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Logger logger; public void initialize() { initLogger(); } private void initLogger() { logger = Logger.getLogger("not.the.real.mapplet"); }
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There are, admittedly, rare cases when initialization code must be run only once in a program s lifetime. But in such cases, the static initializer block still isn t the answer, as we return to the question of error handling. In such cases, you ll need a more robust design that blocks during initialization, and throws an error if initialization failed.
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7. Use Object-Level Methods and Variables
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Remember the too many chefs spoil the broth problem from 9 The solution to the chef problem is, assuming that 11 of the chefs can t be fired, to give each one his or her own saucepan. Don t make use of static class-level methods and variables. Assign members to objects, not to classes. Let s return to HotelPriceCalculator and its static members, nyInstance and nyHotels. These are both symptomatic of a dysfunctional design. Why on earth would a price calculator class need to contain a reference to a bunch of New York hotels Some further digging suggests that this code was added in by a well-meaning programmer who wanted to test the price calculator with specific data, but hadn t heard of unit tests. So we have the remnants of test code mixed in with the product code, obfuscating the design. The code serves no other purpose, so in this case at least the answer s easy: delete it!
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6. Avoid the Singleton Design Pattern
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In 9 we donned our pantomime outfits and jumped around the stage encouraging you to boo and hiss at the Singleton design pattern; we also demonstrated why this particular pattern is so problematic. But for all of that, there is frequently a need for a class that has only one instance. There are two solutions to the problem of creating a single-instance object. Which to use depends upon whether you genuinely need a single-instance-only object. Here are the two solutions: Don t make your object a Singleton (capital S): Allow any class to create and dispose of instances of your one-object class. Apply some other framework or pattern: Deem your class to be a singleton (lowercase S), but use some other framework or pattern to achieve that goal. For example, apply the Spring/Inversion of Control pattern, which maintains the object s lifecycle for you. Or apply a Registry class that your
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CHAPTER 10 DESIGN FOR EASIER TESTING
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run-time code can use to get at a single instance of a class without preventing unit tests from instantiating their own limited-scope copy.4 Quite often, an object that must be a singleton in the deployed product doesn t have the same requirement when it s being unit-tested. In this case, Option 2 is the better option. In all other cases, give yourself a break and go for Option 1. For PriceStreamer, we ve gone with Option 2. This does mean that an extra Registry class is involved, but the improvements in stability, extensibility, testability, all those ilities, make it worthwhile especially as it can also be used to store any other deemed-singleton objects that you need. The registry class isn t used by the unit tests; they can simply create their own instance via the PriceStreamer s public constructor. You don t need a big framework to do what s needed, though. Object registries are easy to implement. Here s a simple one in its entirety: public class SimpleRegistry { public static final Map<Class, Object> lookup = new HashMap<Class, Object>(); } To further simplify things, any object that uses the registry can do a static import at the top of the class. Here is an example: import static com.softwarereality.nottherealmapplet.SimpleRegistry.*; In the program s startup code, add this line to create your single instance: lookup.put(PriceStreamer.class, new PriceStreamer());
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Tip You can further decouple this design by defining an interface for PriceStreamer, and using the interface as the lookup key instead of the concrete class.
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And then getting your single instance is as simple as doing this: PriceStreamer streamer = (PriceStreamer) lookup.get(PriceStreamer.class); This approach limits the use of static members to a single place in the code. Of course, it still violates the principle of encapsulation that we ve discussed in this chapter. However, the reason it gets away with it is that the sole purpose of the registry class s existence is to create and keep a single instance
4 A registry of objects is a single go-to place, a one-stop shop for all your single-instance needs. A registry is the basis for Cairngorm, Adobe s official application framework for Flex. Sometimes denigrated as being the reintroduction of global variables, it does at least solve the occasional case where you genuinely need a singleton object. Spring Framework takes a different approach, by allowing you to mark a Java bean as a singleton; the framework takes care of the lifecycle details without preventing unit tests from creating their own copies.
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