package com.nicchagil.study;public interface Flyweight {}

package com.nicchagil.study;public class User implements Flyweight {private String id;public User(String id) {super();this.id = id;}public String getId() {return id;}@Overridepublic String toString() {StringBuilder builder = new StringBuilder();builder.append("User [id=").append(id).append("]");return builder.toString();}/* 注意，不应该有此方法，及类似的可改变对象内容的逻辑，因为不允许对象的内容发生变更 *//*public void setId(String id) {this.id = id;}*/}

package com.nicchagil.study;import java.util.HashMap;
import java.util.Map;public class UserFlyweightFactory {private static Map<String, Flyweight> flyweights = new HashMap<String, Flyweight>();public static Flyweight getInstance(String key) {/* 简易写法 */synchronized (flyweights) {if (flyweights.get(key) == null) {flyweights.put(key, new User(key));}}/* 双重判断写法 *//*if (flyweights.get(key) == null) {synchronized (flyweights) {if (flyweights.get(key) == null) {flyweights.put(key, new User(key));}}}*/return flyweights.get(key);}}

package com.nicchagil.study;public class HowToUse {public static void main(String[] args) {Flyweight flyweight1 = UserFlyweightFactory.getInstance("1");Flyweight flyweight2 = UserFlyweightFactory.getInstance("2");Flyweight flyweight3 = UserFlyweightFactory.getInstance("1");System.out.println("flyweight1 == flyweight2 -> " + (flyweight1 == flyweight2));System.out.println("flyweight1 == flyweight3 -> " + (flyweight1 == flyweight3));}}