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Java Platform Overview | Getting Started | Step-by-Step Programming
Learning Paths | References & Resources | Certification | Supplements

Before you can develop an application written in the Java programming language, you will need the Java 2 Platform Standard Edition (J2SE) development kit. It has the necessary Java Virtual Machine¹ (JVM), core Application Programming Interfaces (API)s, and the compiler you'll need for most and perhaps all of your development.

Note: Mac users should go to Apple's Mac OS X Java Runtime Environment . Then return for Java programming information and tutorials.

For some projects, you'll want additional bundles. Some of the more popular downloads are listed below. If you're not certain what you need, start with J2SE. You'll discover other tools as you become familiar with the Java platform.

• Java 2 Platform Standard Edition (J2SE): This kit is necessary for developing all applications, except those designed for consumer devices (See the Micro Edition). J2SE comes bundled with the compiler, a runtime environment, and core API.
• Java 2 Platform, Enterprise Edition (J2EE): This packages includes an application server, web server, J2EE APIs, support for Enterprise JavaBeans, Java Servlets API, and JavaServer Pages (JSP) technology. Use J2EE with the J2SE.
• JavaServer Web Development Kit (JSWDK): If you're interested in writing and testing servlets or JavaServer Pages, download this kit along with the J2SE bundle. If you want to work with Enterprise JavaBeans, then you should download the J2EE package instead.
• Java 2 Platform, Micro Edition: If you are interested in developing programs for Palm Pilots, screen phones, and other consumer devices, this kit provides tools for compiling, deployment and device configuration, and APIs that are specialized for each type of device.

The development kits above include the APIs necessary to whatever type of applications you develop in the Java programming language. The APIs and compiler are explained briefly below.

• Java APIs are libraries of compiled code let you add ready-made and customizable functionality to your programs to save coding time.



• Java programs are executed within a program called the JVM. Rather than running directly on the native operating system, the program is interpreted by the JVM for the native operating system. This is key to making your programs portable from one platform to another. In other words, you can develop your programs on a Solaris, Linux, Macintosh, or Windows, then run it on another server or platform.

Once you have the development kits you need, you are ready to begin writing code in the Java programming language. Programs are written in three basic flavors: applets, applications, and servlets/JSP pages. Applets run in the JVM built into a web browser; applications run in the JVM installed on a computer system; and servlets/JSP run in the JVM installed on a web server.

While applets and applications usually have some kind of user interface coupled to backend functionality, servlets provide backend functionality only. The user interface for a servlet is usually an HTML form in a browser that invokes the servlet, but any applet or application that opens a hypertext transfer protocol (HTTP) request can call a servlet. JSP, on the other hand, combines the servlet and HTML into a single component. Instead of using an HTML page with a separate servlet, you use an HTML page with regular HTML tags in combination with scriptlets, short bits of code. The entire page is then processed into a servlet when it's accessed or submitted and the results return to the same HTML page that contains the code.

Java programs run on JVMs in other places too. For example, PersonalJava applications power home appliances, Java Card applications run on smart cards, smart rings, and other devices with limited memory, and Java TV applications run in television settop boxes.

When you download the J2SE or J2EE, you have the APIs and tools you need to write enterprise-worthy applets, applications, and servlets. To add specialized functionality, such as data encryption, or 3D graphics, you need to download additional APIs and tools, optional packages. You can find a list of currently supported optional packages on the Products & APIs page of the java.sun.com web site. The servlet API is a standard extension that comes with the Java 2 SDK download. When a newer version of this package is avilable ahead of the next Java 2 SDK release, it is made available by separate download through the Early Access Program.

Once you've downloaded the development kit you've chosen, see Getting Started for detailed instructions on how to set up your Solaris, Windows, or Mac environment.

The Products & APIs page of the java.sun.com web site provides links to information and software for all currently available Java 2 SDK products.

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¹ As used on this web site, the terms "Java virtual machine" or "JVM" mean a virtual machine for the Java platform. [Return]