从超类调用一个子类方法，该方法在另一个子类中不可用

Question

I have a superclass Person and two subclasses Author and Member as follows

public class Author extends Person {    
  public Author(String fName, String lName, int noOfPublications){
      ----// set 
  }    
   public String getnoOfPub(){
     return noOfPublications;
  }
}

public class Member extends Person {

public Member(String fName, String lName, int memberNo){
----// set 
}    
 public String getMembNo(){
        return memberNo;
    }
}

I want to access the non common methods from main class or How can I get access to getnoOfPub and getMembNo methods from superclass person?

Answer 1

You can gain access to the non-common methods by checking the type of the object and casting it to the type of the relevant sub-class, but that's not something you'd want to do in the code of the base class.

In a method of Person, you can write :

public void someMethod ()
{
    String id = null;
    if (this instanceof Member) {
        Member member = (Member) this;
        id = member.getMembNo ();
    }
}

While this is valid syntax, it's a very bad practice, since the base class shouldn't know about its sub-classes, since the sub-classes can be written long after the base-class was written, so the writers of the base-class can't assume they know all the sub-classes that would extend their base-class.

Answer 2

it is difficult to answer the question because it is very broad. but i'll answer the common mistakes:

1. it might be that you want a method in the superclass to be override in the sub class.
2. you need to cast from Person to the subclasses Member or Author and the use your methods.
3. you need to initiate a variable of Member or Author and then use the methods.
4. not very likely but might be - you want the method to be static