assert aPassenger != null;
date = new Date();
passenger = aPassenger;
}

public abstract Point getLocation();
public Date getDate(){
return new Date( date.getTime() ); //←ここだけこう変える
}

public Passenger getPassenger() {
return passenger;
}
}