从另一个类调用子类方法时出错

Question

I have three classes:

public class BikeSystem {

    static Bicycle[] bicycleArray = new Bicycle[100];
    static int currentBikes = 0;

    public static void addUnicycle() {

        bicycleArray[currentBikes] = new Unicycle();
        currentBikes++; 
    }

    public static void addUniWheel() {
        for (int i=0; i < currentBikes; i++) {
            if (bicycleArray[i] instanceof Unicycle)
                bicycleArray[i].addWheel();
        }
     }
}

public class Bicycle {

    // some variables

}

public class Unicycle extends Bicycle {

    private int wheels;

    public boolean addWheel() {
        wheels++;
        return true;
    }


}

However, I keep getting a "cannot find symbol" error when I try to call the bicycleArray[i].addWheel() method in my BikeSystem class. How do I get around this?

Answer 1

You should explicitly state that you are operating on an instance of Unicycle in order to gain access to the addWheel() method. You can do this by casting.

((Unicycle) bicycleArray[i]).addWheel();

Obviously, if you try to cast to Unicycle on an instance of an object that is not a Unicycle or a further subclass, you will get a ClassCastException

Answer 2

bicycleArray is declared as an array of Bicycle and there is no addWheel method defined by it.

You're trying something similar as:

Bicycle bicycle = new Bicycle();
bicycle = new Unicycle();
bicycle.addWheel(); // err