java父类转子类的一个方法

import com.alibaba.fastjson.JSONObject;

public class test {

public static void main(String[] args) {
B b = new B();
b.setB("1");
String ob = JSONObject.toJSONString(b);
A a = (A)JSONObject.parseObject(ob, A.class);
a.setA("2");
System.out.println(a.getA());
}
}

class A extends B {
private String a;

public String getA() {
return a;
}

public void setA(String a) {
this.a = a;
}
}

class B {
private String b;

public String getB() {
return b;
}

public void setB(String b) {
this.b = b;
}
}