D.2. Program Modules in Java

You write Java programs by combining new methods and classes with predefined ones 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’ll learn how to group your own classes into packages in Appendix F. 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 tasks.

Methods (called functions or procedures in some languages) help you modularize a program by separating its tasks into self-contained units. You’ve declared methods in every program you’ve written. The statements in the method bodies are written only once, are hidden from other methods and can be reused from several locations in a program.

One motivation for modularizing a program into methods is the divide-and-conquer approach, which makes program development more manageable by constructing programs from small, simple pieces. Another is software reusability—using 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 define how to read data from the keyboard—Java provides these capabilities in the methods of class Scanner. A third motivation is to avoid repeating code. Dividing a program into meaningful methods makes the program easier to debug and maintain.