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Java: Book Excerpt

How to Get Started with Java and NetBeans

Book Excerpt: Essential Java Skills (Part 1)

When Sun's developers created Java, they tried to keep the syntax for Java similar to the syntax for C++ so it would be easy for C++ programmers to learn Java. In addition, they designed Java so its applications can be run on any computer platform. In contrast, C++ needs to have a specific compiler for each platform. Java was also designed to automatically handle many operations involving the creation and destruction of memory. This is a key reason why it's easier to develop programs and write bug-free code with Java than with C++. To provide these features, the developers of Java had to sacrifice some speed (or performance) when compared to C++. For many types of applications, however, Java's relative slowness is not an issue.

Microsoft's Visual C# language is similar to Java in many ways. Like Java, C# uses a syntax that's similar to C++ and that automatically handles memory operations. However, in practice, C# code only runs on Windows. Because of that, C# is a good choice for developing applications for a Windows-only environment. However, Java is a better choice if you need to develop crossplatform applications.

Applications, Applets, and Servlets
Figure 1 describes the three types of programs that you can create with Java. First, you can use Java to create applications that run directly on your computer. These are also known as desktop applications.

When you create these desktop applications, you can use a graphical user interface (GUI) to get user input and perform a calculation as shown at the top left of this figure. In chapter 15, you'll learn how to create these types of applications. Until then, you'll learn how to create another type of desktop application known as a console application. This type of application runs in the console, or command prompt, that's available from your operating system. An example of a console application is shown at the top right of this figure.

One of the unique characteristics of Java is that you can use it to create a special type of web-based application known as an applet. For instance, this figure shows an applet that works the same way as the applications above it. The main difference between an application and an applet is that an applet can be stored in an HTML page and can run inside a Java-enabled browser. As a result, you can distribute applets via the Internet or an intranet. In chapter 17, you'll learn how to create and deploy applets.

Although applets can be useful for creating a complex user interface within a browser, they have their limitations. First, you usually need to install a plug-in on each client machine, which isn't ideal for some types of applications. Second, since an applet runs within a browser on the client, it's not ideal for working with resources that run on the server, such as enterprise databases.

To provide access to enterprise databases, many developers use Java EE to create applications that are based on servlets. A servlet is a special type of Java application that runs on the server and can be called by a client, which is usually a web browser. This is also illustrated in this figure. Here, you can see that the servlet works much the same way as the applet. The main difference is that the code for the application runs on the server.

When a web browser calls a servlet, the servlet performs its task and returns the result to the browser, typically in the form of an HTML page. For example, suppose a browser requests a servlet that displays all unprocessed invoices that are stored in a database. Then, when the servlet is executed, it reads data from the database, formats that data within an HTML page, and returns the HTML page to the browser.

When you create a servlet-based application like the one shown here, all the processing takes place on the server and only HTML is returned to the browser. That means that anyone with an Internet or intranet connection, a web browser, and adequate security clearance can access and run a servlet-based application. Because of that, you don't need to install any special software on the client. To make it easy to store the results of a servlet within an HTML page, the Java EE specification provides for JavaServer Pages (JSPs). Most developers use JSPs together with servlets when developing server-side Java applications.

Although servlets and JSPs aren't presented in this book, we cover this topic in a companion book, Murach's Java Servlets and JSP. For more information about this book, please visit our web site at www.murach.com.

A GUI application and a console application

An applet

A servlet

Figure 1: Applications, applets, and servlets

The Code for the Console Version of the Future Value Application
To give you an idea of how the code for a Java application works, Figure 2 presents the code for the console version of the Future Value application that you saw in figure 1-2.

If you have experience with other programming languages, you may be able to understand much of this code already. If not, don't worry! You'll learn how all of this code works in the next few chapters. For now, here's a brief explanation of this code.

Most of the code for this application is stored in a class named FutureValueApp. This class begins with an opening brace ({) and ends with a closing brace (}). Within this class, two methods are defined. These methods also begin with an opening brace and end with a closing brace, and they are indented to clearly show that they are contained within the class.

The first method, named main, is the main method for the application. The code within this method is executed automatically when you run the application. In this case, the code displays the data the user sees on the console, accepts the data the user enters at the console, and calculates the future value.

The second method is named calculateFutureValue. This method is called from the main method and calculates the future value based on the data the user enters.

The code for the Future Value application

import java.util.Scanner;
import java.text.NumberFormat;
public class FutureValueApp
{
public static void main(String[] args)
{
System.out.println("\nWelcome to the Future Value Calculator\n");
Scanner sc = new Scanner(System.in);
String choice = "y";
while (choice.equalsIgnoreCase("y"))
{
// get the input from the user
System.out.print("Enter monthly investment: ");
double monthlyInvestment = sc.nextDouble();
System.out.print("Enter yearly interest rate: ");
double interestRate = sc.nextDouble();
System.out.print("Enter number of years: ");
int years = sc.nextInt();
// calculate the future value
double monthlyInterestRate = interestRate/12/100;
int months = years * 12;
double futureValue = calculateFutureValue(
monthlyInvestment, monthlyInterestRate, months);
// format and display the result
NumberFormat currency = NumberFormat.getCurrencyInstance();
System.out.println("Future value: " +
currency.format(futureValue) + "\n");
// see if the user wants to continue
System.out.print("Continue? (y/n): ");
choice = sc.next();
System.out.println();
}
}
private static double calculateFutureValue(double monthlyInvestment,
double monthlyInterestRate, int months)
{
double futureValue = 0;
for (int i = 1; i <= months; i++)
futureValue = (futureValue + monthlyInvestment) *
(1 + monthlyInterestRate);
return futureValue;
}
}

Figure 2: The code for the console version of the Future Value application

How Java Compiles and Interprets Code
When you develop a Java application, you create one or more classes. For each class, you write the Java statements that direct the operation of the class. Then, you use a Java tool to translate the Java statements into instructions that can be run by the computer. This process is illustrated in Figure 3.

To start, you enter and edit the Java source code for a class. These are the Java statements like the ones you saw in Figure 2 that tell the application what to do. Then, you use the Java compiler to compile the source code into a format known as Java bytecodes. At this point, the bytecodes can be run on any platform that has a Java interpreter to interpret (or translate) the Java bytecodes into code that can be understood by the underlying operating system.

Since Java interpreters are available for all major operating systems, you can run Java on most platforms. This is what gives Java applications their platform independence. In contrast, C++ requires a specific compiler for each type of platform that its programs are going to run on. When a platform has a Java interpreter installed on it, it can be considered an implementation of a Java virtual machine (JVM).

In addition, most modern web browsers can be Java enabled. This allows applets, which are bytecodes that are downloaded from the Internet or an intranet, to run within a web browser. To make this work, Sun developed (and Oracle now maintains) the Java Plug-in. This piece of software is similar to other browser plug-ins such as Apple QuickTime. It allows the browser to run the current version of the Java interpreter.

Figure 3: How Java compiles and interprets Code Description

  • When you develop a Java application, you develop one or more classes.
  • You can use a Java IDE or any text editor to create, edit, and save the source code for a Java class. Source code files have the java extension.
  • The Java compiler translates Java source code into a platform-independent format known as Java bytecodes. Files that contain Java bytecodes have the class extension.
  • The Java interpreter executes Java bytecodes. Since Java interpreters exist for all major operating systems, Java bytecodes can be run on most platforms. A Java interpreter is an implementation of a Java virtual machine (JVM).
  • Most modern web browsers can be Java enabled. This lets applets run within these browsers. Oracle provides a tool known as the Java Plug-in that allows you to specify the version of the Java interpreter that you want to use.

•   •   •

This excerpt is from the book Murach's Java Programming by Joel Murach: http://www.murach.com/books/javp/index.htm

More Stories By Joel Murach

Joel Murach has been writing and editing books about computer programming for over 10 years. During that time, he has written extensively on a wide range of Java, .NET, web, and database technologies. When he's not programming or writing books about programming, he can be found surfing or writing music.

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