向上转型编译出问题，继承应该没问题

package polymorphism;

import polymorphism.cycle.*;

public class E01_Upcasting {
    public static void ride(Cycle c) {}

    public static void main(String[] args) {
        
        ride(new Cycle()); // No upcasting
        ride(new Unicycle()); // Upcast
        ride(new Bicycle()); // Upcast
        ride(new Tricycle()); // Upcast
            
    }
}



//: polymorphism/cycle/Cycle.java
package polymorphism.cycle;
public class Cycle {
} ///:~
//: polymorphism/cycle/Unicycle.java
package polymorphism.cycle;
public class Unicycle extends Cycle {
} ///:~
//: polymorphism/cycle/Bicycle.java
package polymorphism.cycle;
public class Bicycle extends Cycle {
} ///:~
//: polymorphism/cycle/Tricycle.java
package polymorphism.cycle;
public class Tricycle extends Cycle {
}