Defining Classes II - Absolute Java - page 304

EXAMPLE: A Person Class
It is common to have instance variables of a class type. The class Person defined in
Display 5.19 has two instance variables of type Date . So, the class Person has instance
variables of a class type. (The class Date was defined in Chapter 4 , Display 4.11. We
have reproduced the relevant portions of the date class definition in Display 5.20 .)
In fact, all the instance variables for the class Person are of class types. An object
of the class Person has the basic data about people that is found in such places as
on tombstones and in author listings in library catalogues. It describes a person by
giving the person's name, date of birth, and date of death. If the person is still alive,
then the value null is used as the date of death. (So null is good.) A simple program
illustrating the class Person is given in Display 5.21. We will discuss a few details
about the class Person here, but most of the various methods in the class Person will
be discussed as we cover the corresponding topic in the following subsections.
Normally, a class definition should include a no-argument constructor. However,
there are cases where a no-argument constructor makes little sense. For example, the
wrapper classes such as Integer and Double do not have no-argument constructors, as
we explained in the Pitfall subsection “A Wrapper Class Does Not Have a No-Argument
Constructor,” which appeared earlier in this chapter. The class Person also does not
have a no-argument constructor for a reason. A person may have no date of death,
but a person always has a date of birth. A no-argument constructor should initialize all
instance variables, but there is no suitable value to initialize the instance variable born
unless it is provided as an argument to the constructor. In particular, it makes no sense
to initialize the instance variable born to null ; that would indicate that the person
was never born. It makes little sense to have a person who was never born, so it makes
little sense to have a no-argument constructor for the class Person . Note that because
we defined some constructors for the class Person but did not define a no-argument
constructor, it follows that the class Person does not have a no-argument constructor.
Since we are assuming that an object of the class Person always has a birth date
(which is not null ), the following should always be true of an object of the class Person :
An object of the class Person has a date of birth (which is not null ), and if the object
has a date of death, then the date of death is equal to or later than the date of birth.
If you check the definition of the class Person , you will see that this statement
is always true. It is true of every object created by a constructor, and all the other
methods preserve the truth of this statement. In fact, the private method consistent
was designed to provide a check for this property. A statement, such as the above,
that is always true for every object of the class is called a class invariant .
Note that the definition of equals for the class Person includes an invocation
of equals for the class String and an invocation of the method equals for the
class Date. Java determines which equals method is being invoked from the type
of its calling object. Because the instance variable name is of type String , the
invocation name.equals(...) is an invocation of the method equals for the class
String . Because the instance variable born is of type Date , the invocation born.
equals(...) is an invocation of the method equals for the class Date .
Similarly, the definition of the method toString for the class Person includes
invocations of the method toString for the class Date .