What Can Java Do?

Probably the most well-known Java programs are Java applets. An applet is a Java program that adheres to certain conventions that allow it to run within a Java-enabled browser. Here's you can see an applet that runs an animation of Java's mascot, Duke, waving at you:

However, Java is not just for writing cute, entertaining applets for the World Wide Web ("Web"). Java is a general-purpose, high-level programming language and a powerful software platform. Using the generous Java API, you can write many types of programs.
The most common types of programs are probably applets and applications, where a Java application is a standalone program that runs directly on the Java platform. A special kind of application known as a server serves and supports clients on a network. Examples of servers include Web servers, proxy servers, mail servers, print servers, and boot servers. Another specialized program is a servlet. Servlets are similar to applets in that they are runtime extensions of applications. Instead of working in browsers, though, servlets run within Java servers, configuring or tailoring the server.
How does the Java API support all of these kinds of programs? With packages of software components that provide a wide range of functionality. The core API is the API included in every full implementation of the Java platform. The core API gives you the following features:

The Essentials: Objects, strings, threads, numbers, input and output, data structures, system properties, date and time, and so on.
Applets: The set of conventions used by Java applets.
Networking: URLs, TCP and UDP sockets, and IP addresses.
Internationalization: Help for writing programs that can be localized for users worldwide. Programs can automatically adapt to specific locales and be displayed in the appropriate language.
Security: Both low-level and high-level, including electronic signatures, public/private key management, access control, and certificates.
Software components: Known as JavaBeans, can plug into existing component architectures such as Microsoft's OLE/COM/Active-X architecture, OpenDoc, and Netscape's Live Connect.
Object serialization: Allows lightweight persistence and communication via Remote Method Invocation (RMI).
Java Database Connectivity (JDBC): Provides uniform access to a wide range of relational databases.
Java not only has a core API, but also standard extensions. The standard extensions define APIs for 3D, servers, collaboration, telephony, speech, animation, and more. See Touring the Java Packages for a more complete list of the core API and standard extensions, along with information on where to find out about each area of the API.
This book covers the Java language and parts of the core API that we expect beginning to intermediate Java programmers to use most frequently. If you need additional information not found in this book, you can explore the online tutorial for additional coverage, located at this URL:
http://java.sun.com/docs/books/tutorial/index.html

The Java Phenomenon