请问这段代码为什么输出null，求大神解答程序执行的顺序？

Question

public class xieCheng {
private String baseName="Base";
public xieCheng(){
this.callName();
}
public void callName(){
System.out.println(baseName);
}
}

public class Sub extends xieCheng {
private String baseName="Sub";
public void callName(){
System.out.println(baseName);
}

public static void main(String[] args){
xieCheng xc=new Sub();
xc.callName();
}
}

Answer 1