Does the class below implement the singleton pattern? I'm really confused.
class Customer { static private Map costumers = new HashMap(); private Customer() { } public static Customer getInstance(String name) { Customer instance = (Customer) costumers.get(name); if (instance == null) { instance = new Customer(); costumers.put(name, instance); } return instance; } } Thanks.
No, it doesn't. Thread-safety and type-safety issues aside, I can very easily get two different instances of Customer:
Customer x = Customer.getInstance("first"); Customer y = Customer.getInstance("second"); System.out.println(x == y); // false Therefore it's not a singleton. It's a sort of factory/flyweight pattern hybrid, but it's certainly not the singleton pattern.
Its not. In the given example, a new Customer instance is created for each different Customer name. Which should not happen in case of singleton. There sould be only one Customer instance created and shared for all the customer names. The Customer instance needs to be private static and it shoud be created and assigned only once.
public class SingltonPattern { private static SingltonPattern instance = null; protected SingltonPattern() {} public static SingltonPattern getInstance() { if (instance == null) { // Thread Safe - Second layer synchronized (SingltonPattern.class) { if (instance == null) { instance = new SingltonPattern(); } } } return instance; } Here is a pattern for Singlton DP.
instance variable directly. It's very rare that you need this more complicated and error-prone pattern. (Even if you want laziness with the ability to call other static methods without initializing, a nested class is a simpler approach.) And if you're not addressing the OP's question, it doesn't belong as an answer to the question, IMO.