Java Inheritance: How to override instance variables/fields from parent class?

Question

Update: I can change the variables from being private, static, or final.

I have a parent class and a child class. I want to re-use a method in the parent class. Normally, this is as easy as super.methodFromParentClass() and you're done. However, when I do this, the method I want to re-use is using instance variable data from the Parent class, which is wrong or rather, I do not want this behavior. I have DIFFERENT initialized data in the child class that needs to get passed into the method I want to reuse. If you look at the method I want to re-use (below is just an example to make it simple, but the idea is the same), I am creating multiple objects in there that use the instance variables of the class its called in. So you can see why when I call super.methodIWantToReuse it won't work, because it will take the Parent data and pass it into the objects, even though I really want it to pass the data I initialize in the child class. My real example is also creating way more objects and I have way more instance variables, so I really want to re-use this code (DRY principle) if at all possible.

How can I get around this? Would using getters i.e. getFirstName() and overriding them in the Child class, thus using Runtime Polymorphism when I call super.methodIWantToReuse(), would grab/use the Child class instance variable data be the only way???

public class ParentClass {
   private static final String firstName = "Billy Ray";
   private static final String lastName = "Cyrus";
   private static final int age = 58;
   private static final String city = "Hunstville";

public boolean methodIWantToReuse() {
   Object1 obj1 = new Object(firstName, lastName);
   
   Object2 obj2 = new Object(age,city);

   Object3 obj3 = new Object(obj1, obj2);

   Object4 obj4 = new Object(obj3);

   // Passing in the objects created above as argument, which have the Parent instance variable data
   return someRandomMethodHere(obj4);
}

public class ChildClass {
    private static final String firstName = "Miley";
    private static final String lastName = "Cyrus";
    private static final int age = 27;
    private static final String city = "Los Angeles";

public boolean methodIWantToReuse() {
   // DOESN'T WORK CORRECTLY, because ends up using the instance variable data of PARENT class, but it 
   // needs to use CHILD class instance variable data

   super.methodIWantToReuse();
}

Your parent class instance variables are Private to that, so you can't update them from Child class. So rather you use parameterize method or create Protected setter getter for instance variables. In you your case the variables are final so you actually can'tr even update them. So technically that;s not possible to use child class variables in parent class. — Gaurav Jeswani
– Gaurav Jeswani, Commented Aug 17, 2020 at 4:37
@GauravJeswani I can remove "private" and "final" modifiers. — PainIsAMaster
– PainIsAMaster, Commented Aug 17, 2020 at 6:00
@AshishKamble I want to use the CHILD isntance variables, NOT the parent. super.VarName would just call the parent instance variables? — PainIsAMaster
– PainIsAMaster, Commented Aug 17, 2020 at 6:03
@ennth I have added my answer below. You can check the solution. — Gaurav Jeswani
– Gaurav Jeswani, Commented Aug 17, 2020 at 6:14

Gaurav Jeswani · Accepted Answer · 2020-08-17 06:38:24Z

3

Your parent class instance variables are Private to that, so you can't update them from Child class. So rather you use parameterize method or create Protected setter/getter for instance variables (or protected variable itself). In you your case the variables are final so you actually can't even update them. So technically that's not possible to use child class variables in parent class.

If you update your variable to protected and remove static/final modifiers (as you mentioned in comments that you can). Before calling method from parent class, update variable data before calling super method. You can do it as below:

Approach 1 : Updating data in parent class before calling parent class method.

Parent Class:

public class ParentClass {

    protected String firstName = "Billy Ray";
    protected String lastName = "Cyrus";
    protected int age = 58;
    protected String city = "Hunstville";

    public boolean methodIWantToReuse() {
        // Passing in the objects created above as argument, which have the Parent
        // instance variable data
         Object1 obj1 = new Object(firstName, lastName);

         Object2 obj2 = new Object(age,city);

         Object3 obj3 = new Object(obj1, obj2);

         Object4 obj4 = new Object(obj3);
        return someRandomMethodHere(obj4);;
    }
}

Child Class:

public class ChildClass extends ParentClass {
    protected String firstName = "Miley";
    protected String lastName = "Cyrus";
    protected int age = 27;
    protected String city = "Los Angeles";

    public boolean methodIWantToReuse() {
        // Update data in Parent class first

        super.firstName = firstName;
        super.lastName = lastName;
        super.age = age;
        super.city = city;
        return super.methodIWantToReuse();
    }
}

Approach 2 : If you want to use parameterized method to make it stateless, you can do it as below:

Parent Class:

public class ParentClass {

    protected String firstName = "Billy Ray";
    protected String lastName = "Cyrus";
    protected int age = 58;
    protected String city = "Hunstville";

    public boolean methodIWantToReuse() {
        
        return methodIWantToReuse(this.firstName, this.lastName, this.age, this.city);
    }

    public boolean methodIWantToReuse(String firstName, String lastName, int age, String city) {
        // Passing in the objects created above as argument, which have the Parent
        // instance variable data
         Object1 obj1 = new Object(firstName, lastName);

         Object2 obj2 = new Object(age,city);

         Object3 obj3 = new Object(obj1, obj2);

         Object4 obj4 = new Object(obj3);
        return someRandomMethodHere(obj4);;
    }
}

Child Class:

public class ChildClass extends ParentClass {
    protected String firstName = "Miley";
    protected String lastName = "Cyrus";
    protected int age = 27;
    protected String city = "Los Angeles";

    public boolean methodIWantToReuse() {
        // Update data in Parent class first
        return super.methodIWantToReuse(this.firstName, this.lastName, this.age, this.city);
    }
}

NOTE: It's not good practice to keep local variables name same as the class level variables. But kept it here same for just understanding.

edited Aug 17, 2020 at 6:38

answered Aug 17, 2020 at 6:13

Gaurav Jeswani

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4 Comments

PainIsAMaster Over a year ago

But if the Parent class was CALLED after this, (after child sets variables to child values i.e. super.firstName = firstName), wouldn't the Parent class THEN use the wrong data? Would I need sometype of "cleanup" to bring back the Parent data?

Gaurav Jeswani Over a year ago

Yes, it will have new data only. If that is the case, better to use parameterized method in super class.

Gaurav Jeswani Over a year ago

@ennth Updated answer with that approach also.

PainIsAMaster Over a year ago

Thanks. calling super. and changing the varnames seems like bad practice, especially if something in my program calls the PARENT class AFTER, and it has wrong values. I guess I will use runtime polymorphism here and just override the getters. Thanks for your help.

M. Khodadadi · Accepted Answer · 2020-08-17 05:35:13Z

3

You can not override fields of a class. Only methods can be overridden. In your case you have to use getters and override them in sub class.

answered Aug 17, 2020 at 5:35

M. Khodadadi

313 bronze badges

2 Comments

PainIsAMaster Over a year ago

Or I could just re-initialize the variables in the Child class like super.varName = varName RIGHT BEFORE I call super.methodIWantToReuse() inside the child class, and I think it will grab the child variables that I want.

M. Khodadadi Over a year ago

You declare final fields(constants) and it is impossible to assign new value to them. mainly I tried to say: "fields can not be overridden".

Oskar Grosser · Accepted Answer · 2020-08-17 05:36:31Z

In case you really mean instance variables instead of your static variables (or class variables) as shown in your example, you could make them accessible for your subclass by changing the access modifier and removing the final keyword.

If, however, you actually mean static variables, you cannot reassign them in each subclass as they would all share the same static variables defined by the ParentClass, meaning the last loaded class would be the only result you get by calling your ParentClass#methodIWantToReuse.

Best would be to use OOP to your advantage by instantiating new individual objects with the required arguments, and using them.

By this I mean instead of doing this:

public class Example {
  public static class ParentClass {
    protected String name;
    protected int age;
    
    public ParentClass() {
      name = "The parent";
      age = 35;
    }
    
    public String methodIWantToReuse() {
      return name + " is " + age + " years old.";
    }
  }
  
  public static class AChildClass extends ParentClass {
    public AChildClass() {
      name = "Alice";
      age = 13;
    }
  }
  
  public static class AnotherChildClass extends ParentClass {
    public AnotherChildClass() {
      name = "Bob";
      age = 21;
    }
  }
  
  public static void main(String[] args) {
    // Prints "The parent is 35 years old."
    System.out.println(new ParentClass().methodIWantToReuse());
    // Prints "Alice is 13 years old."
    System.out.println(new AChildClass().methodIWantToReuse());
    // Prints "Bob is 21 years old."
    System.out.println(new AnotherChildClass().methodIWantToReuse());
  }
}

Do this:

public class Example {
  public static class ParentClass {
    protected String name;
    protected int age;
    
    // Variables instantiated here to not cause confusion
    public ParentClass() {
      name = "The parent";
      age = 35;
    }
    
    public String methodIWantToReuse() {
      return name + " is " + age + " years old.";
    }
  }
  
  public static class ChildClass extends ParentClass {
    public ChildClass(String name, int age) {
      this.name = name;
      this.age = age;
    }
  }
  
  public static void main(String[] args) {
    // Prints "The parent is 35 years old."
    System.out.println(new ParentClass().methodIWantToReuse());
    // Prints "Alice is 13 years old."
    System.out.println(new ChildClass("Alice", 13).methodIWantToReuse());
    // Prints "Bob is 21 years old."
    System.out.println(new ChildClass("Bob", 21).methodIWantToReuse());
  }
}

This should also be along the lines of the DRY principle, as you want to reuse your code as efficient as possible instead of coding technically the same over and over again.

As you can see, there was no need for me to override ParentClass#methodIWantToReuse or call the ChildClass' super's implementation.

I kind of wanted to override having to recreate the objects though? then, I'm not really reusing the method and kinda re-writing it anyway (my real code has way MORE objects and crap). I think I can just remove private/static/final and re-initialize the variables like super.VarName= VarName inside childclass

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