Guava學習筆記之Maps(1)：Maps.uniqueIndex(Iterable, Function)

Guava官方文檔 https://github.com/google/guava/wiki/CollectionUtilitiesExplained

官方文檔這樣描述：

[`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：描述了這樣一種常見情況：有一大堆對象，每個對象都有一些獨特的屬性，能夠根據該獨特屬性查找到對應對象。

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屬性能唯一確定一個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為key，WebUser為值的一個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
}

可以進一步簡化：

ImmutableMap<String, WebUser> map = Maps.uniqueIndex(users,WebUser::getNickname);

結果：

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屬性不能唯一確定一個WebUser（有兩個元素的nickname是"one"）
ArrayList<WebUser> users = Lists.newArrayList(new WebUser(1,"one"),new WebUser(2,"one"),new WebUser(1,"three"),new WebUser(1,"four"));

結果：

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.

由此可見，必須確保key的唯一性。