从抽象类Java继承后的Access属性

Question

I cannot access a field of a class that is a concrete type inheriting from an abstract class.

In Java I create a class of External student that extends Student

 */
public class ExternalStudent extends Student  {
String currentSchool;

  public ExternalStudent(String name, Integer age, String studentIdentifier, String currentSchool) {
    super(name, age, studentIdentifier);
    this.currentSchool = currentSchool; 
  }
}

where student is

public abstract class Student {

    //Attributes
    String studentIdentifier;
    Integer age;
    String name;

    //Associations
    List<Subject> subject =  new ArrayList<Subject>();
    PersonalDetails personaldetails;

    //Methods
    public void setSubject () {
        this.subject.add(new Subject("Name"));
    }

    //Constructors
    public Student(String name, Integer age, String studentIdentifier){
       this.age = age;
       this.name = name;
       this.studentIdentifier = studentIdentifier;
    }
}

and external student is set up by my class Application

public class ApplicationC {
    //Attributes
    private String personalStatement;
    private String applicationForm;

    //Associations
    Registration registration;
    Student student;
    ApplicationTest applicationtest;

    //Methods
    public void setApplicationResult(String result){
        this.applicationtest = new ApplicationTest(result);
    }

    //Constructor
    public ApplicationC (String personalStatement, String name){
        this.registration = new Registration();
        this.student = new ExternalStudent("Tom",16,"78954","DHS");
    }
}

I've set up a simple test class

public void testPostCondition() throws ParseException{   
 ApplicationC instance = new ApplicationC("test statement","test name");
    instance.setApplicationResult("pass");    
    assertEquals("pass",instance.applicationtest.result);
         instance.student.age = 16;
     instance.student.studentIdentifier = "78954";
     instance.student.name = "Tom";
     instance.student.currentSchool = "test"; //Error as field does not exist
}

But I cannot access the current school of the student instance (who must be an externalStudent). How can I access this field in order to test my code?

Answer 1

In ApplicationC , the student field is declared with the Student class :

Student student;

Methods available on an objects relies on the declared type, not the object really instantiated.
And currentSchool is only declared in the subclass ExternalStudent .
So, you cannot access it in this way.

A workaround is downcasting Student to ExternalStudent :

 ((ExternalStudent)instance.student).studentIdentifier = "78954";

And generally, it is better to check the type of the instance before doing it :

 if (instance.student instanceof ExternalStudent){
    ((ExternalStudent)instance.student).studentIdentifier = "78954";
 }

As a general advice, in Java, you should favor the private modifier for fields and if you need to manipulate the base class and access to some fields specific to the subclass, you could define a method in the base class that returns null or Optional and override it in the subclass with the return of the field.
It avoids cast that may be error prone and that often are symptoms of a conception problem.

Answer 2

Your instance is an AplicationC,
So, "instance.student" is a "Student".
"Student" does not have the "currentSchool" property.
to get to it
* add "currentSchool" property to "Student"
or
* cast your "instance.student" to "ExternalStudent"

note: you will need to handle all the exceptions and over-head of casting etc'

Hope this helps