INHERITANCE IN JAVA SECOND PART

THE ACLASS AND BCLASS

We have made the attributes non-private again; you will see why in a moment. Let’s look at the change to the subclass.

We have redefined the attribute st, same type and same name as the one in the superclass. In fact redefining that attribute, BClass
hides the top-level variable. Let’s look at the class of test.

When we create the instance of BClass superclass and subclass constructor are invoked, respectively. The method viewAttr() which we invoke via the instance bc, correctly displays the attribute st of the superclass, when we get to the println the attribute st invoked is that of the subclass.

You may be wondering, what does keyword super have to do with this? Let’s see what happens by using super to access the st attribute of the superclass from the subclass. The call to super is not only limited in the constructor, but we can use it in any other method.

Let’s look at the test class.

CClass has a constructor that takes three parameters; it is called super with two parameters a and b that are used to initialize the top-level attributes a for AClass, b for BClass. The c parameter is used to initialize the CClass instance attribute. We have access to the attributes of AClass and BClass because let us remember they are not private. Let’s look at the test class.

VARIABLES, REFERENCE, INHERITANCE

We define two methods, one in the superclass and one in the subclass.

Let’s look at the test class.

If we now try to make the assignment b=a that is, we assign a b the superclass reference we get a compiler error.

If we do the opposite a=b this time we have no problem.

We try with the superclass instance variable to call the BClass method. We still get an error.

This means that even if the superclass has the same reference as the subclass it cannot access it as it can only access its own members. Let us summarize what we have seen so far and derive a general rule from it. We have two variables a and b of type AClass and BClass that point to objects of the same classes. At first we have this situation (see figure).

Then we placed b = a and we saw that it is not possible to assign the variable b to an object of the superclass. (see figure)

Then we swapped the references and saw that it is possible to assign a variable of the superclass to an object of its subclass. However, even doing this the variable a of the superclass continues to see the AClass object.

This brings us to this final conclusion. It is only the type of the variable i.e. the reference type that determines which members can be accessed through this variable, while the object type is irrelevant. This means that once the variable a has been defined to be of type AClass even if we assign it an object created as an instance of BClass, this object is seen however as an object of type AClass so we can access only the members defined in the superclass.