More about inheritance - Objects First with Java: A Practical Introduction Using BlueJ - page 318

technique for calculating hash codes, but we recommend the interested reader to see Joshua
Bloch's Effective Java , whose technique we use here. 4 Essentially, an integer value should be
computed making use of the values of the fields that are compared by the overridden equals
method. Here is a hypothetical hashCode method that uses the values of an integer field called
count and a String field called name to calculate the code:

public int hashCode()
{
int result = 17; // An arbitrary starting value.
// Make the computed value depend on the order in which
// the fields are processed.
result = 37 * result + count;
result = 37 * result + name.hashCode();
return result;
}

9.9 Protected access

In Chapter 8, we noted that the rules on public and private visibility of class members apply
between a subclass and its superclass, as well as between classes in different inheritance hier-
archies. This can be somewhat restrictive, because the relationship between a superclass and
its subclasses is clearly closer than it is with other classes. For this reason, object-oriented lan-
guages often define a level of access that lies between the complete restriction of private access
and the full availability of public access. In Java, this is called protected access and is provided
by the protected keyword as an alternative to public and private . Code 9.5 shows an
example of a protected accessor method, which we could add to class Post .

Concepts:

Declaring a field or a
method protected
allows direct access
to it from (direct or
indirect) subclasses.

Code 9.5
An example of a
protected method

protected long getTimeStamp()
{

return timestamp;

}

Protected access allows access to the fields or methods within a class itself and from all its
subclasses, but not from other classes. 5 The getTimeStamp method shown in Code 9.5 can be
called from class Post or any subclasses, but not from other classes. Figure 9.8 illustrates this.
The oval areas in the diagram show the group of classes that are able to access members in class
SomeClass .

4 At the time of writing, sample chapters that contain material relevant to this topic are available at
http://java.sun.com/developer/Books/effectivejava/ .
5 In Java, this rule is not as clear-cut as described here, because Java includes an additional level of vis-
ibility, called package level , but with no associated keyword. We will not discuss this further, and it is
more general to consider protected access as intended for the special relationship between superclass
and subclass.