The Internet and Beyond
Computers can be connected together on networks. A computer on a network can communicate with other computers on the same network by exchanging data and files or by sending and receiving messages. Computers on a network can even work together on a large computation.
Today, millions of computers throughout the world are connected to a single huge network called the Internet. New computers are being connected to the Internet every day, both by wireless communication and by physical connection using technologies such as DSL, cable modems, and Ethernet.
There are elaborate protocols for communication over the Internet. A protocol is simply a detailed specification of how communication is to proceed. For two computers to communicate at all, they must both be using the same protocols. The most basic protocols on the Internet are the Internet Protocol (IP), which specifies how data is to be physically transmitted from one computer to another, and the Transmission Control Protocol (TCP), which ensures that data sent using IP is received in its entirety and without error. These two protocols, which are referred to collectively as TCP/IP, provide a foundation for communication. Other protocols use TCP/IP to send specific types of information such as web pages, electronic mail, and data files.
All communication over the Internet is in the form of packets. A packet consists of some data being sent from one computer to another, along with addressing information that indicates where on the Internet that data is supposed to go. Think of a packet as an envelope with an address on the outside and a message on the inside. (The message is the data.) The packet also includes a "return address," that is, the address of the sender. A packet can hold only a limited amount of data; longer messages must be divided among several packets, which are then sent individually over the Net and reassembled at their destination.
Every computer on the Internet has an IP address, a number that identifies it uniquely among all the computers on the Net. (Actually, the claim about uniqueness is not quite true, but the basic idea is valid, and the full truth is complicated.) The IP address is used for addressing packets. A computer can only send data to another computer on the Internet if it knows that computer's IP address. Since people prefer to use names rather than numbers, most computers are also identified by names, called domain names. For example, the main computer of the Mathematics Department at Hobart and William Smith Colleges has the domain name math.hws.edu. (Domain names are just for convenience; your computer still needs to know IP addresses before it can communicate. There are computers on the Internet whose job it is to translate domain names to IP addresses. When you use a domain name, your computer sends a message to a domain name server to find out the corresponding IP address. Then, your computer uses the IP address, rather than the domain name, to communicate with the other computer.)
The Internet provides a number of services to the computers connected to it (and, of course, to the users of those computers). These services use TCP/IP to send various types of data over the Net. Among the most popular services are instant messaging, file sharing, electronic mail, and the World-Wide Web. Each service has its own protocols, which are used to control transmission of data over the network. Each service also has some sort of user interface, which allows the user to view, send, and receive data through the service.
For example, the email service uses a protocol known as SMTP (Simple Mail Transfer Protocol) to transfer email messages from one computer to another. Other protocols, such as POP and IMAP, are used to fetch messages from an email account so that the recipient can read them. A person who uses email, however, doesn't need to understand or even know about these protocols. Instead, they are used behind the scenes by computer programs to send and receive email messages. These programs provide the user with an easy-to-use user interface to the underlying network protocols.
The World-Wide Web is perhaps the most exciting of network services. The World-Wide Web allows you to request pages of information that are stored on computers all over the Internet. A Web page can contain links to other pages on the same computer from which it was obtained or to other computers anywhere in the world. A computer that stores such pages of information is called a web server. The user interface to the Web is the type of program known as a web browser. Common web browsers include Microsoft Edge, Internet Explorer, Firefox, Chrome, and Safari. You use a Web browser to request a page of information. The browser sends a request for that page to the computer on which the page is stored, and when a response is received from that computer, the web browser displays it to you in a neatly formatted form. A web browser is just a user interface to the Web. Behind the scenes, the web browser uses a protocol called HTTP (HyperText Transfer Protocol) to send each page request and to receive the response from the web server.
Now just what, you might be thinking, does all this have to do with Java? In fact, Java is intimately associated with the Internet and the World-Wide Web. When Java was first introduced, one of its big attractions was the ability to write applets. An applet is a small program that is transmitted over the Internet and that runs on a web page. Applets made it possible for a web page to perform complex tasks and have complex interactions with the user. Alas, applets have suffered from a variety of problems, and they have fallen out of use. There are now other options for running programs on Web pages.
But applets were only one aspect of Java's relationship with the Internet. Java can be used to write complex, stand-alone applications that do not depend on a Web browser. Many of these programs are network-related. For example many of the largest and most complex web sites use web server software that is written in Java. Java includes excellent support for network protocols, and its platform independence makes it possible to write network programs that work on many different types of computer. You will learn about Java's network support in Chapter 11.
Its support for networking is not Java's only advantage. But many good programming languages have been invented only to be soon forgotten. Java has had the good luck to ride on the coattails of the Internet's immense and increasing popularity.
As Java has matured, its applications have reached far beyond the Net. The standard version of Java already comes with support for many technologies, such as cryptography, data compression, sound processing, and three-dimensional graphics. And programmers have written Java libraries to provide additional capabilities. Complex, high-performance systems can be developed in Java. For example, Hadoop, a system for large scale data processing, is written in Java. Hadoop is used by Yahoo, Facebook, and other Web sites to process the huge amounts of data generated by their users.
Furthermore, Java is not restricted to use on traditional computers. Java can be used to write programs for many smartphones (though not for the iPhone). It is the primary development language for Android-based devices. (Android uses Google's own version of Java and does not use the same graphical user interface components as standard Java.) Java is also the programming language for the Amazon Kindle eBook reader and for interactive features on Blu-Ray video disks.