Introduction to Object-Oriented Programming in Java: Understanding Objects and Classes, Schemes and Mind Maps of Computer science

Download Introduction to Object-Oriented Programming in Java: Understanding Objects and Classes and more Schemes and Mind Maps Computer science in PDF only on Docsity! Thinking in Java Fourth Edition Bruce Eckel President, MindView, Inc. Introduction to Objects “We cut nature up, organize it into concepts, and ascribe significances as we do, largely because we are parties to an agreement that holds throughout our speech community and is codified in the patterns of our language … we cannot talk at all except by subscribing to the organization and classification of data which the agreement decrees.” Benjamin Lee Whorf (1897-1941) The genesis of the computer revolution was in a machine. The genesis of our programming languages thus tends to look like that machine. But computers are not so much machines as they are mind amplification tools (“bicycles for the mind,” as Steve Jobs is fond of saying) and a different kind of expressive medium. As a result, the tools are beginning to look less like machines and more like parts of our minds, and also like other forms of expression such as writing, painting, sculpture, animation, and filmmaking. Object-oriented programming (OOP) is part of this movement toward using the computer as an expressive medium. This chapter will introduce you to the basic concepts of OOP, including an overview of development methods. This chapter, and this book, assumes that you have some programming experience, although not necessarily in C. If you think you need more preparation in programming before tackling this book, you should work through the Thinking in C multimedia seminar, downloadable from www.MindView.net. This chapter is background and supplementary material. Many people do not feel comfortable wading into object-oriented programming without understanding the big picture first. Thus, there are many concepts that are introduced here to give you a solid overview of OOP. However, other people may not get the big picture concepts until they’ve seen some of the mechanics first; these people may become bogged down and lost without some code to get their hands on. If you’re part of this latter group and are eager to get to the specifics of the language, feel free to jump past this chapter—skipping it at this point will not prevent you from writing programs or learning the language. However, you will want to come back here eventually to fill in your knowledge so you can understand why objects are important and how to design with them. The progress of abstraction All programming languages provide abstractions. It can be argued that the complexity of the problems you’re able to solve is directly related to the kind and quality of abstraction. By “kind” I mean, “What is it that you are abstracting?” Assembly language is a small abstraction of the underlying machine. Many so-called “imperative” languages that followed (such as FORTRAN, BASIC, and C) were abstractions of assembly language. These languages are big improvements over assembly language, but their primary abstraction still requires you to think in terms of the structure of the computer rather than the structure of the problem you are trying to solve. The programmer must establish the association between the machine model (in the “solution space,” which is the place where you’re implementing that solution, such as a computer) and the model of the problem that is actually being solved (in the   Once a class is established, you can make as many objects of that class as you like, and then manipulate those objects as if they are the elements that exist in the problem you are trying to solve. Indeed, one of the challenges of object-oriented programming is to create a one-to- one mapping between the elements in the problem space and objects in the solution space. But how do you get an object to do useful work for you? There needs to be a way to make a request of the object so that it will do something, such as complete a transaction, draw something on the screen, or turn on a switch. And each object can satisfy only certain requests. The requests you can make of an object are defined by its interface, and the type is what determines the interface. A simple example might be a representation of a light bulb: Light lt = new Light(); lt.on(); The interface determines the requests that you can make for a particular object. However, there must be code somewhere to satisfy that request. This, along with the hidden data, comprises the implementation. From a procedural programming standpoint, it’s not that complicated. A type has a method associated with each possible request, and when you make a particular request to an object, that method is called. This process is usually summarized by saying that you “send a message” (make a request) to an object, and the object figures out what to do with that message (it executes code). Here, the name of the type/class is Light, the name of this particular Light object is lt, and the requests that you can make of a Light object are to turn it on, turn it off, make it brighter, or make it dimmer. You create a Light object by defining a “reference” (lt) for that object and calling new to request a new object of that type. To send a message to the object, you state the name of the object and connect it to the message request with a period (dot). From the standpoint of the user of a predefined class, that’s pretty much all there is to programming with objects. The preceding diagram follows the format of the Unified Modeling Language (UML). Each class is represented by a box, with the type name in the top portion of the box, any data members that you care to describe in the middle portion of the box, and the methods (the functions that belong to this object, which receive any messages you send to that object) in the bottom portion of the box. Often, only the name of the class and the public methods are shown in UML design diagrams, so the middle portion is not shown, as in this case. If you’re interested only in the class name, then the bottom portion doesn’t need to be shown, either. An object provides services While you’re trying to develop or understand a program design, one of the best ways to think about objects is as “service providers.” Your program itself will provide services to the user, and it will accomplish this by using the services offered by other objects. Your goal is to 18 Thinking in Java Bruce Eckel Introduction to Objects 19  produce (or even better, locate in existing code libraries) a set of objects that provide the ideal services to solve your problem. A way to start doing this is to ask, “If I could magically pull them out of a hat, what objects would solve my problem right away?” For example, suppose you are creating a bookkeeping program. You might imagine some objects that contain pre-defined bookkeeping input screens, another set of objects that perform bookkeeping calculations, and an object that handles printing of checks and invoices on all different kinds of printers. Maybe some of these objects already exist, and for the ones that don’t, what would they look like? What services would those objects provide, and what objects would they need to fulfill their obligations? If you keep doing this, you will eventually reach a point where you can say either, “That object seems simple enough to sit down and write” or “I’m sure that object must exist already.” This is a reasonable way to decompose a problem into a set of objects. Thinking of an object as a service provider has an additional benefit: It helps to improve the cohesiveness of the object. High cohesion is a fundamental quality of software design: It means that the various aspects of a software component (such as an object, although this could also apply to a method or a library of objects) “fit together” well. One problem people have when designing objects is cramming too much functionality into one object. For example, in your check printing module, you may decide you need an object that knows all about formatting and printing. You’ll probably discover that this is too much for one object, and that what you need is three or more objects. One object might be a catalog of all the possible check layouts, which can be queried for information about how to print a check. One object or set of objects can be a generic printing interface that knows all about different kinds of printers (but nothing about bookkeeping—this one is a candidate for buying rather than writing yourself). And a third object could use the services of the other two to accomplish the task. Thus, each object has a cohesive set of services it offers. In a good object-oriented design, each object does one thing well, but doesn’t try to do too much. This not only allows the discovery of objects that might be purchased (the printer interface object), but it also produces new objects that might be reused somewhere else (the catalog of check layouts). Treating objects as service providers is a great simplifying tool. This is useful not only during the design process, but also when someone else is trying to understand your code or reuse an object. If they can see the value of the object based on what service it provides, it makes it much easier to fit it into the design. The hidden implementation It is helpful to break up the playing field into class creators (those who create new data types) and client programmers4 (the class consumers who use the data types in their applications). The goal of the client programmer is to collect a toolbox full of classes to use for rapid application development. The goal of the class creator is to build a class that exposes only what’s necessary to the client programmer and keeps everything else hidden. Why? Because if it’s hidden, the client programmer can’t access it, which means that the class creator can change the hidden portion at will without worrying about the impact on anyone else. The hidden portion usually represents the tender insides of an object that could easily be corrupted by a careless or uninformed client programmer, so hiding the implementation reduces program bugs. In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the client programmer, who is also a programmer, but one who is putting together an application by using your library, possibly to build a bigger library. If all the members of a class are available to everyone, then the client programmer can do anything with that class and there’s no way to enforce rules. Even though you might really prefer that the client programmer not directly                                                              4 I’m indebted to my friend Scott Meyers for this term. manipulate some of the members of your class, without access control there’s no way to prevent it. Everything’s naked to the world. So the first reason for access control is to keep client programmers’ hands off portions they shouldn’t touch—parts that are necessary for the internal operation of the data type but not part of the interface that users need in order to solve their particular problems. This is actually a service to client programmers because they can easily see what’s important to them and what they can ignore. The second reason for access control is to allow the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. For example, you might implement a particular class in a simple fashion to ease development, and then later discover that you need to rewrite it in order to make it run faster. If the interface and implementation are clearly separated and protected, you can accomplish this easily. Java uses three explicit keywords to set the boundaries in a class: public, private, and protected. These access specifiers determine who can use the definitions that follow. public means the following element is available to everyone. The private keyword, on the other hand, means that no one can access that element except you, the creator of the type, inside methods of that type. private is a brick wall between you and the client programmer. Someone who tries to access a private member will get a compile-time error. The protected keyword acts like private, with the exception that an inheriting class has access to protected members, but not private members. Inheritance will be introduced shortly. Java also has a “default” access, which comes into play if you don’t use one of the aforementioned specifiers. This is usually called package access because classes can access the members of other classes in the same package (library component), but outside of the package those same members appear to be private. Reusing the implementation Once a class has been created and tested, it should (ideally) represent a useful unit of code. It turns out that this reusability is not nearly so easy to achieve as many would hope; it takes experience and insight to produce a reusable object design. But once you have such a design, it begs to be reused. Code reuse is one of the greatest advantages that object-oriented programming languages provide. The simplest way to reuse a class is to just use an object of that class directly, but you can also place an object of that class inside a new class. We call this “creating a member object.” Your new class can be made up of any number and type of other objects, in any combination that you need to achieve the functionality desired in your new class. Because you are composing a new class from existing classes, this concept is called composition (if the composition happens dynamically, it’s usually called aggregation). Composition is often referred to as a “has-a” relationship, as in “A car has an engine.” 20 Thinking in Java Bruce Eckel