java的继承现象

public class q
{
    public static void main(String[] args)
    {
        A a = new A();
        B b = new B();
        a.s = "[AA]";
        b.s = "[BB]";
        a = b;
        System.out.println(a.s);
        System.out.println(b.s);
        System.out.println(a.getS());
        System.out.println(b.getS());
        System.out.println("---------");
        ((A)b).s = "[AA]";
        System.out.println(a.s);
        System.out.println(b.s);
        System.out.println(a.getS());
        System.out.println(b.getS());
    }
}

class A
{
    String s = "[A]";
    String getS()
    {
        return s;
    }
}

class B extends A
{
    String s = "";
    String getS()
    {
        return s;
    }
}


输出：

E:\java>java q
[A]
[BB]
[BB]
[BB]
---------
[AA]
[BB]
[BB]
[BB]