Java学习笔记（一）の考虑用静态工厂方法代替构造器 (一)

静态工厂方法与构造器不同的第一大优势在于，它们有名称。

静态工厂方法与构造器不同的第二大优势在于，不必在每次调用它们的时候都创建一个新对象。

静态工厂方法与构造器不同的第三大优势在于，它们可以返回原返回类型的任何子类型的对象。

静态工厂方法的第四大优势在于，在创建参数化类型实例的时候，它们使代码变得更加简洁。

静态工厂方法的主要缺点在于，类如果不含有公有的或受保护的构造器，就不能被子类化。

静态工厂方法的第二个缺点在于，它们与去他的静态方法实际上没有任何区别。

服务提供者框架：

[java] // Service provider framework sketch - Service interface

public interface Service {
// Service-specific methods go here
}
// Service provider framework sketch - Service interface

public interface Service {
// Service-specific methods go here
}
[java] // Service provider framework sketch - Service provider interface

public interface Provider {
Service newService();
}
// Service provider framework sketch - Service provider interface

public interface Provider {
Service newService();
}[java] // Service provider framework sketch

// Noninstantiable class for service registration and access

import java.util.*;
import java.util.concurrent.*;

public class Services {
private Services() { } // Prevents instantiation (Item 4)

// Maps service names to services
private static final Map providers =
new ConcurrentHashMap();
public static final String DEFAULT_PROVIDER_NAME = "";

// 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();
}
}
// Service provider framework sketch

// Noninstantiable class for service registration and access

import java.util.*;
import java.util.concurrent.*;

public class Services {
private Services() { } // Prevents instantiation (Item 4)

// Maps service names to services
private static final Map providers =
new ConcurrentHashMap();
public static final String DEFAULT_PROVIDER_NAME = "";

// 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();
}
}
[java] // Simple test program for service provider framework

public class Test {
public static void main(String[] args) {
// Providers would execute these lines
Services.registerDefaultProvider(DEFAULT_PROVIDER);
Services.registerProvider("comp", COMP_PROVIDER);
Services.registerProvider("armed", ARMED_PROVIDER);

// Clients would execute these lines
Service s1 = Services.newInstance();