Java Reference
In-Depth Information
repetition structure
. When we introduce Java's control-structure implementations, we'll
refer to them in the terminology of the
Java Language Specification
as “control statements.”
Sequence Structure in Java
The sequence structure is built into Java. Unless directed otherwise, the computer executes
Java statements one after the other in the order in which they're written—that is, in se-
quence. The
activity diagram
in Fig. 4.1 illustrates a typical sequence structure in which
two calculations are performed in order. Java lets you have as many actions as you want in
a sequence structure. As we'll soon see, anywhere a single action may be placed, we may
place several actions in sequence.
Corresponding Java statement:
total = total + grade;
add grade to total
Corresponding Java statement:
counter = counter + 1;
add 1 to counter
Fig. 4.1
|
Sequence-structure activity diagram.
A UML activity diagram models the
workflow
(also called the
activity
) of a portion
of a software system. Such workflows may include a portion of an algorithm, like the
sequence structure in Fig. 4.1. Activity diagrams are composed of symbols, such as
action-
state symbols
(rectangles with their left and right sides replaced with outward arcs),
dia-
monds
and
small circles
. These symbols are connected by
transition
arrows
, which rep-
resent the
flow of the activity
—that is, the
order
in which the actions should occur.
Like pseudocode, activity diagrams help you develop and represent algorithms.
Activity diagrams clearly show how control structures operate. We use the UML in this
chapter and Chapter 5 to show control flow in control statements. Online Chapters 33-
34 use the UML in a real-world automated-teller-machine case study.
Consider the sequence-structure activity diagram in Fig. 4.1. It contains two
action
states
, each containing an
action expression
—for example, “add grade to total” or “add 1
to counter”—that specifies a particular action to perform. Other actions might include
calculations
or
input/output
operations.
The
arrows
in
the
activity diagram represent
transitions
, which
indicate
the
order
in which the actions represented by the action states
occur. The program that implements the activities illustrated by the diagram in Fig. 4.1
first adds
grade
to
total
, then adds
1
to
counter
.
The
solid circle
at the top of the activity diagram represents the
initial state
—the
beginning
of the workflow
before
the program performs the modeled actions. The
solid
circle surrounded by a hollow circle
at the bottom of the diagram represents the
final
state
—the
end
of the workflow
after
the program performs its actions.
Figure 4.1 also includes rectangles with the upper-right corners folded over. These are
UML
notes
(like comments in Java)—explanatory remarks that describe the purpose of sym-