Writing Classes - Java Software Solutions: Foundations of Program Design - page 167

Instance Data
Note that in the Die class, the constant MAX and the variable faceValue are declared
inside the class but not inside any method. The location at which a variable is
declared defines its scope, which is the area within a program in which that vari-

able can be referenced. By being declared at the class level (not within a method),
these variables and constants can be referenced in any method of the class.
Attributes such as the variable faceValue are called instance
data because new memory space is reserved for that variable every
time an instance of the class that is created. Each Die object has its
own faceValue variable with its own data space. That's how each
Die object can have its own state. We see that in the output of the
RollingDice program: one die has a face value of 5 and the other has a face value
of 2. That's possible only because the memory space for the faceValue variable
is created for each Die object.
We can depict this situation as follows:

KEY CONCEPT
The scope of a variable, which deter-
mines where it can be referenced,
depends on where it is declared.

die1

faceValue 5

die2

faceValue 2

The die1 and die2 reference variables point to (that is, contain the address
of) their respective Die objects. Each object contains a faceValue variable
with its own memory space. Thus each object can store different values for its
instance data.
Java automatically initializes any variables declared at the class level. For
example, all variables of numeric types such as int and double are initialized to
zero. However, despite the fact that the language performs this automatic initial-
ization, it is good practice to initialize variables explicitly (usually in a construc-
tor) so that anyone reading the code will clearly understand the intent.

UML Class Diagrams
Throughout this topic we use UML diagrams to visualize relationships among
classes and objects. UML stands for the Unified Modeling Language, which
has become the most popular notation for representing the design of an object-
oriented program.
Several types of UML diagrams exist, each designed to show specific aspects
of object-oriented programs. We focus primarily on UML class diagrams in this
book to show the contents of classes and the relationships among them.