看《Java Concurrency in Practice》时注意到里面一句话:
Runnable is a fairly limiting abstraction; run cannot return a value or throw checked exceptions
成功引起了我的好奇心,为什么Runnable的run方法不能抛出 checked exception呢?我们先看下Runnable类的代码:
public interface Runnable {
public abstract void run();
}
可见其run方法的定义确定没有抛出任何异常。再做个实验验证下。程序如下,可以通过编译,运行时会报RuntimeException,但是如果把抛出的RuntimeException换成checked exception,比如IOException,那就不能通过编译了:
abstract class AClass {
public static void main(String[] args){
Runnable r = new Runnable() {
public void run() throws RuntimeException{
throw new RuntimeException("rr");
}
};
new Thread(r).start();
}
}
为了寻求一般化的规律,又进一步做了一个实验:
class Sick extends Exception{
}
class FeverSick extends Sick{
}
class JointSick extends Sick{
}
class Children extends People{
public void coldAir() throws FeverSick{
}
}
class Adults extends People{
public void coldAir() throws RuntimeException/*,Exception ,IOException */{
}
}
class Olds extends People{
public void coldAir() throws JointSick{
}
}
public class People {
public void coldAir() throws Sick {
}
}
有结论如下:当子类覆盖父类方法时,若抛出checked exceptions,则Exception必须是父类方法抛出的Exception的子类(或同类)。RuntimeException不受此限制。