Access modifiers:

One of the key refinements of our object-oriented programming skills in this unit is learning how we can specify that only certain classes of objects may invoke a method and only certain classes may access an instance variable. There are two kinds of access to class members: quali?ed or simple, using what we call a qualified name or a simple name. A name used with an object reference and a dot notation is a qualified name, for example john.getNumOfAccesses(); a name without a reference and dot notation is a simple name, for example getNumOfAccesses(). 

Inheritance and member accessibility

If we say a member is inherited we mean that the subclass has the same access to it as if it were defined in the subclass. It can be accessed using its simple name. This kind of access is available only via an inheritance relationship. Any other class would have to have a reference to an object, and use a quali?ed name. It is useful to be able to create different levels of information hiding, allowing us to share information between certain classes and deny other classes access to details they should not and do not need to know. The classes that you have seen in the course so far were all constructed in such a way that the instance variables have been private and therefore have not been available for access outside their class, but the methods have been public and therefore have been available to be accessed. This is generally best programming practice. However, there are circumstances when other levels of access should be allowed.

There are three keywords associated with controlling levels of access to class members in Java: public, protected and private. These are known as access modi?ers. However, there is actually a fourth level of access, 'default'. There is no 'default' keyword; default access arises when none of the access modifiers is specified. The most accessible (least restrictive) level is public, followed by protected, default and then private. This means that if you fail to specify an access level, by default you get a fairly restricted access to class methods and instance variables. Only the private access level is more restrictive. In the following sections we will look at variations on how we might declare members of the User class, the effect this has on whether classes can access the members of User objects and also which members of the User class would be inherited by any of its subclasses.

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