指出下面程序的运行结果

class A{
static{
System.out.println("1");
}
public A(){
System.out.println("2");
}
}

class B extends A{
static{
System.out.println("a");
}
public B(){
System.out.println("b");
}
}
public class Hello {
public static void main(String[] args){
A ab = new B();
ab = new B();
}
}