Read Notes:[Effective Java] Consider static factory methods instead of Constructors
Providing a static method instead of a public constructor has both advantages and disadvantages.
One advantage of static factory methods is that, unlike constructors,they have names.
A Second advantage of static factory methods is that, unlike constructors, they are not require to create a new object each time they are invoked.
A Third advantage of static factory methods is that, unlike constructors, they can return an object of any sub-type of their return type.
A fourth advantage of static factory methods is that they reduce the verbosity of creating parameterized type instances.
// 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 instantion
// 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()
}
// Service access API
public Service newInstance() {
return newInstance(DEFAULT_PROVIDER_NAME);
}
public Service newInstance(String name) {
Provider p = providers.get(name);
if (p == null) {
throw new IllegalArgumentException("No provider register with name:" + name);
}
return p.newService();
}
}The main disadvantage of providing only static factory methods is that classes without public or protected constructors can not be sub-classed.
A second disadvantage of static factory methods is that they are not readily distinguishable from other static methods.
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原文：http://kinken.blog.51cto.com/4569049/1532801