Java code

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";  
        }  
    }  
}

Download

The result of this is null
Step 1 Set as test
Step 2 Set as null
Step 3 Print out null

If not String value = null ; Just String value; Download

Step 1 Set to test
Step 2 Do nothing, because there is already a value, no need to set it to the default null value
Step 3 Print out null

So there is a difference between not setting a value for a field and setting it to null.