8.1 Choosing Classes Other classes are abstractions of real-life entities. BankAccount Cash Register...

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8.1 ■ Choosing Classes Other classes are abstractions of real-life entities. • BankAccount • Cash Register 337 For these classes, the properties of a typical object are easy to understand. A Rectangle object has a width and height. Given a BankAccount object, you can deposit and withdraw money. Generally, concepts from the part of the universe that a program concerns, such as science, business, or a game, make good classes. The name for such a class should be a noun that describes the concept. Some of the stan- dard Java class names are a bit strange, such as Ellipse2D.Double, but you can choose better names for your own classes. Another useful category of classes can be described as actors. Objects of an actor class do some kinds of work for you. Examples of actors are the Scanner class of Chapter 4 and the Random class in Chapter 6. A Scanner object scans a stream for numbers and strings. A Random object generates random numbers. It is a good idea to choose class names for actors that end in "-er" or "-or". (A better name for the Random class might be RandomNumber Generator.) Very occasionally, a class has no objects, but it contains a collection of related static methods and constants. The Math class is a typical example. Such a class is called a utility class. Finally, you have seen classes with only a main method. Their sole purpose is to start a program. From a design perspective, these are somewhat degenerate exam- ples of classes. What might not be a good class? If you can't tell from the class name what an object of the class is supposed to do, then you are probably not on the right track. For example, your homework assignment might ask you to write a program that prints paychecks. Suppose you start by trying to design a class PaycheckProgram. What would an object of this class do? An object of this class would have to do everything that the homework needs to do. That doesn't simplify anything. A bet- ter class would be Paycheck. Then your program can manipulate one or more Paycheck objects. Another common mistake, particularly by students who are used to writing pro- grams that consist of functions, is to turn an action into a class. For example, if your homework assignment is to compute a paycheck, you may consider writing a class ComputePaycheck. But can you visualize a "ComputePaycheck" object? The fact that "ComputePaycheck" isn't a noun tips you off that you are on the wrong track. On the other hand, a Paycheck class makes intuitive sense. The word "paycheck" is a noun. You can visualize a paycheck object. You can then think about useful meth- ods of the Paycheck class, such as computeTaxes, that help you solve the assignment. SELF CHECK 1. What is the rule of thumb for finding classes? 2. Your job is to write a program that plays chess. Might ChessBoard be an appropriate class? How about MovePiece? 8.1 ■ Choosing Classes Other classes are abstractions of real-life entities. • BankAccount • Cash Register 337 For these classes, the properties of a typical object are easy to understand. A Rectangle object has a width and height. Given a BankAccount object, you can deposit and withdraw money. Generally, concepts from the part of the universe that a program concerns, such as science, business, or a game, make good classes. The name for such a class should be a noun that describes the concept. Some of the stan- dard Java class names are a bit strange, such as Ellipse2D.Double, but you can choose better names for your own classes. Another useful category of classes can be described as actors. Objects of an actor class do some kinds of work for you. Examples of actors are the Scanner class of Chapter 4 and the Random class in Chapter 6. A Scanner object scans a stream for numbers and strings. A Random object generates random numbers. It is a good idea to choose class names for actors that end in "-er" or "-or". (A better name for the Random class might be RandomNumber Generator.) Very occasionally, a class has no objects, but it contains a collection of related static methods and constants. The Math class is a typical example. Such a class is called a utility class. Finally, you have seen classes with only a main method. Their sole purpose is to start a program. From a design perspective, these are somewhat degenerate exam- ples of classes. What might not be a good class? If you can't tell from the class name what an object of the class is supposed to do, then you are probably not on the right track. For example, your homework assignment might ask you to write a program that prints paychecks. Suppose you start by trying to design a class PaycheckProgram. What would an object of this class do? An object of this class would have to do everything that the homework needs to do. That doesn't simplify anything. A bet- ter class would be Paycheck. Then your program can manipulate one or more Paycheck objects. Another common mistake, particularly by students who are used to writing pro- grams that consist of functions, is to turn an action into a class. For example, if your homework assignment is to compute a paycheck, you may consider writing a class ComputePaycheck. But can you visualize a "ComputePaycheck" object? The fact that "ComputePaycheck" isn't a noun tips you off that you are on the wrong track. On the other hand, a Paycheck class makes intuitive sense. The word "paycheck" is a noun. You can visualize a paycheck object. You can then think about useful meth- ods of the Paycheck class, such as computeTaxes, that help you solve the assignment. SELF CHECK 1. What is the rule of thumb for finding classes? 2. Your job is to write a program that plays chess. Might ChessBoard be an appropriate class? How about MovePiece?

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1 The rule of thumb for finding classes is to look for reallife entities or concepts and to try to name the class based on a noun that describes the c... View the full answer