java - 为什么使用java中的多态性从父类打印子私有字段时为空?

As far as In understand he getS method from child class BB overrides the same method from the parent class AA. Yet although both classes have initialized the field s which is returned by getS it's printed as null. Why is this happening?

这是代码:

public class AA {
    public String getS() {
        return s;
    }
    private String s = "hello1";
    public AA() {
        System.out.println(service() + getS());
    }
    public static String service() {
        return "A service ";
    }
}

public class BB extends AA {
    private String s = "hello2";
    @Override
    public String getS() {
        return s;
    }
    public static String service() {
        return "B service ";
    }
}

public class CC {
    public static void main(String[] args) {
        BB b =new BB(); //prints "A service null"
    }
}

推荐答案

当我们调用 new SomeClass()

1. new 运算符首先创建 SomeClass 的对象，但该对象的所有字段都设置为默认值(0、''、false、null).

BB(){
    super(); //calls AA()
    s = "hello2"; // by default `s` holds `null` value 
                  // all initialization is moved to constructors 
                  // after superconstructor call
}