学步园

两点需要注意：

1 子类的构造函数可以扔出比超类更广泛的异常，但必须包含超类的构造函数扔出的异常

2 子类的方法扔出的异常一定要比超类的“更窄”，这样向上转型时才不会出错

下面是一个例子：

//球类异常基类
class BaseballException extends Exception {}
//犯规
class Foul extends BaseballException {}
//打架
class Strike extends BaseballException {}

//一局球
abstract class Inning {
	public Inning() throws BaseballException {}
	public void event() throws BaseballException {
		//Doesn't actually have to throw anything
	}
	public void atBat() throws Strike, Foul {
		System.out.println("atBat() in superclass Inning");
	}
	public void walk() {} //Throws no checked exceptions
}

class StormException extends Exception {}
class RainedOut extends StormException {}
class PopFoul extends Foul {}

interface Storm {
	public void event() throws RainedOut;
	public void rainHard() throws RainedOut;
}

public class StormyInning extends Inning implements Storm {
	//构造函数可以扔出新的异常,但必须仍出父类的异常
	public StormyInning() throws RainedOut, BaseballException {}
	public StormyInning(String s) throws Foul, BaseballException {}
	
	public void walk() {}
	
	public void rainHard() throws RainedOut {}
	
	//下面这样写编译器会报错
	//public void event() throws RainedOut {}
	//这样写也是错的
	//public void event() throws BaseballException {}
	//子类可以不扔出异常(比超类更窄),这样向上转型时就不会出错
	public void event() {}
	//重载的方法可以扔出比超类少的异常或扔出超类异常的子类
	public void atBat() throws PopFoul {
		System.out.println("atBat() in subclass StormyInning");
	}
	
	
	public static void main(String[] args) {
		try {
			StormyInning si = new StormyInning();
			si.atBat();
		} catch(PopFoul e) {
			System.out.println("Pop foul");
		} catch (RainedOut e) {
			System.out.println("Rained out");
		} catch (BaseballException e) {
			System.out.println("Generic baseball exception");
		}
		
		try {
			Inning i = new StormyInning();
			i.atBat();
		} catch(Strike e) {
			System.out.println("Strike");
		} catch(Foul e) {
			System.out.println("Foul");
		} catch (RainedOut e) {
			System.out.println("Rained out");
		} catch (BaseballException e) {
			System.out.println("Generic baseball exception");
		}
	}
}
/*
此程序的输出为：
atBat() in subclass StormyInning
atBat() in subclass StormyInning

即使子类向上转型为超类，然后调用超类的某个方法，其内部也会调用这个子类的方法，这是多态的体现
*/