Subclasses - Multimedia Introduction to Programming Using Java - page 154

In addition, it should have two other properties:

• It is consistent : x.equals(y) consistently returns true or consistently
returns false , provided no information used in equals comparisons on
the object is modified.
• For any folder x , x.equals( null ) is false .
The spec of function equals says that it is generally necessary to override
method hashCode whenever equals is overridden to maintain the general con-
tract for method hashCode , which states that equal objects must have equal hash
codes. But the topic of method hashCode is outside the scope of this text.

An example of overriding equals
In Sec. 3.2.4, we discussed aliasing and equality and wrote this boolean
function in class Employee :
/** = " This Employee and e contain the same fields " */
public boolean equals(Employee e) {
return name == e.name
&& start == e.start
&& salary == e.salary;

}
We now rewrite this function so that it overrides function equals of class
Object . Thus, its parameter must be Object . Further, we must make sure that e
is not null and that its real class is Employee . Here is the function:
/** = "e is an Employee , with the same fields as this Employee" */
public boolean equals(Object e) {
return e!= null
&& e instanceOf Employee
&& name == e.name
&& start == e.start
&& salary == e.salary;

}

4.4 Access modifiers

This section need be studied only if you are going to write your own packages
(see Chap. 11).
As you know, a private component in a class C can be accessed only in class
C , and not even in subclasses of C . A public component can be accessed any-
where. Classification private is extremely restrictive; public is extremely liber-
al. Java has two other access schemes, which we call protected and package , that
fall between these two extremes. Below, we list all four schemes, from least to
most restrictive:

Activity
4-2.6