(^18) | Introduction to Object-Oriented Programming
(^4) Java actually allows us to write more general asynchronous programs using a construct called a thread.
Threaded programs are beyond the scope this text. We restrict our use of asynchronous structures to
handling events.
The repetitive control structure, the loop, repeats operations while certain
conditions are met.
The subprogram allows us to organize our code into units that correspond to spe-
cific object behaviors; Java calls these units methods.
Asynchronous control lets us write code that handles events, such as the user
clicking a button on the screen with the mouse.
Each of these ways of structuring operations controls the order in which the computer ex-
ecutes the operations, which is why they are called control structures.
Suppose you’re driving a car. Going down a straight stretch of road is like following a se-
quenceof instructions. When you come to a fork in the road, you must decide which way to
go and then take one or the other branch of the fork. The computer does something similar
when it encounters a selectioncontrol structure (sometimes called a branchor decision) in a
program. Sometimes you have to go around the block several times to find a place to park.
The computer does the same sort of thing when it encounters a loop.
A subprogram is a named sequence of instructions written as a separate unit. When
the computer executes an instruction that refers to the name of the subprogram, the code
for the subprogram executes. When the subprogram finishes, execution resumes at the in-
struction that originally referred to the subprogram. Suppose, for example, that every day you
go to work at an office. The directions for getting from home to work form a method called
“Go to the office.” It makes sense, then, for someone to give you directions to a meeting by
saying, “Go to the office, then go four blocks west”—without listing all the steps needed to
get to the office.
Responding to asynchronous events is like working as a pizza delivery person. You wait
around the dispatch station with all of the other delivery people. The dispatcher calls your
name and gives you some pizzas and a delivery address. You deliver the pizzas and return
to the dispatch station. At the same time, other delivery people may be out driving.^4 The
term asynchronousmeans “not at the same time.” In this context, it refers to the fact that the
user can, for example, click the mouse on the screen at any time while the application is run-
ning. The mouse click does not have to happen at some particular time corresponding to cer-
tain instructions within the code.
Object-Oriented Programming Languages
Early programming languages focused their attention on the operations and control struc-
tures of programming. These procedurallanguages paid little explicit attention to the rela-
tionships between the operations and the data. At that time, a typical computer program used