Java How to Program Fourth Edition - DCC

270 Methods Chapter 6
and implements ActionListener. A class can inherit existing attributes and behaviors
(data and methods) from another class specified to the right of keyword extends in the
class definition. In addition, a class can implement one or more interfaces. An interface
specifies one or more behaviors (i.e., methods), which you must define in your class definition.
Implementing interface ActionListener forces us to define a method with the
first line
public void actionPerformed( ActionEvent actionEvent )
in our Craps class. This method’s task is to process a user’s interaction with the JButton
(called Roll Dice on the user interface). When the user presses the button, this method will
be called automatically in response to the user interaction. This process is called event handling.
The event is the user interaction (i.e., pressing the button). The event handler is method
actionPerformed. We discuss the details of this interaction and method
actionPerformed shortly. Chapter 9, Object-Oriented Programming, discusses interfaces
in detail. For now, as you develop your own applets that have graphical user interfaces,
mimic the features that support event handling of the GUI components we present.
The game of craps is reasonably involved. The player may win or lose on the first roll,
or may win or lose on any roll. Line 14 creates variables that define the three states of a
game of craps: Game won (WON), game lost (LOST) or continue rolling the dice (CON-
TINUE). Keyword final at the beginning of the declaration indicates that these are constant
variables. When a program declares a final variable, the program must initialize
the variable before using the variable and cannot modify the variable thereafter. If the variable
is an instance variable, this initialization normally occurs in the variable’s declaration.
The initialization also can occur in a special method of a class called a constructor (discussed
in Chapter 8). Constant variables are often called named constants or read-only
variables. We provide more details on keyword final in Chapter 7 and Chapter 8.
Common Programming Error 6.12
After declaring and initializing a final variable, attempting to assign another value to that
variable is a syntax error. 6.12
Good Programming Practice 6.5
Use only uppercase letters (with underscores between words) in the names of final variables.
This format makes these constants stand out in a program. 6.5
Good Programming Practice 6.6
Using meaningfully named final variables rather than integer constants (such as 2) makes
programs more readable. 6.6
Lines 17–20 declare several instance variables that are used throughout the Craps
applet. Variable firstRoll is a boolean variable that indicates whether the next roll
of the dice is the first roll in the current game. Variable sumOfDice maintains the sum of
the dice for the last roll. Variable myPoint stores the “point” if the player does not win or
lose on the first roll. Variable gameStatus keeps track of the current state of the game
(WON, LOST or CONTINUE).
Lines 23–25 declare references to the GUI components used in this applet’s graphical
user interface. References die1Label, die2Label, sumLabel and pointLabel all
refer to JLabel objects. A JLabel contains a string of characters to be displayed on the
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