Advantage
Unlike constructors, they have names. (BigInteger.probablePrime vs BigInteger(int, int, Random) They are not required to create a new object each time they're invoked(Flyweight pattern). This ensures that a.equals(b) if and only if a = = b. Then the client can use = = instead of equals method which gets better performance to Equals method. It can return an object of any subtype of their return type.(This gives you great flexibility in choosing the class of the returned object. )/********************sample code ******************/
// Service provider framework sketch
// Service interface
public interface Service {
... // Service-specific methods go here
}
// Service provider interface
public interface Provider {
Service newService();
}
// Noninstantiable class for service registration and access
public class Services {
private Services() {
} // Prevents instantiation (Item 4)
// Maps service names to services
private static final Map<String, Provider> providers = new ConcurrentHashMap<String, Provider>();
public static final String DEFAULT_PROVIDER_NAME = "<def>";
// Provider registration API
public static void registerDefaultProvider(Provider p) {
registerProvider(DEFAULT_PROVIDER_NAME, p);
}
public static void registerProvider(String name, Provider p) {
providers.put(name, p);
}
// Service access API
public static Service newInstance() {
return newInstance(DEFAULT_PROVIDER_NAME);
}
public static Service newInstance(String name) {
Provider p = providers.get(name);
if (p == null)
throw new IllegalArgumentException(
"No provider registered with name: " + name);
return p.newService();
}
}
Reduce the verbosity of creating parameterized type instances.Disadvantages The class without public or protected constructors cannot be subclassed. They are not readily distinguishable from other static methods.
Here are some common names for static factory methods:
• valueOf—Returns an instance that has, loosely speaking, the same value as its
parameters. Such static factories are effectively type-conversion methods.
• of—A concise alternative to valueOf, popularized by EnumSet(Item 32).
• getInstance—Returns an instance that is described by the parameters but
cannot be said to have the same value. In the case of a singleton, getInstance
takes no parameters and returns the sole instance.
• newInstance—LikegetInstance, except that newInstanceguarantees that
each instance returned is distinct from all others.
• getType—Like getInstance, but used when the factory method is in a different class. Type indicates the type of object returned by the factory method.
• newType—Like newInstance, but used when the factory method is in a different class. Type indicates the type of object returned by the factory method.
转载于:https://www.cnblogs.com/haokaibo/p/consider-static-factory-methods-instead-of-constructors.html
