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22 Part I: Getting Started instance of the Accountclass) has an account number, a type, and a balance. So, the bottom row of the table is an object with account number 16-1723813344- 7. This same object has type Savings and a balance of 247.38. If you opened a new account, you would have another object, and the table would grow an additional row. The new object would be an instance of the same Accountclass. What s Next? This chapter is filled with general descriptions of things. A general description is good when you re just getting started, but you don t really understand things until you get to know some specifics. That s why the next several chapters deal with specifics. So please, turn the page. The next chapter can t wait for you to read it.
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Chapter 1: All About Java 21 Close your eyes for a minute and think about what it means for something to be a chair. . . . A chair has a seat, a back, and legs. Each seat has a shape, a color, a degree of softness, and so on. These are the properties that a chair possesses. What I describe is chairness the notion of something being a chair. In object- oriented terminology, I m describing the chair class. Now peek over the edge of this book s margin and take a minute to look around your room. (If you re not sitting in a room right now, fake it.) Several chairs are in the room, and each chair is an object. Each of these objects is an example of that ethereal thing called the Chairclass. So that s how it works the class is the idea of chairness, and each individual chair is an object. A class isn t quite a collection of things. Instead, a class is the idea behind a certain kind of thing. When I talk about the class of chairs in your room, I m talking about the fact that each chair has legs, a seat, a color, and so on. The colors may be different for different chairs in the room, but that doesn t matter. When you talk about a class of things, you re focusing on the properties that each of the things possesses. It makes sense to think of an object as being a concrete instance of a class. In fact, the official terminology is consistent with this thinking. If you write a Java program in which you define a Chairclass, each actual chair (the chair that you re sitting on, the empty chair right next to you, and so on) is called an instance of the Chairclass. Here s another way to think about a class. Imagine a table displaying all three of your bank accounts. (See Table 1-1.) Table 1-1 A Table of Accounts Account Number Type Balance 16-13154-22864-7 Checking 174.87 1011 1234 2122 0000 Credit 471.03 16-17238-13344-7 Savings 247.38 Think of the table s column headings as a class, and think of each row of the table as an object. The table s column headings describe the Accountclass. According to the table s column headings, each account has an account number, a type, and a balance. Rephrased in the terminology of object- oriented programming, each object in the Accountclass (that is, each
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20 Part I: Getting Started Figure 1-1: Terminology in object- oriented programming. The house class is Superclass Parent thesuperclass of the three-bedroom house class, theparent class of the three-bedroom house class, thesuperclass of the four-bedroom house class, theparent class of the four-bedroom house class. Subclass Child four-bedroom house class The three-bedroom house class The four-bedroom house class extends the house class, extends the house class, inherits the features of the house class,inherits the features of the house class, is a subclass of the house class, is a subclass of the house class, is a child class of the house class. is a child class of the house class. In a confident tone, you reply, We don t have to mess with the original house blueprint. If someone wants a Jacuzzi in his living room, we can make a new, small blueprint describing only the new living room and call this the Jacuzzi- in-living-room house blueprint. Then, this new blueprint can refer to the original house blueprint for info on the rest of the house (the part that s not in the living room). In the language of object-oriented programming, the Jacuzzi-inliving- room house blueprint still extends the original house blueprint. The Jacuzzi blueprint is still a subclass of the original house blueprint. In fact, all the terminology about superclass, parent class, and child class still applies. The only thing that s new is that the Jacuzzi blueprint overrides the living room features in the original house blueprint. In the days before object-oriented languages, the programming world experienced a crisis in software development. Programmers wrote code, then discovered new needs, and then had to trash their code and start from scratch. This happened over and over again because the code that the programmers were writing couldn t be reused. Object-oriented programming changed all this for the better (and, as Burd said, Java is A Great Object-Oriented Language ). Refining your understanding of classes and objects When you program in Java, you work constantly with classes and objects. These two ideas are really important. That s why, in this chapter, I hit you over the head with one analogy after another about classes and objects.
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Chapter 1: All About Java 19 instructions for building bedrooms. To make things worse, the instructions could be scattered among pages 234, 394 410, 739, 10, and 2. If the builder had to decipher other peoples complicated instructions, the task would be ten times harder. Starting with a class, however, is like starting with a blueprint. If you decide to have both three- and four-bedroom houses, you can start with a blueprint called the house blueprint that has a ground floor and a second floor, but has no indoor walls drawn on the second floor. Then, you make two more second- floor blueprints one for the three-bedroom house and another for the four- bedroom house. (You name these new blueprints the three-bedroom house blueprint and the four-bedroom house blueprint.) Your builder colleagues are amazed with your sense of logic and organization, but they have concerns. They pose a question. You called one of the blueprints the three-bedroom house blueprint. How can you do this if it s a blueprint for a second floor and not for a whole house? You smile knowingly and answer, The three-bedroom house blueprint can say, For info about the lower floors, see the original house blueprint. That way, the three-bedroom house blueprint describes a whole house. The four- bedroom house blueprint can say the same thing. With this setup, we can take advantage of all the work we already did to create the original house blueprint and save lots of money. In the language of object-oriented programming, the three- and four-bedroom house classes are inheriting the features of the original house class. You can also say that the three- and four-bedroom house classes are extending the original house class. (See Figure 1-1.) The original house class is called the superclass of the three- and four-bedroom house classes. In that vein, the three- and four-bedroom house classes are subclasses of the original house class. Put another way, the original house class is called the parent class of three- and four-bedroom house classes. The three- and four-bedroom house classes are child classes of the original house class. (See Figure 1-1.) Needless to say, your home-builder colleagues are jealous. A crowd of home- builders is mobbing around you to hear about your great ideas. So, at that moment, you drop one more bombshell: By creating a class with subclasses, we can reuse the blueprint in the future. If someone comes along and wants a five-bedroom house, we can extend our original house blueprint by making a five-bedroom house blueprint. We ll never have to spend money for an original house blueprint again. But, says a colleague in the back row, what happens if someone wants a different first-floor design? Do we trash the original house blueprint or start scribbling all over the original blueprint? That ll cost big bucks, won t it?
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18 Part I: Getting Started that features of C++ and threw them in the trash. Then he added features that made the development of objects smoother and easier. All in all, Gosling created a language whose object-oriented philosophy is pure and clean. When you program in Java, you have no choice but to work with objects. That s the way it should be. (continued) Now notice that I put the word classes first. How dare I do this! Well, maybe I m not so crazy. Think again about a housing development that s under construction. Somewhere on the lot, in a rickety trailer parked on bare dirt, is a master list of characteristics known as a blueprint. An architect s blueprint is like an object-oriented programmer s class. A blueprint is a list of characteristics that each house will have. The blueprint says, siding. The actual house object has gray siding. The blueprint says, kitchen cabinet. The actual house object has Louis XIV kitchen cabinets. The analogy doesn t end with lists of characteristics. Another important parallel exists between blueprints and classes. A year after you create the blueprint, you use it to build ten houses. It s the same with classes and objects. First, the programmer writes code to describe a class. Then when the program runs, the computer creates objects from the (blueprint) class. So that s the real relationship between classes and objects. The programmer defines a class, and from the class definition, the computer makes individual objects. What s so good about an object-oriented language? Based on the previous section s story about home building, imagine that you have already written a computer program to keep track of the building instructions for houses in a new development. Then, the big boss decides on a modified plan a plan in which half the houses have three bedrooms, and the other half have four. If you use the old FORTRAN/C style of computer programming, your instructions look like this: Dig a ditch for the basement. Lay concrete around the sides of the ditch. Put two-by-fours along the sides for the basement s frame. … This would be like an architect creating a long list of instructions instead of a blueprint. To modify the plan, you have to sort through the list to find the
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Chapter 1: All About Java 17 The winding road from FORTRAN to Java Back in the mid-1950s, a team of people created a programming language named FORTRAN. It was a good language, but it was based on the idea that you should issue direct, imperative commands to the computer. Do this, computer. Then do that, computer. (Of course, the com- mands in a real FORTRAN program were much more precise than Do this or Do that. ) In the years that followed, teams developed many new computer languages, and many of the languages copied the FORTRAN Do this/Do that model. One of the more popular Do this/Do that languages went by the one-letter name C. Of course, the Do this/Do that camp had some renegades. In languages named SIMULA and Smalltalk, programmers moved the imperative Do this commands into the back- ground and concentrated on descriptions of data. In these languages, you didn t come right out and say, Print a list of delinquent accounts. Instead, you began by saying, This is what it means to be an account. An account has a name and a balance. Then you said, This is how you ask an account whether it s delinquent. Suddenly, the data became king. An account was a thing that had a name, a balance, and a way of telling you whether it was delinquent. Languages that focus first on the data are called object-oriented programming languages. These object-oriented languages make excellent pro- gramming tools. Here s why: Thinking first about the data makes you a good computer programmer. You can extend and reuse the descriptions of data over and over again. When you try to teach old FORTRAN programs new tricks, however, the old programs show how brittle they are. They break. In the 1970s, object-oriented languages like SIMULA and Smalltalk became buried in the computer hobbyist magazine articles. In the meantime, languages based on the old FOR- TRAN model were multiplying like rabbits. So in 1986, a fellow named Bjarne Stroustrup created a language named C++. The C++ lan- guage became very popular because it mixed the old C language terminology with the improved object-oriented structure. Many com- panies turned their backs on the old FOR- TRAN/C programming style and adopted C++ as their standard. But C++ had a flaw. Using C++, you could bypass all the object-oriented features and write a program by using the old FORTRAN/C programming style. When you started writing a C++ accounting program, you could take either fork in the road: You could start by issuing direct Do this commands to the computer, saying the mathematical equivalent of Print a list of delinquent accounts, and make it snappy. You could take the object-oriented approach and begin by describing what it means to be an account. Some people said that C++ offered the best of both worlds, but others argued that the first world (the world of FORTRAN and C) shouldn t be part of modern programming. If you gave a programmer an opportunity to write code either way, the programmer would too often choose to write code the wrong way. So in 1995, James Gosling of Sun Microsystems created the language named Java. In creating Java, Gosling borrowed the look and feel of C++. But Gosling took most of the old Do this/Do (continued)
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14 Part I: Getting Started Getting Perspective: Where Java Fits In Here s a brief history of modern computer programming: 1954 1957: FORTRAN is developed. FORTRAN was the first modern computer programming language. For scientific programming, FORTRAN is a real racehorse. Year after year, FORTRAN is a leading language among computer programmers throughout the world. 1959: COBOL is created. The letter B in COBOL stands for Business, and business is just what COBOL is all about. The language s primary feature is the processing of one record after another, one customer after another, or one employee after another. Within a few years after its initial development, COBOL became the most widely used language for business data processing. Even today, COBOL represents a large part of the computer programming industry. 1972: Dennis Ritchie at AT&T Bell Labs develops the C programming language. The look and feel that you see in this book s examples come from the C programming language. Code written in C uses curly braces, ifstatements, forstatements, and so on. In terms of power, you can use C to solve the same problems that you can solve by using FORTRAN, Java, or any other modern programming language. (You can write a scientific calculator program in COBOL, but doing that sort of thing would feel really strange.) The difference between one programming language and another isn t power. The difference is ease and appropriateness of use. That s where the Java language excels. 1986: Bjarne Stroustrup (again at AT&T Bell Labs) develops C++. Unlike its C language ancestor, the language C++ supports object-oriented programming. This represents a huge step forward. (See the next section in this chapter.) May 23, 1995: Sun Microsystems releases its first official version of the Java programming language. Java improves upon the concepts in C++. Java s Write Once, Run Anywhere philosophy makes the language ideal for distributing code across the Internet. In addition, Java is a great general-purpose programming language. With Java, you can write windowed applications, build and explore databases, control handheld devices, and more. Within five short years, the Java
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Chapter 1: All About Java 13 public static void main(String args[]) { double checkAmount = 1257.63; out.print( Pay to the order of ); out.print( Dr. Barry Burd ); out.print( $ ); out.println(checkAmount); } } Why You Should Use Java It s time to celebrate! You ve just picked up a copy of Java For Dummies, 4th Edition, and you re reading Chapter 1. At this rate, you ll be an expert Java programmer in no time at all, so rejoice in your eventual success by throwing a big party. To prepare for the party, I ll bake a cake. I m lazy, so I ll use a ready-to-bake cake mix. Let me see . . . add water to the mix, and then add butter and eggs . . . Hey, wait! I just looked at the list of ingredients. What s MSG? And what about propylene glycol? That s used in antifreeze, isn t it? I ll change plans and make the cake from scratch. Sure, it s a little harder. But that way, I get exactly what I want. Computer programs work the same way. You can use somebody else s program or write your own. If you use somebody else s program, you use whatever you get. When you write your own program, you can tailor the program especially for your needs. Writing computer code is a big, worldwide industry. Companies do it, freelance professionals do it, hobbyists do it, all kinds of people do it. A typical big company has teams, departments, and divisions that write programs for the company. But you can write programs for yourself or someone else, for a living or for fun. In a recent estimate, the number of lines of code written each day by programmers in the United States alone exceeds the number of methane molecules on the planet Jupiter.* Take almost anything that can be done with a computer. With the right amount of time, you can write your own program to do it. (Of course, the right amount of time may be very long, but that s not the point. Many interesting and useful programs can be written in hours or even minutes.) * I made up this fact all by myself.
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12 Part I: Getting Started When you start dealing efficiently with ideas, something very nice happens. Suddenly, all the overhead is gone. Instead of pushing paper and trees, you re pushing numbers and concepts. Without the overhead, you can do things much faster, and do things that are far more complex than ever before. What You Can Do with Java It would be so nice if all this complexity was free, but unfortunately, it isn t. Someone has to think hard and decide exactly what the computer will be asked to do. After that thinking is done, someone has to write a set of instructions for the computer to follow. Given the current state of affairs, you can t write these instructions in English or any other language that people speak. Science fiction is filled with stories about people who say simple things to robots and get back disastrous, unexpected results. English and other such languages are unsuitable for communication with computers for several reasons: An English sentence can be misinterpreted. Chew one tablet three times a day until finished. It s difficult to weave a very complicated command in English. Join flange A to protuberance B, making sure to connect only the outermost lip of flange A to the larger end of the protuberance B, while joining the middle and inner lips of flange A to grommet C. An English sentence has lots of extra baggage. Sentence has unneeded words. English is difficult to interpret. As part of this Publishing Agreement between John Wiley & Sons, Inc. ( Wiley ) and the Author ( Barry Burd ), Wiley shall pay the sum of one-thousand-two-hundred-fifty-seven dollars and sixty-three cents ($1,257.63) to the Author for partial submittal of Java For Dummies, 4th Edition ( the Work ). To tell a computer what to do, you have to speak a special language and write terse, unambiguous instructions in that language. A special language of this kind is called a computer programming language. A set of instructions written in such a language is called a program. When they re looked at as a big blob, these instructions are called software or code. Here s what code looks like when it s written in Java: import static java.lang.System.out; class PayBarry {
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Chapter 1 All about Java In This Chapter What Java is Where Java came from Why Java is so cool How to orient yourself to object-oriented programming Say what you want about computers. As far as I m concerned, computers are good for just two simple reasons: When computers do work, they feel no resistance, no stress, no boredom, and no fatigue. Computers are our electronic slaves. I have my computer working 24/7 doing calculations for SETI@home the search for extraterrestrial intelligence. Do I feel sorry for my computer because it s working so hard? Does the computer complain? Will the computer report me to the National Labor Relations Board? No. I can make demands, give the computer its orders, and crack the whip. Do I (or should I) feel the least bit guilty? Not at all. Computers move ideas, not paper. Not long ago, when you wanted to send a message to someone, you hired a messenger. The messenger got on his or her horse and delivered your message personally. The message was on paper, parchment, a clay tablet, or whatever physical medium was available at the time. This whole process seems wasteful now, but that s only because you and I are sitting comfortably at the dawn of the electronic age. The thing is that messages are ideas. Physical things like ink, paper, and horses have little or nothing to do with real ideas. These physical things are just temporary carriers for ideas (temporary because people used them to carry ideas for several centuries). But, in truth, the ideas themselves are paperless, horseless, and messengerless. So the neat thing about computers is that they carry ideas efficiently. They carry nothing but the ideas, a couple of photons, and a little electrical power. They do this with no muss, no fuss, and no extra physical baggage.
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