java - 在运行 Java 多态性代码示例时遇到问题

Question

我复制了一个自行车示例，并想将其更改为汽车示例，但我无法运行 PolyCar 类的扩展类 Mountainbike。如果我注释掉 Mountainbike，PolyCar 就会运行。谁能告诉我为什么该程序无法识别山地自行车

这是用于运行我的 PolyCar 程序的 TestCar 程序

//import PolyCar.MountainBike;
//import PolyCar.RoadBike;

public class TestCars {
      public static void main(String[] args){
        PolyCar bike01, bike02, bike03;

        bike01 = new PolyCar(20, 10, 1);
        bike02 = new MountainBike(20, 10, 5, "Dual");
//      bike03 = new RoadBike(40, 20, 8, 23);

        bike01.printDescription();
        bike02.printDescription();
//      bike03.printDescription();
      }
    }

这是我的 PolyCar 代码

public class PolyCar {

    // the PolyCar class has three fields
    public int cadence;
    public int gear;
    public int speed;

    // the PolyCar class has one constructor
    public PolyCar(int startCadence, int startSpeed, int startGear) {
        gear = startGear;
        cadence = startCadence;
        speed = startSpeed;
    }

    // the PolyCar class has four methods
    public void setCadence(int newValue) {
        cadence = newValue;
    }

    public void setGear(int newValue) {
        gear = newValue;
    }

    public void applyBrake(int decrement) {
        speed -= decrement;
    }

    public void speedUp(int increment) {
        speed += increment;
    }
public void printDescription(){
    System.out.println("\nBike is " + "in gear " + this.gear
        + " with a cadence of " + this.cadence +
        " and travelling at a speed of " + this.speed + ". ");
}
public class MountainBike extends PolyCar {
    private String suspension;

    public MountainBike(
               int startCadence,
               int startSpeed,
               int startGear,
               String suspensionType){
        super(startCadence,
              startSpeed,
              startGear);
        this.setSuspension(suspensionType);
    }

    public String getSuspension(){
      return this.suspension;
    }

    public void setSuspension(String suspensionType) {
        this.suspension = suspensionType;
    }

    public void printDescription() {
        super.printDescription();
        System.out.println("The " + "MountainBike has a" +
            getSuspension() + " suspension.");
    }
} 
public class RoadBike extends PolyCar{
    // In millimeters (mm)
    private int tireWidth;

    public RoadBike(int startCadence,
                    int startSpeed,
                    int startGear,
                    int newTireWidth){
        super(startCadence,
              startSpeed,
              startGear);
        this.setTireWidth(newTireWidth);
    }

    public int getTireWidth(){
      return this.tireWidth;
    }

    public void setTireWidth(int newTireWidth){
        this.tireWidth = newTireWidth;
    }

    public void printDescription(){
        super.printDescription();
        System.out.println("The RoadBike" + " has " + getTireWidth() +
            " MM tires.");
    }
}

}

Answer 1

您已将 MountainBike 和 RoadBike 封装在 PolyCar 类中；它们应该位于单独的类文件中。您的解决方案非常简单，只需在声明其他两个子类之前移动最后一个花括号来结束 PolyCar 类即可。

目前您有:

public class PolyCar
{
    ...
    public class MountainBike extends PolyCar{
        ...
    }

    public class RoadBike extends PolyCar{
        ...
    }
}

更改为:

class PolyCar {
    ...  
} 

class RoadBike extends PolyCar{
    ....
}

class RoadBike extends PolyCar{
    ...
}

工作教程点示例 here