0x01 什么是Monads

简单说，Monad就是一种设计模式，表示将一个运算过程，通过函数拆解成互相连接的多个步骤。你只要提供下一步运算所需的函数，整个运算就会自动进行下去。
下面这两篇文章通过图解的方式讲解了什么是Monads

• 文章一
• 文章二

0x02 为什么要使用Monads？

一段不优雅的例子

编程时检查参数是否为null是最常见的功能。以下面的代码为例子：

public static class Userdetails{
    public Address address;
    public Name name;
    
}

public static class Name{
    public String firstName;
    public String lastName;        
}

public static class Address{
    public String houseNumber;
    public Street street;
    public City city;
    
}

public static class Street{
    public String name;        
}

public static class City{
    public String name;        
}

现在我想获得user.address.street.name，常见代码如下：

 if(user == null )
    return null;
else if(user.address == null)
    return null;
else if(user.address.street == null)
    return null;
else
    return user.address.street.name;

这段检测代码淹没了真正取值的代码，阅读性非常不优雅

使用Monads来实现的例子

为了使用Lambda表达式，先定义一个函数接口: SingleArgExpression

package function;

/**
 * Created by haicheng.lhc on 13/04/2017.
 *
 * @author haicheng.lhc
 * @date 2017/04/13
 */
@FunctionalInterface
public interface SingleArgExpression<P, R> {

    public R function(P param);
}

然后创建一个类：Option

package function;

import java.lang.reflect.ParameterizedType;

/**
 * Created by haicheng.lhc on 13/04/2017.
 *
 * @author haicheng.lhc
 * @date 2017/04/13
 */
public class Option<T> {

    T value;

    public Option(T value) {
        this.value = value;
    }


    public <E> Option<E> flatMap(SingleArgExpression<T, Option<E>> mapper) {
        if (value == null) {
            return new Option<E>(null);
        }
        return mapper.function(value);

    }


    @Override
    public boolean equals(Object rhs) {
        if (rhs instanceof Option) {
            Option o = (Option) rhs;
            if (value == null) {
                return (o.value == null);
            } else {
                return value.equals(o.value);
            }
        } else {
            return false;
        }

    }

    @Override
    public int hashCode() {
        return value == null ? 0 : value.hashCode();
    }

    public T get() {
        System.out.println("hello");
        return value;
    }
}

接下来创建我们的测试类:OptionExample

package function;

/**
 * Created by haicheng.lhc on 13/04/2017.
 *
 * @author haicheng.lhc
 * @date 2017/04/13
 */
public class OptionExample {

    public static class Userdetails {

        public Option<Address> address = new Option<>(null);
        public Option<Name> name = new Option<>(null);

    }


    public static class Name {
        public Option<String> firstName = new Option<String>(null);
        public Option<String> lastName = new Option<String>(null);
    }


    public static class Address {
        public Option<String> houseNumber;
        public Option<Street> street;
        public Option<City> city;

    }


    public static class Street {
        public Option<String> name = new Option<>("alibaba");
    }


    public static class City {
        public Option<String> name;
    }

    public static void main(String[] args) {

        Option<Userdetails> userOpt = new Option<Userdetails>(new Userdetails());

        //And look how simple it is now
        String streetName = userOpt.flatMap(user -> user.address)
            .flatMap(address1 -> address1.street)
            .flatMap(street -> street.name)
            .get();
        System.out.println(streetName);

    }
}

可以看出，通过这个方法获取user.address.street.name的时候，代码是非常简洁易懂的。

思路解析：
create a class Option that represents an optional value. And lets then have a map method that will run a lambda on its wrapped value and return another option. If the wrapped value is null, it will return an Option containing null without processing the lambda, thus avaoiding a null pointer exception

0x03 参考文章

https://www.javacodegeeks.com/2014/03/functional-programming-with-java-8-lambda-expressions-monads.html