Styjun

I'm not sure how exactly to phrase my question.
So, I have an interface reference and I'm creating a new object. The new object obviously implements said interface. The initial class inherits another class. That sub-class inherits the super class. However, I cannot access data from super class from the main method without casting the reference first. I'll show an example below

 public class a 

 public int getSomeData1() 
 return someData;
 


public class b extends a implements someInterface 
 // Some behavior. 


public class c extends b implements someInterface 
 // Some behavior. 


public class Main 

 public static void main(String[] args) 
 someInterface obj = new b();

 obj.someData1(); // I cannot access someData1(). 

 c anotherObj = new c();

 c.getSomeData1(); // This works however. 
 

How can I have obj.someData1() actually get the data from class a rather than casting it to a.

Just remember the rule that method invocations allowed by the compiler are based solely on the declared type of the reference, regardless of the object type.

If it is not very clear, here is another version of this rule: what is on the left side defines methods you can call, no matter what is on the right :)

Here are a few examples to make it more clear:

public interface Animal 
 void voice();


public class Dog implements Animal 
 public void voice() 
 System.out.println("bark bark");
 

 public void run() 
 // impl
 

When you create a dog like this:

Animal dog1 = new Dog();

The reference type which is Animal defines which methods are allowed for you to call. So basically you can only call:

dog1.voice();

When you create a dog like this:

Dog dog2 = new Dog();

The reference type which is Dog, so you are allowed to call:

dog2.voice();
dog2.run();

This rule remains also when you have class inheritance, not only when you implement an interface. Let's say we have something like:

public class SpecialDog extends Dog 
 public void superPower() 

And those are examples of what you can call:

Animal dog1 = new SpecialDog();
dog1.voice(); // only this

Dog dog2 = new SpecialDog();
// here you can call everything that Dog contains
dog2.voice();
dog2.run();

SpecialDog dog3 = new SpecialDog();
// here you can call all 3 methods
// this is the SpecialDog method
dog3.superPower();

// those 2 are inherited from Dog, so SpecialDog also has them
dog3.voice();
dog3.run();

In other cases, you need to upcast/downcast to be able to call some specific method.

Happy Hacking :)