Programming and Problem Solving with Java

(^154) | Selection and Encapsulation

4.1 Flow of Control

The order in which statements execute is called the flow of control. In a sense, the

computer is under the control of one statement at a time. After a statement exe-

cutes, control turns over to the next statement (like a baton being passed in a re-

lay race).

The flow of control is normally sequential (see Figure 4.1). That is, when one

statement is finished executing, control passes to the next statement in the code.

Where we want the flow of control to be nonsequential, we use control structures,

or special statements that transfer control to a statement other than the one that

physically comes next. As we saw earlier, method calls are control structures that

alter the flow of control so that a separate sequence of statements can be executed.

Selection

We use a selection (or branching) control structure when we want the com-

puter to choose between alternative actions. To do so, we make an asser-

tion, a claim that is either true or false. If the assertion is true, the computer

executes one statement. If it is false, it executes another (see Figure 4.2). The

computer’s ability to solve practical problems is a product of its ability to

make decisions and execute different sequences of instructions.

The Payrollapplication in Chapter 3 is unable to recognize an invalid amount of hours

worked, such as a negative number. How can the computer respond to erroneous input? It

can decide whether a negative number has been entered. It does so by testing the assertion

that the number is less than zero. If the assertion is true, the computer follows the instruc-

tions for displaying an error message. If the assertion is false, the computer simply computes

the pay. Before we examine selection control structures in Java, let’s look closely at how we

get the computer to make decisions.

4.2 Conditions and Logical Expressions

To ask a question in Java, we don’t phrase it as a question; rather, we state it as an assertion.

If our assertion is true, the answer to the question is yes. If our assertion is false, the answer

to the question is no. The need to simplify assertions to true-false form stems from the fact

Flow of control The order in

which the computer executes

statements

Control structure A statement

used to alter the normally

sequential flow of control

statement 1

statement 2

statement 3

statement 4

Flow of

control

Figure 4.1 Sequential Control