biking17.java

thinking in java 经典书的习题答案 希望对大家有帮助啊

// polymorphism/biking/Biking17.java
// TIJ4 Chapter Polymorphism, Exercise 17, page 310
/* Using the Cycle hierarchy from Exercise 1, add a balance method() to 
* Unicycle and Bicycle, but not to Tricycle. Create instances of all three
* types and upcast them to an array of Cycle. Try to call balance() on each
* element of the array an observe the results. Downcast and call balance() and
* observe what happens.
*/
package polymorphism.biking;
import static org.greggordon.tools.Print.*;

class Cycle {
	private String name = "Cycle";
	public static void travel(Cycle c) {
		println("Cycle.ride() " + c);
	}
	public String toString() {
		return this.name;
	}
}

class Unicycle extends Cycle {
	private String name = "Unicycle";
	public void balance() { println("Balance Unicycle"); }
	public String toString() {
		return this.name;
	}	
}

class Bicycle extends Cycle {
	private String name = "Bicycle";
	public void balance() { println("Balance Bicycle"); }
	public String toString() {
		return this.name;
	}	

}

class Tricycle extends Cycle {
	private String name = "Tricycle";
	public String toString() {
		return this.name;
	}	
}

public class Biking17 {
	public static void main(String[] args) {
		Cycle[] ride = {
			new Unicycle(),
			new Bicycle(),
			new Tricycle(),
		};
		// Compile time error: cannot find balance() method in Cycle:
		// for(Cycle c : ride) {
		//	c.balance();
		// }
		((Unicycle)ride[0]).balance();
		((Bicycle)ride[1]).balance();
		// Compile time error: no balance() in Tricycle:
		// ((Tricycle)ride[2]).balance();
		// RTTI: ClassCastException: Tricycle cannot be cast to Bicycle:
		// ((Bicycle)ride[2]).balance();	
	}
}