Effective Java: Programming Language Guide in Java

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Effective Java: Programming Language Guide
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In the JCE, the system administrator registers an implementation class by editing a wellknown Properties file, adding an entry that maps a string key to the corresponding class name Clients use a static factory method that takes the key as a parameter The static factory method looks up the Class object in a map initialized from the Properties file and instantiates the class using the ClassnewInstance method The following implementation sketch illustrates this technique:
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// Provider framework sketch public abstract class Foo { // Maps String key to corresponding Class object private static Map implementations = null; // Initializes implementations map the first time it's called private static synchronized void initMapIfNecessary() { if (implementations == null) { implementations = new HashMap(); // Load implementation class names and keys from // Properties file, translate names into Class // objects using ClassforName and store mappings
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public static Foo getInstance(String key) { initMapIfNecessary(); Class c = (Class) implementationsget(key); if (c == null) return new DefaultFoo(); try { return (Foo) cnewInstance(); } catch (Exception e) { return new DefaultFoo(); }
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The main disadvantage of static factory methods is that classes without public or protected constructors cannot be subclassed The same is true for nonpublic classes returned by public static factories For example, it is impossible to subclass any of the convenience implementation classes in the Collections Framework Arguably this can be a blessing in disguise, as it encourages programmers to use composition instead of inheritance (Item 14) A second disadvantage of static factory methods is that they are not readily distinguishable from other static methods They do not stand out in API documentation in the way that constructors do Furthermore, static factory methods represent a deviation from the norm Thus it can be difficult to figure out from the class documentation how to instantiate a class that provides static factory methods instead of constructors This disadvantage can be reduced by adhering to standard naming conventions These conventions are still evolving, but two names for static factory methods are becoming common:
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Effective Java: Programming Language Guide valueOf Returns an instance that has, loosely speaking, the same value as its parameters Static factory methods with this name are effectively type-conversion operators getInstance Returns an instance that is described by its parameters but cannot be said to have the same value In the case of singletons, it returns the sole instance This name is common in provider frameworks
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In summary, static factory methods and public constructors both have their uses, and it pays to understand their relative merits Avoid the reflex to provide constructors without first considering static factories because static factories are often more appropriate If you've weighed the two options and nothing pushes you strongly in either direction, it's probably best to provide a constructor simply because it's the norm
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Item 2: Enforce the singleton property with a private constructor
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A singleton is simply a class that is instantiated exactly once [Gamma98, p 127] Singletons typically represent some system component that is intrinsically unique, such as a video display or file system There are two approaches to implementing singletons Both are based on keeping the constructor private and providing a public static member to allow clients access to the sole instance of the class In one approach, the public static member is a final field:
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// Singleton with final field public class Elvis { public static final Elvis INSTANCE = new Elvis(); private Elvis() { } } // Remainder omitted
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The private constructor is called only once, to initialize the public static final field ElvisINSTANCE The lack of public or protected constructors guarantees a monoelvistic universe: Exactly one Elvis instance will exist once the Elvis class is initialized no more, no less Nothing that a client does can change this In a second approach, a public static factory method is provided instead of the public static final field:
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Effective Java: Programming Language Guide // Singleton with static factory public class Elvis { private static final Elvis INSTANCE = new Elvis(); private Elvis() { } public static Elvis getInstance() { return INSTANCE; } } // Remainder omitted
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All calls to the static method, ElvisgetInstance, return the same object reference, and no other Elvis instance will ever be created The main advantage of the first approach is that the declarations of the members comprising the class make it clear that the class is a singleton: the public static field is final, so the field will always contain the same object reference There may also be a slight performance advantage to the first approach, but a good JVM implementation should be able to eliminate it by inlining the call to the static factory method in the second approach The main advantage of the second approach is that it gives you the flexibility to change your mind about whether the class should be a singleton without changing the API The static factory method for a singleton returns the sole instance of the class but could easily be modified to return, say, a unique instance for each thread that invokes the method On balance, then, it makes sense to use the first approach if you're absolutely sure that the class will forever remain a singleton Use the second approach if you want to reserve judgment in the matter To make a singleton class serializable ( 10), it is not sufficient merely to add
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implements Serializable to its declaration To maintain the singleton guarantee, you must also provide a readResolve method (Item 57) Otherwise, each deserialization of a serialized
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instance will result in the creation of a new instance, leading, in the case of our example, to spurious Elvis sightings To prevent this, add the following readResolve method to the Elvis class:
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// readResolve method to preserve singleton property private Object readResolve() throws ObjectStreamException { /* * Return the one true Elvis and let the garbage collector * take care of the Elvis impersonator */ return INSTANCE; }
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A unifying theme underlies this Item and Item 21, which describes the typesafe enum pattern In both cases, private constructors are used in conjunction with public static members to ensure that no new instances of the relevant class are created after it is initialized In the case of this item, only a single instance of the class is created; in Item 21, one instance is created
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