Java Class Libraries

Java programs consist of pieces called classes. Classes include pieces called methods that perform tasks and return information when they complete them. Programmers can create each piece they need to form Java programs. However, most Java programmers take advantage of the rich collections of existing classes in the Java class libraries, which are also known as the Java APIs (Application Programming Interfaces). Thus, there are really two aspects to learning the Java "world." The first is the Java language itself, so that you can program your own classes, and the second is the classes in the extensive Java class libraries. Throughout this book, we discuss many library classes. Class libraries are provided primarily by compiler vendors, but many are supplied by independent software vendors (ISVs).

Software Engineering Observation 1.1

Use a building-block approach to create programs. Avoid reinventing the wheeluse existing pieces wherever possible. Called software reuse, this practice is central to object-oriented programming.

We include many tips such as these Software Engineering Observations throughout the book to explain concepts that affect and improve the overall architecture and quality of software systems. We also highlight other kinds of tips, including Good Programming Practices (to help you write programs that are clearer, more understandable, more maintainable and easier to test and debugor remove programming errors), Common Programming Errors (problems to watch out for and avoid), Performance Tips (techniques for writing programs that run faster and use less memory), Portability Tips (techniques to help you write programs that can run, with little or no modification, on a variety of computersthese tips also include general observations about how Java achieves its high degree of portability), Error-Prevention Tips (techniques for removing bugs from your programs and, more important, techniques for writing bug-free programs in the first place) and Look and Feel Observations (techniques to help you design the "look" and "feel" of your applications' user interfaces for appearance and ease of use). Many of these are only guidelines. You will, no doubt, develop your own preferred programming style.

Software Engineering Observation 1.2

When programming in Java, you will typically use the following building blocks: Classes and methods from class libraries, classes and methods you create yourself and classes and methods that others create and make available to you.

The advantage of creating your own classes and methods is that you know exactly how they work and you can examine the Java code. The disadvantage is the time-consuming and potentially complex effort that is required.

Performance Tip 1.1

Using Java API classes and methods instead of writing your own versions can improve program performance, because they are carefully written to perform efficiently. This technique also shortens program development time.

Portability Tip 1.1

Using classes and methods from the Java API instead of writing your own improves program portability, because they are included in every Java implementation.

Software Engineering Observation 1.3

Extensive class libraries of reusable software components are available over the Internet and the Web, many at no charge.

To download the Java API documentation, go to the Sun Java site java.sun.com/j2se/5.0/download.jsp.