How Object-Oriented Programming Works in Java

Drawer QR Code ISO/IEC18004 in Java How Object-Oriented Programming Works

Like subprograms, objects divide a large program into smaller, interchangeable parts The main difference is that subprograms divide a program into separate tasks whereas objects divide a program into real-world items For example, consider a hotel reservation program used by the front desk when a guest checks in Dividing this problem into tasks might create the following: Subprogram #1: RoomAvailable (Checks if a hotel room is available) Subprogram #2: RoomBeds (Checks if the room has 1 or 2 beds) Subprogram #3: RoomType (Checks if it s a smoking or a nonsmoking room) Subprogram #4: RoomPrice (Checks the price) Dividing this problem into objects, you could create the following: Object #1: Guest Object #2: Front desk clerk Object #3: Hotel room Figure 7-1 shows how a task-oriented solution might break a program into multiple subprograms The main program works by running each subprogram, one at a time, with each subprogram performing a specific task (such as determining whether a room is smoking or nonsmoking)

Main Program True or False Room available Subprogram RoomAvailable 1 or 2 beds available How many beds does the room have Smoking or Non-smoking Smoking or non-smoking room $158 What is the price Subprogram RoomPrice Subprogram RoomType Subprogram RoomBeds

Figure 7-2 shows an equivalent object-oriented solution to the same program where each object represents a real-world item Rather than having a single main program controlling multiple subprograms (like one boss controlling a dozen subordinates), OOP divides a program into multiple objects that pass messages to one another (like having a bunch of workers cooperating with one another as equals) Although both subprograms and objects solve the same problem, they use different solutions Object-oriented programming is basically a different way of thinking about how to solve problems

Figure 7-2: OOP divides a large program into objects that behave like their real-world counterparts

Objects aren t an alternative to subprograms Subprograms solve a single task Objects just organize related subprograms together

There s no single right way to divide a large program into objects Two programmers tackling the same problem will likely divide the same program into different objects The way you define your objects reflects how you view a particular problem Every object consists of two parts, as shown in Figure 7-3: Data (also called properties) Subprograms (also called methods)

Objects contain two types of data: Public data is accessible by other parts of the program Private data within the object is hidden from the rest of the program Objects contain two types of subprograms: Public subprograms allow other parts of a program to control an object Private subprograms are used by an object to calculate a result needed by its public subprograms (methods) The difference between public and private data and subprograms is accessibility: Public data and subprograms are what the rest of a program can see and use in an object: Public data typically describes the object in some way For example, a video game program might create an object that represents a monster This object may need data, representing X and Y coordinates, to define the monster s location on-screen Public subprograms (methods) allow other parts of a program to manipulate an object For example, an object representing a monster

might include a Move subprogram that can change the value of the object s X and Y coordinates (to determine where to display the cartoon monster on-screen) Private data and subprograms are what an object uses to do something useful, so the object doesn t need to allow other parts of the program to access this information The Monster object might contain a private subprogram that calculates exactly how the Monster object moves Because other parts of the program don t need to know exactly how the Monster object calculates its movement, this type of information would be private and hidden from the rest of the program Ultimately, OOP is another way to make programming easier Just as highlevel languages (like BASIC) simplify programming by using real-life commands (such as PRINT), so does OOP simplify organizing programs by modeling real-life items The three advantages that objects have over ordinary subprograms include Encapsulation Inheritance Polymorphism Object-oriented programming provides tools for making programming easier, but it s still possible to write horrible software with OOP Think of OOP like lines painted on the highway If you follow the lines, you probably arrive safely at your destination, but if you ignore the lines and do whatever you want, you probably crash your car Like traffic lines painted on the road, OOP guides you into writing software that can be created and modified easily, but you can still mess up things if you re not careful