Program Modules in Java

Three kinds of modules exist in Javamethods, classes and packages. Java programs are written by combining new methods and classes that the programmer writes with predefined methods and classes available in the Java Application Programming Interface (also referred to as the Java API or Java class library) and in various other class libraries. Related classes are typically grouped into packages so that they can be imported into programs and reused. You will learn how to group your own classes into packages in Chapter 8. The Java API provides a rich collection of predefined classes that contain methods for performing common mathematical calculations, string manipulations, character manipulations, input/output operations, database operations, networking operations, file processing, error checking and many other useful operations. The Java API classes are part of the J2SE Development Kit (JDK) 5.0.

Good Programming Practice 6.1

Familiarize yourself with the rich collection of classes and methods provided by the Java API (java.sun.com/j2se/5.0/docs/api/index.html). In Section 6.8, we present an overview of several common packages. In Appendix G, we explain how to navigate the Java API documentation.

Software Engineering Observation 6.1

Don't try to reinvent the wheel. When possible, reuse Java API classes and methods. This reduces program development time and avoids introducing programming errors.

Methods (called functions or procedures in other programming languages) allow the programmer to modularize a program by separating its tasks into self-contained units. You have declared methods in every program you have written. These methods are sometimes referred to as programmer-declared methods. The actual statements in the method bodies are written only once, reused from perhaps several locations in a program and are hidden from other methods.

There are several motivations for modularizing a program by means of methods. One is the divide-and-conquer approach, which makes program development more manageable by constructing programs from small, simple pieces. Another is software reusabilityusing existing methods as building blocks to create new programs. Often, you can create programs mostly from standardized methods rather than by building customized code. For example, in earlier programs, we did not have to define how to read data values from the keyboardJava provides these capabilities in class Scanner. A third motivation is to avoid repeating code. Dividing a program into meaningful methods makes the program easier to debug and maintain.

Software Engineering Observation 6.2

To promote software reusability, every method should be limited to performing a single, well-defined task, and the name of the method should express that task effectively. Such methods make programs easier to write, debug, maintain and modify.

Error-Prevention Tip 6.1

A small method that performs one task is easier to test and debug than a larger method that performs many tasks.

Software Engineering Observation 6.3

If you cannot choose a concise name that expresses a method's task, your method might be attempting to perform too many diverse tasks. It is usually best to break such a method into several smaller method declarations.

As you know, a method is invoked by a method call, and when the called method completes its task, it either returns a result or simply control to the caller. An analogy to this program structure is the hierarchical form of management (Figure 6.1). A boss (the caller) asks a worker (the called method) to perform a task and report back (i.e., return) the results after completing the task. The boss method does not know how the worker method performs its designated tasks. The worker may also call other worker methods, unbeknownst to the boss. This "hiding" of implementation details promotes good software engineering. Figure 6.1 shows the boss method communicating with several worker methods in a hierarchical manner. The boss method divides the responsibilities among the various worker methods. Note that worker1 acts as a "boss method" to worker4 and worker5.

Figure 6.1. Hierarchical boss-method/worker-method relationship.