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CHAPTER 5: Composition
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Accessor Methods
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Programmers are rarely satisfied with the programs they write, because software is never finished. There s always one more bug to fix, one more feature to add, or one more way to make the program bigger, stronger, or faster. So it s no surprise that CarParts isn t perfect yet. We can improve it and make its code more flexible by using accessor methods. The code for this new version can be found in the 05.02 CarParts-Accessors folder. An experienced programmer looking at Car s init method might say, Why is the car creating its own tires and engine The program would be much better if you could customize the car to use different kinds of tires (such as snow tires for the winter months) or various types of engines (fuel injected rather than carbureted). It would be nice if we could instruct the car to use a particular tire or engine. We could then let users mix and match car parts to create custom vehicles. We can make this happen by adding accessor methods. An accessor method is one that reads or changes a specific attribute for an object. For instance, setFillColor: in Shapes-Object is an accessor method. If we added a new method to change the engine in a Car object, it would be an accessor method. This particular kind of accessor method is called a setter method, because it sets a value on an object. You might hear the term mutator used for a method that changes an object state. You ve probably already guessed that another kind of accessor method is a getter. A getter method provides a way for code that uses an object to access its attributes. In a racing game, the physics logic would want to access attributes of the car s tires to figure out if the car will skid on wet pavement at its current speed.
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NOTE
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You should always use any provided accessor methods when manipulating another object s attributes never reach into an object and change values directly. For example, main() should not directly access the Car s engine instance variable (using car->engine) to change its engine. Instead, your code should use a setter method to make the change. Accessor methods are yet another example of indirection at work. By accessing the car s engine indirectly via an accessor method, you re allowing for flexibility in the car s implementation.
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Let s add some setter and getter methods to Car so the code that uses it has control over the kinds of tires and engine used. Here is the new interface for Car, with the new items in bold:
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CHAPTER 5: Composition
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@interface Car : NSObject { Engine *engine; Tire *tires[4]; } - (Engine *) engine; - (void) setEngine: (Engine *) newEngine;
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- (Tire *) tireAtIndex: (int) index; - (void) setTire: (Tire *) tire atIndex: (int) index; - (void) print; @end // Car
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The set of instance variables hasn t changed, but there are two new pairs of methods: engine and setEngine: deal with the engine attributes, and tireAtIndex: and setTire:atIndex: work with the tires. Accessor methods almost always come in pairs, one to set the value and one to get it. Occasionally, having only a getter (for a read-only attribute, like the size of a file on disk) or only a setter (like setting a secret password) might make sense, but most often, you ll be writing both setters and getters. Cocoa has conventions for naming accessor methods. When you re writing accessor methods for your own classes, you should follow these conventions so that you and other people reading your code won t get confused. Setter methods are named after the attribute they change, preceded by the word set. Here are examples of names of setter methods: setEngine:, setStringValue: , setFont:, setFillColor:, and setTextLineHeight:. Getter methods are simply named after the attribute they return. The getters corresponding to the preceding setters would be named engine, stringValue, font, fillColor, and textLineHeight. Don t use the word get in the name of the method. For example, methods named getStringValue and getFont would violate the convention. Some languages, such as Java, have different conventions that use get in the name of accessor methods, but if you re writing Cocoa code, don t use it.
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