Frequently Ask Questions - Java Sample Program

Frequently Ask Questions

Q: I have heard about a special type of Java program called a servlet. What is it?

A: A servlet is a small program that executes on the server. Just as applets dynamically extend the functionality of a web browser, servlets dynamically extend the functionality of a web server. It is helpful to understand that as useful as applets can be, they are just one half of the client/server equation. Not long after the initial release of Java it became obvious that Java would also be useful on the server side. The result was the servlet. Thus, with the advent of the servlet, Java spanned both sides of the client/server connection. Although the creation of servlets is beyond the scope of this beginner’s guide, they are something that you will want to study further as you advance in Java programming. (Coverage of servlets can be found in my book Java: The Complete Reference, published by Oracle Press/McGraw-Hill.)


Q: To address the issues of portability and security, why was it necessary to create a new computer language such as Java; couldn’t a language like C++ be adapted? In other words, couldn’t a C++ compiler that outputs bytecode be created?

A: While it would be possible for a C++ compiler to generate something similar to bytecode rather than executable code, C++ has features that discourage its use for the creation of Internet programs—the most important feature being C++’s support for pointers. A pointer is the address of some object stored in memory. Using a pointer, it would be possible to access resources outside the program itself, resulting in a security breach. Java does not support pointers, thus eliminating this problem.


Q: You state that object-oriented programming is an effective way to manage large programs. However, it seems that it might add substantial overhead to relatively small ones. Since you say that all Java programs are, to some extent, object-oriented, does this impose a penalty for smaller programs

A: No. As you will see, for small programs, Java’s object-oriented features are nearly transparent. Although it is true that Java follows a strict object model, you have wide latitude as to the degree to which you employ it. For smaller programs, their “object-orientedness” is barely perceptible. As your programs grow, you will integrate more object-oriented features effortlessly.


Q: Why does Java have different data types for integers and floating-point values? That is, why aren’t all numeric values just the same type?

A: Java supplies different data types so that you can write efficient programs. For example, integer arithmetic is faster than floating-point calculations. Thus, if you don’t need fractional values, then you don’t need to incur the overhead associated with types float or double. Second, the amount of memory required for one type of data might be less than that required for another. By supplying different types, Java enables you to make best use of system resources. Finally, some algorithms require (or at least benefit from) the use of a specific type of data. In general, Java supplies a number of built-in types to give you the greatest flexibility.


Q: Does the use of a code block introduce any run-time inefficiencies? In other words, does Java actually execute the { and }?

A: No. Code blocks do not add any overhead whatsoever. In fact, because of their ability to simplify the coding of certain algorithms, their use generally increases speed and efficiency. Also, the { and } exist only in your program’s source code. Java does not, per se, execute the { or }.


Q: You say that there are four integer types: int, short, long, and byte. However, I have heard that char can also be categorized as an integer type in Java. Can you explain?

A: The formal specification for Java defines a type category called integral types, which includes byte, short, int, long, and char. They are called integral types because they all hold whole-number, binary values. However, the purpose of the first four is to represent numeric integer quantities. The purpose of char is to represent characters. Therefore, the principal uses of char and the principal uses of the other integral types are fundamentally different. Because of the differences, the char type is treated separately in this book


Q: Why does Java use Unicode?

A: Java was designed for worldwide use. Thus, it needs to use a character set that can represent all the world’s languages. Unicode is the standard character set designed expressly for this purpose. Of course, the use of Unicode is inefficient for languages such as English, German, Spanish, or French, whose characters can be contained within 8 bits. But such is the price that must be paid for global portability.


Q: Is a string consisting of a single character the same as a character literal? For example, is “k” the same as ‘k’?

A: No. You must not confuse strings with characters. A character literal represents a single letter of type char. A string containing only one letter is still a string. Although strings consist of characters, they are not the same type.

Obtaining the Java Development Kit - Java Sample Program

Obtaining the Java Development Kit

 Now that the theoretical underpinning of Java has been explained, it is time to start writing Java programs. Before you can compile and run those programs, however, you must have the Java Development Kit (JDK) installed on your computer. The JDK is available free of charge from Oracle. At the time of this writing, the current release of the JDK is JDK 7.

 The JDK can be downloaded from www.oracle.com/technetwork/java/javase/downloads/index.html. Just go to the download page and follow the instructions for the type of computer that you have. After you have installed the JDK, you will be able to compile and run programs. The JDK supplies two primary programs. 
The first is javac, which is the Java compiler. The second is java, which is the standard Java interpreter and is also referred to as the application launcher.

One other point: The JDK runs in the command prompt environment and uses command-line tools. It is not a windowed application. It is also not an integrated development environment (IDE).

NOTE:
 In addition to the basic command-line tools supplied with the JDK, there are several high-quality IDEs available for Java, such as NetBeans and Eclipse. An IDE can be very helpful when developing and deploying commercial applications. As a general rule, you can also use an IDE to compile and run the programs in this book if you so choose. However, the instructions presented in this book for compiling and running a Java program describe only the JDK command-line tools. The reasons for this are easy to understand. First, the JDK is readily available to all readers. Second, the instructions for using the JDK will be the same for all readers. Furthermore, for the simple programs presented in this book, using the JDK command-line tools is usually the easiest approach. If you are using an IDE, you will need to follow its instructions. Because of differences between IDEs, no general set of instructions can be given

Inheritance - Java Sample Program

Inheritance

 Inheritance is the process by which one object can acquire the properties of another object. This is important because it supports the concept of hierarchical classification. If you think about it, most knowledge is made manageable by hierarchical (i.e., top-down) classifications. For example, a Red Delicious apple is part of the classification apple, which in turn is part of the fruit class, which is under the larger class food. That is, the food class possesses certain qualities (edible, nutritious, etc.) which also, logically, apply to its subclass, fruit. In addition to these qualities, the fruit class has specific characteristics (juicy, sweet, etc.) that distinguish it from other food. The apple class defines those qualities specific to an apple (grows on trees, not tropical, etc.). A Red Delicious apple would, in turn, inherit all the qualities of all preceding classes, and would define only those qualities that make it unique.

 Without the use of hierarchies, each object would have to explicitly define all of its characteristics. Using inheritance, an object need only define those qualities that make it unique within its class. It can inherit its general attributes from its parent. Thus, it is the inheritance mechanism that makes it possible for one object to be a specific instance of a more general case.

Polymorphism - Java Sample Program

Polymorphism

 Polymorphism (from Greek, meaning “many forms”) is the quality that allows one interface to access a general class of actions. The specific action is determined by the exact nature of the situation. A simple example of polymorphism is found in the steering wheel of an automobile. The steering wheel (i.e., the interface) is the same no matter what type of actual steering mechanism is used. That is, the steering wheel works the same whether your car has manual steering, power steering, or rack-and-pinion steering. 

 Therefore, once you know how to operate the steering wheel, you can drive any type of car.

The same principle can also apply to programming. For example, consider a stack (which is a first-in, last-out list). You might have a program that requires three different types of stacks. One stack is used for integer values, one for floating-point values, and one for characters. In this case, the algorithm that implements each stack is the same, even though the data being stored differs. In a non-object-oriented language, you would be required to create three different sets of stack routines, with each set using different names. However, because of polymorphism, in Java you can create one general set of stack routines that works for all three specific situations. This way, once you know how to use one stack, you can use them all.

 More generally, the concept of polymorphism is often expressed by the phrase “one interface, multiple methods.” This means that it is possible to design a generic interface to a group of related activities. Polymorphism helps reduce complexity by allowing the same interface to be used to specify a general class of action. It is the compiler’s job to select the specific action (i.e., method) as it applies to each situation. You, the programmer, don’t need to do this selection manually. You need only remember and utilize the general interface.

Encapsulation - Java Sample Program

Encapsulation

 Encapsulation is a programming mechanism that binds together code and the data it manipulates, and that keeps both safe from outside interference and misuse. In an object-oriented language, code and data can be bound together in such a way that a self-contained black box is created. Within the box are all necessary data and code. When code and data are linked together in this fashion, an object is created. In other words, an object is the device that supports encapsulation.

 Within an object, code, data, or both may be private to that object or public. Private code or data is known to and accessible by only another part of the object. That is, private code or data cannot be accessed by a piece of the program that exists outside the object. When code or data is public, other parts of your program can access it even though it is defined within an object. Typically, the public parts of an object are used to provide a controlled interface to the private elements of the object.

 Java’s basic unit of encapsulation is the class. Although the class will be examined in great detail later in this book, the following brief discussion will be helpful now. A class defines the form of an object. It specifies both the data and the code that will operate on that data. Java uses a class specification to construct objects. Objects are instances of a class. Thus, a class is essentially a set of plans that specify how to build an object.

 The code and data that constitute a class are called members of the class. Specifically, the data defined by the class are referred to as member variables or instance variables. The code that operates on that data is referred to as member methods or just methods. Method is Java’s term for a subroutine. If you are familiar with C/C++, it may help to know that what a Java programmer calls a method, a C/C++ programmer calls a function.