前言

"若存在则取回，若不存在则创建后取回"

这种逻辑在日常的编程中极为常见。例如带有缓存功能的对象创建工厂。

平常实现方法

首先假设我们有一个工厂类，可以创建对象，并缓存对象，对象缓存下来下次就不需要再创建一次，减少开销。工厂类大致如下：

java/**
 *
 *
 * @author beanlam
 * @date 2015年7月1日 上午10:46:47
 * @version 1.0
 *
 */
public class Factory {
    private static ConcurrentMap<String, Person> men = new ConcurrentHashMap<String, Person>();
    private static ConcurrentMap<String, Person> women = new ConcurrentHashMap<String, Person>();

    public static Man getManByName(String name) {
        if (men.get(name) == null) {
            Man newMan = new Man(name);
            men.putIfAbsent(name, newMan);
        }
        return (Man)men.get(name);
    }

    public static Woman getWomanByName(String name) {
        if (women.get(name) == null) {
            Woman newWoman = new Woman(name);
            women.putIfAbsent(name, newWoman);
        }

        return (Woman)women.get(name);
    }
}

class Person {
    String name;
    Person (String name) {
        this.name = name;
    }
}

class Man extends Person {
    Man(String name) {
        super(name);
    }
}

class Woman extends Person {
    Woman(String name) {
        super(name);
    }
}

另外一种实现

java/**
 *
 *
 * @author beanlam
 * @date 2015年7月1日 上午10:46:47
 * @version 1.0
 *
 */
public class Factory {
    private static ConcurrentMap<String, Person> men = new ConcurrentHashMap<String, Person>();
    private static ConcurrentMap<String, Person> women = new ConcurrentHashMap<String, Person>();

    public static Man getManByName(final String name) {
        return (Man)get(men, name, new Creatable<Person>() {

            @Override
            public Person create() {
                // TODO Auto-generated method stub
                return new Man(name);
            }
        });
    }

    public static Woman getWomanByName(final String name) {
        return (Woman)get(women, name, new Creatable<Person>() {

            @Override
            public Person create() {
                // TODO Auto-generated method stub
                return new Woman(name);
            }
        });
    }

    private static Person get(ConcurrentMap<String, Person> which, String name, Creatable<Person> creatable) {
        Person person = which.get(name);
        if (person == null) {
            person = creatable.create();
            Person prePerson = which.putIfAbsent(name, person);

            if (prePerson != null) {
                person = prePerson;
            }
        }

        return person;
    }

    private interface Creatable<V> {
        V create();
    }
}

class Person {
    String name;
    Person (String name) {
        this.name = name;
    }
}

class Man extends Person {
    Man(String name) {
        super(name);
    }
}

class Woman extends Person {
    Woman(String name) {
        super(name);
    }
}