Control Structures

Normally, statements in a program are executed one after the other in the order in which they are written. This process is called sequential execution. Various Java statements, which we will soon discuss, enable the programmer to specify that the next statement to execute is not necessarily the next one in sequence. This is called transfer of control.

During the 1960s, it became clear that the indiscriminate use of transfers of control was the root of much difficulty experienced by software development groups. The blame was pointed at the goto statement (used in most programming languages of the time), which allows the programmer to specify a transfer of control to one of a very wide range of possible destinations in a program. The notion of so-called structured programming became almost synonymous with "goto elimination." [Note: Java does not have a goto statement; however, the word goto is reserved by Java and should not be used as an identifier in programs.]

The research of Bohm and Jacopini[1] had demonstrated that programs could be written without any goto statements. The challenge of the era for programmers was to shift their styles to "goto-less programming." Not until the 1970s did programmers start taking structured programming seriously. The results were impressive. Software development groups reported shorter development times, more frequent on-time delivery of systems and more frequent within-budget completion of software projects. The key to these successes was that structured programs were clearer, easier to debug and modify, and more likely to be bug free in the first place.

[1] Bohm, C., and G. Jacopini, "Flow Diagrams, Turing Machines, and Languages with Only Two Formation Rules," Communications of the ACM, Vol. 9, No. 5, May 1966, pp. 336371.

Bohm and Jacopini's work demonstrated that all programs could be written in terms of only three control structuresthe sequence structure, the selection structure and the repetition structure. The term "control structures" comes from the field of computer science. When we introduce Java's implementations of control structures, we will 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 are written, that is, in sequence. The activity diagram in Fig. 4.1 illustrates a typical sequence structure in which two calculations are performed in order. Java lets us have as many actions as we want in a sequence structure. As we will soon see, anywhere a single action may be placed, we may place several actions in sequence.

Figure 4.1. Sequence structure activity diagram.

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Activity diagrams are part of the UML. An 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, such as the sequence structure in Fig. 4.1. Activity diagrams are composed of special-purpose symbols, such as action-state symbols (rectangles with their left and right sides replaced with arcs curving outward), diamonds and small circles. These symbols are connected by transition arrows, which represent the flow of the activity, that is, the order in which the actions should occur

Like pseudocode, activity diagrams help programmers develop and represent algorithms, although many programmers prefer pseudocode. Activity diagrams clearly show how control structures operate.

Consider the activity diagram for the sequence structure in Fig. 4.1. It contains two action states that represent actions to perform. Each action state contains an action expressionfor 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 located at the top of the activity diagram represents the activity's initial statethe beginning of the workflow before the program performs the modeled actions. The solid circle surrounded by a hollow circle that appears at the bottom of the diagram represents the final statethe 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 symbols in the diagram. Figure 4.1 uses UML notes to show the Java code associated with each action state in the activity diagram. A dotted line connects each note with the element that the note describes. Activity diagrams normally do not show the Java code that implements the activity. We use notes for this purpose here to illustrate how the diagram relates to Java code. For more information on the UML, see our optional case study, which appears in the Software Engineering Case Study sections at the ends of Chapters 18 and 10, or visit www.uml.org.

Selection Statements in Java

Java has three types of selection statements (discussed in this chapter and Chapter 5). The if statement either performs (selects) an action if a condition is true or skips the action, if the condition is false. The if...else statement performs an action if a condition is true and performs a different action if the condition is false. The switch statement (Chapter 5) performs one of many different actions, depending on the value of an expression.

The if statement is a single-selection statement because it selects or ignores a single action (or, as we will soon see, a single group of actions). The if...else statement is called a double-selection statement because it selects between two different actions (or groups of actions). The switch statement is called a multiple-selection statement because it selects among many different actions (or groups of actions).

Repetition Statements in Java

Java provides three repetition statements (also called looping statements) that enable programs to perform statements repeatedly as long as a condition (called the loop-continuation condition) remains true. The repetition statements are the while, do...while and for statements. (Chapter 5 presents the do...while and for statements.) The while and for statements perform the action (or group of actions) in their bodies zero or more timesif the loop-continuation condition is initially false, the action (or group of actions) will not execute. The do...while statement performs the action (or group of actions) in its body one or more times.

The words if, else, switch, while, do and for are Java keywords. Recall that keywords are used to implement various Java features, such as control statements. Keywords cannot be used as identifiers, such as variable names. A complete list of Java keywords appears in Appendix C.

Summary of Control Statements in Java

Java has only three kinds of control structures, which from this point forward we refer to as control statements: the sequence statement, selection statements (three types) and repetition statements (three types). Every program is formed by combining as many sequence, selection and repetition statements as is appropriate for the algorithm the program implements. As with the sequence statement in Fig. 4.1, we can model each control statement as an activity diagram. Each diagram contains an initial state and a final state that represent a control statement's entry point and exit point, respectively. Single-entry/single-exit control statements make it easy to build programsthe control statements are "attached" to one another by connecting the exit point of one to the entry point of the next. This procedure is similar to the way in which a child stacks building blocks, so we call it control-statement stacking. We will learn that there is only one other way in which control statements may be connectedcontrol-statement nestingin which a control statement appears inside another control statement. Thus, algorithms in Java programs are constructed from only three kinds of control statements, combined in only two ways. This is the essence of simplicity.