public class A{
private static Integer n = 0;
}
public class B extends A implements Runnable {
public void run(){
synchronized(n){
++n;
}
}
}
public class C extends A implements Runnable {
public void run(){
synchronized(n){
--n;
}
}
}
其实这道题目没说要同步，所以不加锁也没问题