Java ??

public class Son extends Father {  
  
    String value = null;//2  
  
    public Son() {  
        super();  //1  
        System.out.println("Son:  " + value);//3  
    }  
  
    public static void main(final String[] args) {  
        new Son();  
    }  
}  
  
  
class Father {  
  
    public Father() {  
        if (this instanceof Son) {  
            Son lower = (Son) this;  
            lower.value = "test";  
        }  
    }  
}  
  
  
class Father {  
  
    public Father() {  
        if (this instanceof Son) {  
            Son lower = (Son) this;  
            lower.value = "test";  
        }  
    }  
}

????

??? null
1?? ???? ??
2?? null? ??
3?? ?? null

??? ?? ?? ?? = null; ?? ??? ?; ????

1?? ???? ??
2?? ?? ?? ???? ???? ?? ???. ????? ?? null ?
3???? null? ?????

??? ?? ?? ???? ?? ?? null? ???? ??? ??? ????.