Java 存在继承和多态性问题

I also need a overridden calclatefees method to calculate late fees for overdue movies.

 public class Action extends Movie {

    protected double latecost;
    protected double latefees;

    public Action (String title, String rating, int id, int rentTime, 
            double latecost, double latefees ) {

        super(title, rating, id, rentTime);
        this.title = title;
        this.rating= rating; 
        this.id = id; 
        this.rentTime=  8;
        this.latecost =  2;

        System.out.println( "Overridden " + title + rating + " " + id + " " 
                + latecost + " " + latefees);
    }
    public double calclatefees (double latecost) {

        if (rentTime > 3)
            latefees = ((rentTime - 3) * latecost);
        return latefees;}

    @Override
   public String toString () {
    String x = "Movie: " + title + " is rated "  + rating + "
Movie ID           number:" 
            + id + " late fees are $" + latefees;
    return x;
    }
   }    

解决方案

You need to be using the this keyword. I believe that if you do not and you are using the same names in the constructor, it isn't assigning anything. You are telling the title within the parameter to equal whatever you are assigning it, which in the example below is " not overridden ".

for example

public Movie(String title, String racing, int id, int rentTime){
   this.title = " not overridden ";
   // probably should be something like this.title = title after debugging.
   //etc...
}

The this keyword refers to the current object's data members, so this.title would be the Movie's title, not the parameter member title.

You will need to do this with your set methods above also.

public void rating (String rating) {this.rating = rating;}

edit: try this for constructor

public Movie(String title, String rating, int id, int rentTime){
    this.title = " not overridden ";
    this.rating = " NR ";
    this.id = 0;
    this.rentTime = 0;
    System.out.println( " overloaded -" + title + rating + id + rentTime);
 }

after you see the output, if you want to show the actual values you are passing into the other child classes, you have to change it to this:

public Movie(String title, String rating, int id, int rentTime){
    this.title = title;
    this.rating = rating;
    this.id = id;
    this.rentTime = rentTime;
    System.out.println( " overloaded -" + title + rating + id + rentTime);
 }