Java 存在继承和多态性问题

Question

我还需要一个覆盖的 calclatefees 方法来计算过期电影的滞纳金。

 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 + "\nMovie ID           number:" 
            + id + " late fees are $" + latefees;
    return x;
    }
   }    

Answer 1

目前打印：

overloaded - not overridden  NR 00
overridden 

这是完美的。它表明构造函数被特殊对待。他们都先调用父构造函数。

因此，您会看到 Movie 的构造函数打印重载 - 未覆盖 NR 00，然后 Action 的构造函数启动并打印 overridden。

Answer 2

您需要使用 this 关键字。我相信，如果您不这样做并且您在构造函数中使用相同的名称，那么它不会分配任何东西。您告诉参数中的标题等于您分配的任何内容，在下面的示例中是“未覆盖”。

例如

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...
}

this 关键字引用当前对象的数据成员，因此 this.title 将是电影的标题，而不是参数成员标题。

您还需要使用上面设置的方法来执行此操作。

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

编辑：试试这个构造函数

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);
 }

看到输出后，如果要显示传递给其他子类的实际值，则必须将其更改为：

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);
 }