Question

使用Stream API获取一对一映射时遇到一些麻烦。基本上，假设你上课了。

public class Item {
    private final String uuid;

    private Item(String uuid) {
        this.uuid = uuid;
    }

    /**
    * @return universally unique identifier
    */
    public String getUuid() {
        return uuid;
    }
}

我希望Map<String, Item>能够轻松查找。但是考虑到Stream<Item>，似乎并不是一种简单的方法来获得Map<String, Item>。

显然，Map<String, List<Item>>无所谓：

public static Map<String, List<Item>> streamToOneToMany(Stream<Item> itemStream) {
    return itemStream.collect(groupingBy(Item::getUuid));
}

这是更安全更普遍的情况，但我们做知道在这种情况下只会一对一。我无法找到任何可编译的内容 - 我特意尝试将downstream参数设置为Collectors.groupingBy。类似的东西：

// DOESN'T COMPILE
public static Map<String, Item> streamToOneToOne(Stream<Item> itemStream) {
    return itemStream.collect(groupingBy(Item::getUuid, Function.identity()));
}

我错过了什么？

Answer 1

使用Collectors#toMap(Function, Function)，生成每个Item uuid和Item的密钥作为值本身。

public static Map<String, Item> streamToOneToOne(Stream<Item> itemStream) {
    return itemStream.collect(Collectors.toMap(Item::getUuid, Function.identity()));
}

来自javadoc的注释

如果映射的密钥包含重复项（根据 Object.equals(Object)），当IllegalStateException时抛出toMap(Function, Function, BinaryOperator) 执行收集操作。如果映射的键可能有重复，改为使用try{}。

Answer 2

import requests from bs4 import BeautifulSoup import csv import re import time import logging try: def parse_page(data): '#Sleep to avoid excessive requests' time.sleep(1) subsoup = BeautifulSoup(data,"html.parser") rs = requests.get("http://www.bbc.co.uk/sport/0/football/31776459") ssubsoup = BeautifulSoup(rs.content,"html.parser") matchoverview = subsoup.find('div', attrs={'id':'match-overview'}) print '--------------' date = subsoup.find('div', attrs={'id':'article-sidebar'}).findNext('span').text league = subsoup.find('a', attrs={'class':'secondary-nav__link'}).findNext('span').findNext('span').text #HomeTeam info printing homeTeam = matchoverview.find('div', attrs={'class':'team-match-details'}).findNext('span').findNext('a').text homeScore = matchoverview.find('div', attrs={'class':'team-match-details'}).findNext('span').findNext('span').text homeGoalScorers = [] for goals in matchoverview.find('div', attrs={'class':'team-match-details'}).findNext('p').find_all('span'): homeGoalScorers.append(goals.text.replace(u'\u2032', "'")) homeGoals = homeGoalScorers homeGoals2 = ''.join(homeGoals) homeGoals3 = re.sub("[^0-9']","",homeGoals2) homeGoals4 = homeGoals3.replace("'","',") homeGoals5 = homeGoals4.replace("'","H") if homeScore == '0': homeGoals5 ='' #AwayTeam info printing awayTeam = matchoverview.find('div', attrs={'id': 'away-team'}).find('div', attrs={'class':'team-match-details'}).findNext('span').findNext('a').text awayScore = matchoverview.find('div', attrs={'id': 'away-team'}).find('div', attrs={'class':'team-match-details'}).findNext('span').findNext('span').text awayGoalScorers = [] for goals in matchoverview.find('div', attrs={'id': 'away-team'}).find('div', attrs={'class':'team-match-details'}).findNext('p').find_all('span'): awayGoalScorers.append(goals.text.replace(u'\u2032', "'")) awayGoals = awayGoalScorers awayGoals2 = ''.join(awayGoals) awayGoals3 = re.sub("[^0-9']","",awayGoals2) awayGoals4 = awayGoals3.replace("'","',") awayGoals5 = awayGoals4.replace("'","A") if awayScore == '0': awayGoals5 ='' #combine scores scores = homeGoals5+awayGoals5 #Printouts print date print league print '{0} {1} - {2} {3}'.format(homeTeam, homeScore, awayTeam, awayScore) print scores if len(homeTeam) >1: with open('score.txt', 'a') as f: writer = csv.writer(f) writer.writerow([league,date,homeTeam,awayTeam,scores]) except: print "Item Missing" with open('score.txt', 'a') as f: writer = csv.writer(f) writer.writerow(["--Item Missing--"]) pass def all_league_results(): r = requests.get("http://www.bbc.co.uk/sport/football/league-two/results") soup = BeautifulSoup(r.content,"html.parser") # Save Teams for link in soup.find_all("a", attrs={'class': 'report'}): fullLink = 'http://www.bbc.com' + link['href'] time.sleep(2) subr = requests.get(fullLink) logging.basicConfig(filename='LogReport.log',level=logging.DEBUG) logging.debug('DEBUG ERROR:') logging.info('INFO ERROR:') logging.warning('WARNING ERROR:') logging.error('ERROR WARNING:') logging.critical('CRITICAL ERROR:') parse_page(subr.text) def specific_game_results(url): subr = requests.get(url) parse_page(subr.text) #get specific games results #specific_game_results('http://www.bbc.co.uk/sport/0/football/31776459') #get all current league results all_league_results()通过密钥收集项目（复数，groupingBy()）。

您想要List：

toMap()

Answer 3

也许试试

itemStream.stream().collect(toMap(Item::getUuid,Functions.identity());

使用Java流的一对一地图

3 个答案: