Guava学习笔记之Maps(1)：Maps.uniqueIndex(Iterable, Function)

guava官方文档 https://github.com/google/guava/wiki/collectionutilitiesexplained

官方文档这样描述：

[`maps.uniqueindex(iterable, function)`](http://google.github.io/guava/releases/snapshot/api/docs/com/google/common/collect/maps.html#uniqueindex-java.lang.iterable-com.google.common.base.function-) addresses the common case of having a bunch of objects that each have some unique attribute, and wanting to be able to look up those objects based on that attribute. 
大概意思：描述了这样一种常见情况：有一大堆对象，每个对象都有一些独特的属性，能够根据该独特属性查找到对应对象。

demo1:

import com.google.common.collect.immutablemap;
import com.google.common.collect.lists;
import com.google.common.collect.maps;
import java.util.arraylist;
import java.util.function.function;

/**
 * @author: lishuai
 * @date: 2018/12/4 10:20
 * @description
 */
public class application {
    public static void main(string[] args) {
        // nickname属性能唯一确定一个webuser
        arraylist<webuser> users = lists.newarraylist(new webuser(1,"one"),new webuser(2,"two"),new webuser(1,"three"),new webuser(1,"four"));
        // 得到以nickname为key，webuser为值的一个map
        immutablemap<string, webuser> map = maps.uniqueindex(users,new com.google.common.base.function<webuser, string>() {
            @override
            public string apply(webuser user) {
                return user.getnickname();
            }
        });
        system.err.println("map:" + map);
        system.err.println("name:" + map.get("two").getnickname());
    }
}

class webuser {
    private integer sid;
    private string nickname;

    public webuser(integer sid, string nickname) {
        this.sid = sid;
        this.nickname = nickname;
    }

  @override
    public string tostring() {
        return "webuser{" +
                "sid=" + sid +
                ", nickname='" + nickname + '\'' +
                '}';
    }
    
    // set、get省略
}

可以进一步简化：

immutablemap<string, webuser> map = maps.uniqueindex(users,webuser::getnickname);

结果：

map:{one=webuser{sid=1, nickname='one'}, two=webuser{sid=2, nickname='two'}, three=webuser{sid=1, nickname='three'}, four=webuser{sid=1, nickname='four'}}

name:two

demo2:

// nickname属性不能唯一确定一个webuser（有两个元素的nickname是"one"）
arraylist<webuser> users = lists.newarraylist(new webuser(1,"one"),new webuser(2,"one"),new webuser(1,"three"),new webuser(1,"four"));

结果：

exception in thread "main" java.lang.illegalargumentexception: multiple entries with same key: one=webuser{sid=2, nickname='one'} and one=webuser{sid=1, nickname='one'}. to index multiple values under a key, use multimaps.index.

由此可见，必须确保key的唯一性。