Writing Classes - Java Software Solutions: Foundations of Program Design - page 181

SR 4.20 Explain the difference between an actual parameter and a formal
parameter.
SR 4.21 Write a method called getFaceDown for the Die class that returns the
current “face down” value of the die. Hint: On a standard die, the
sum of any two opposite faces is seven.
SR 4.22 In the Transactions program:

a. How many Account objects are created?
b. How many arguments (actual parameters) are passed to the
withdraw method when it is invoked on the acct2 object?
c. How many arguments (actual parameters) are passed to the
addInterest method when it is invoked on the acct3 object?

SR 4.23 Which of the Account class methods would you classify as accessor
methods? As mutator methods? As service methods?

4.5 Constructors Revisited

As we stated in Chapter 2, a constructor is similar to a method that is invoked
when an object is instantiated. When we define a class, we usually define a con-
structor to help us set up the class. In particular, we often use a constructor to
initialize the variables associated with each object.
A constructor differs from a regular method in two ways. First, the name of a
constructor is the same name as the class. Therefore the name of the constructor
in the Die class is Die , and the name of the constructor in the Account class is
Account . Second, a constructor cannot return a value and does not have a return
type specified in the method header.
A common mistake made by programmers is to put a void return
type on a constructor. As far as the compiler is concerned, putting
any return type on a constructor, even void , turns it into a regular
method that happens to have the same name as the class. As such, it cannot be
invoked as a constructor. This leads to error messages that are sometimes difficult
to decipher.
Generally, a constructor is used to initialize the newly instantiated object. For
instance, the constructor of the Die class sets the face value of the die to 1 initially.
The constructor of the Account class sets the values of the instance variables to
the values passed in as parameters to the constructor.
We don't have to define a constructor for every class. Each class has a default
constructor that takes no parameters. The default constructor is used if we don't

KEY CONCEPT
A constructor cannot have any return
type, even void .