Java程式碼?

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";  
        }  
    }  
}

下載

這個的結(jié)果是null?
步驟1? 設(shè)定為test?
步驟2? 設(shè)定為null?
步驟3 列印
步驟1設(shè)定為test?
步驟2? 不做任何事情，因為已經(jīng)有值了，不用設(shè)定為預(yù)設(shè)的null值了?

所以? 一個欄位不設(shè)定值和設(shè)定為null? 是有區(qū)別的。?