{"id":181,"date":"2018-09-28T11:36:53","date_gmt":"2018-09-28T03:36:53","guid":{"rendered":"https:\/\/www.espandy.com\/?p=181"},"modified":"2018-09-28T11:36:53","modified_gmt":"2018-09-28T03:36:53","slug":"stream-collectors-groupingby-%e4%be%8b%e5%ad%90","status":"publish","type":"post","link":"https:\/\/www.espandy.com\/?p=181","title":{"rendered":"Stream Collectors groupingBy \u4f8b\u5b50"},"content":{"rendered":"<\/p>\n

Stream Collectors groupingBy \u4f8b\u5b50 <\/span><\/span><\/p>\n

\u8f6c\u8f7d\uff1a<\/b><\/span>https:\/\/my.oschina.net\/LiuLangEr\/blog\/1632378<\/a><\/p>\n

 <\/a>\u952e\u76d8\u4e0a\u8df3\u821e<\/a> <\/span>\u53d1\u5e03\u4e8e 03\/09 16:06<\/span> <\/span><\/p>\n

\u5728\u8fd9\u7bc7\u6587\u7ae0\u4e2d\uff0c\u6211\u4eec\u5c06\u5411\u60a8\u5c55\u793a\u5982\u4f55\u4f7f\u7528java 8  Stream <\/span>Collectors<\/span> \u5bf9\u5217\u8868\u5206\u7ec4\uff0c\u8ba1\u6570\uff0c\u6c42\u548c\u548c\u6392\u5e8f\u3002<\/span><\/p>\n

1. Group By, Count and Sort<\/span><\/h2>\n

1.1 Group by a <\/span>List<\/span> and display the total count of it.\uff08\u6309\u5217\u8868\u5206\u7ec4\uff0c\u5e76\u663e\u793a\u5176\u603b\u6570\uff09<\/span><\/p>\n

Java8Example1.java<\/span><\/p>\n

package com.mkyong.java8;<\/span><\/p>\n

import<\/span> java.util.Arrays;<\/span><\/p>\n

import<\/span> java.util.List;<\/span><\/p>\n

import<\/span> java.util.Map;<\/span><\/p>\n

import<\/span> java.util.function.<\/span>Function<\/span>;<\/span><\/p>\n

import<\/span> java.util.stream.Collectors;<\/span><\/p>\n

public<\/span> class<\/span> Java8Example1<\/span> {<\/span><\/p>\n

public<\/span> static<\/span> void<\/span> main(<\/span>String<\/span>[] args) {<\/span><\/p>\n

\/\/3 apple, 2 banana, others 1<\/span><\/p>\n

List<<\/span>String<\/span>> items =<\/span><\/p>\n

Arrays.asList(<\/span>"apple"<\/span>,<\/span> "apple"<\/span>,<\/span> "banana"<\/span>,<\/span><\/p>\n

"apple"<\/span>,<\/span> "orange"<\/span>,<\/span> "banana"<\/span>,<\/span> "papaya"<\/span>);<\/span><\/p>\n

Map<\/span><<\/span>String<\/span>, Long> result =<\/span><\/p>\n

items.stream().collect(<\/span><\/p>\n

Collectors.groupingBy(<\/span><\/p>\n

Function<\/span>.identity(), Collectors.counting()<\/span><\/p>\n

)<\/span><\/p>\n

);<\/span><\/p>\n

System.out.println(result);<\/span><\/p>\n

}<\/span><\/p>\n

}<\/span><\/p>\n

output<\/span><\/p>\n

{<\/span><\/p>\n

    papaya=1, orange=1, banana=2, apple=3<\/span><\/p>\n

}<\/span><\/p>\n

1.2 Add sorting.\uff08\u589e\u52a0\u6392\u5e8f\u5b9e\u73b0\uff09<\/span><\/p>\n

Java8Example2.java<\/span><\/p>\n

package com.mkyong.java8;<\/span><\/p>\n

import<\/span> java.util.Arrays;<\/span><\/p>\n

import<\/span> java.util.LinkedHashMap;<\/span><\/p>\n

import<\/span> java.util.List;<\/span><\/p>\n

import<\/span> java.util.Map;<\/span><\/p>\n

import<\/span> java.util.function.<\/span>Function<\/span>;<\/span><\/p>\n

import<\/span> java.util.stream.Collectors;<\/span><\/p>\n

public<\/span> class<\/span> Java8Example2<\/span> {<\/span><\/p>\n

public<\/span> static<\/span> void<\/span> main(<\/span>String<\/span>[] args) {<\/span><\/p>\n

\/\/3 apple, 2 banana, others 1<\/span><\/p>\n

List<<\/span>String<\/span>> items =<\/span><\/p>\n

Arrays.asList(<\/span>"apple"<\/span>,<\/span> "apple"<\/span>,<\/span> "banana"<\/span>,<\/span><\/p>\n

"apple"<\/span>,<\/span> "orange"<\/span>,<\/span> "banana"<\/span>,<\/span> "papaya"<\/span>);<\/span><\/p>\n

Map<\/span><<\/span>String<\/span>, Long> result =<\/span><\/p>\n

items.stream().collect(<\/span><\/p>\n

Collectors.groupingBy(<\/span><\/p>\n

Function<\/span>.identity(), Collectors.counting()<\/span><\/p>\n

)<\/span><\/p>\n

);<\/span><\/p>\n

Map<\/span><<\/span>String<\/span>, Long> finalMap =<\/span> new<\/span> LinkedHashMap<>();<\/span><\/p>\n

\/\/Sort a map and add to finalMap<\/span><\/p>\n

result.entrySet().stream()<\/span><\/p>\n

.sorted(<\/span>Map<\/span>.Entry.<<\/span>String<\/span>, Long>comparingByValue()<\/span><\/p>\n

.reversed()).forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));<\/span><\/p>\n

System.out.println(finalMap);<\/span><\/p>\n

}<\/span><\/p>\n

}<\/span><\/p>\n

output<\/span><\/p>\n

{<\/span><\/p>\n

    apple=3, banana=2, papaya=1, orange=1<\/span><\/p>\n

}<\/span><\/p>\n

2. List Objects<\/span><\/h2>\n

Examples to \u2018group by\u2019 a list of user defined Objects.\uff08\u901a\u8fc7\u201c\u7528\u6237\u5b9a\u4e49\u7684\u5bf9\u8c61\u201d\u5217\u8868\u8fdb\u884c\u5206\u7ec4\u7684\u793a\u4f8b\u3002\uff09<\/span><\/p>\n

2.1 A Pojo.<\/span><\/p>\n

Item.java<\/span><\/p>\n

package<\/span> com.mkyong.java8;<\/span><\/p>\n

import<\/span> java.math.BigDecimal;<\/span><\/p>\n

public<\/span> class<\/span> Item<\/span> {<\/span><\/p>\n

private<\/span> String name;<\/span><\/p>\n

private<\/span> int<\/span> qty;<\/span><\/p>\n

private<\/span> BigDecimal price;<\/span><\/p>\n

\/\/constructors, getter\/setters<\/span><\/p>\n

}<\/span><\/p>\n

2.2 Group by the name + Count or Sum the Qty. \uff08name + Count\u5206\u7ec4\u6216\u8005 \u5bf9 Qty\u6c42\u548c\u5206\u7ec4\uff09<\/span><\/p>\n

Java8Examples3.java<\/span><\/p>\n

package com.mkyong.java8;<\/span><\/p>\n

import<\/span> java.math.BigDecimal;<\/span><\/p>\n

import<\/span> java.util.Arrays;<\/span><\/p>\n

import<\/span> java.util.List;<\/span><\/p>\n

import<\/span> java.util.Map;<\/span><\/p>\n

import<\/span> java.util.stream.Collectors;<\/span><\/p>\n

public<\/span> class<\/span> Java8Examples3<\/span> {<\/span><\/p>\n

public<\/span> static<\/span> void<\/span> main(<\/span>String<\/span>[] args) {<\/span><\/p>\n

\/\/3 apple, 2 banana, others 1<\/span><\/p>\n

List<Item> items = Arrays.asList(<\/span><\/p>\n

new<\/span> Item(<\/span>"apple"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"banana"<\/span>,<\/span> 20<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"19.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"orang"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"29.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"watermelon"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"29.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"papaya"<\/span>,<\/span> 20<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"apple"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"banana"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"19.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"apple"<\/span>,<\/span> 20<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>))<\/span><\/p>\n

);<\/span><\/p>\n

Map<\/span><<\/span>String<\/span>, Long> counting = items.stream().collect(<\/span><\/p>\n

Collectors.groupingBy(Item::getName, Collectors.counting()));<\/span><\/p>\n

System.out.println(counting);<\/span><\/p>\n

Map<\/span><<\/span>String<\/span>, Integer> sum = items.stream().collect(<\/span><\/p>\n

Collectors.groupingBy(Item::getName, Collectors.summingInt(Item::getQty)));<\/span><\/p>\n

System.out.println(sum);<\/span><\/p>\n

}<\/span><\/p>\n

}<\/span><\/p>\n

output<\/span><\/p>\n

\/\/Group by + Count<\/span><\/p>\n

{<\/span><\/p>\n

    papaya=1, banana=2, apple=3, orang=1, watermelon=1<\/span><\/p>\n

}<\/span><\/p>\n

\/\/Group by + Sum qty<\/span><\/p>\n

{<\/span><\/p>\n

    papaya=20, banana=30, apple=40, orang=10, watermelon=10<\/span><\/p>\n

}<\/span><\/p>\n

2.2  Price \u5206\u7ec4 \u2013 <\/span>Collectors.groupingBy<\/span> and <\/span>Collectors.mapping<\/span>\u4f8b\u5b50.<\/span><\/p>\n

Java8Examples4.java<\/span><\/p>\n

package com.mkyong.java8;<\/span><\/p>\n

import<\/span> java.math.BigDecimal;<\/span><\/p>\n

import<\/span> java.util.Arrays;<\/span><\/p>\n

import<\/span> java.util.List;<\/span><\/p>\n

import<\/span> java.util.Map;<\/span><\/p>\n

import<\/span> java.util.Set;<\/span><\/p>\n

import<\/span> java.util.stream.Collectors;<\/span><\/p>\n

public<\/span> class<\/span> Java8Examples4<\/span> {<\/span><\/p>\n

public<\/span> static<\/span> void<\/span> main(<\/span>String<\/span>[] args) {<\/span><\/p>\n

\/\/3 apple, 2 banana, others 1<\/span><\/p>\n

List<Item> items = Arrays.asList(<\/span><\/p>\n

new<\/span> Item(<\/span>"apple"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"banana"<\/span>,<\/span> 20<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"19.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"orang"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"29.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"watermelon"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"29.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"papaya"<\/span>,<\/span> 20<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"apple"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"banana"<\/span>,<\/span> 10<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"19.99"<\/span>)),<\/span><\/p>\n

new<\/span> Item(<\/span>"apple"<\/span>,<\/span> 20<\/span>,<\/span> new<\/span> BigDecimal(<\/span>"9.99"<\/span>))<\/span><\/p>\n

);<\/span><\/p>\n

        <\/span>\/\/group by price<\/span><\/p>\n

Map<\/span><BigDecimal, List<Item>> groupByPriceMap =<\/span><\/p>\n

            items.stream().collect(Collectors.groupingBy(Item::getPrice));<\/span><\/p>\n

System.out.println(groupByPriceMap);<\/span><\/p>\n

        <\/span>\/\/ group by price, uses 'mapping' to convert List<Item> to Set<String><\/span><\/p>\n

Map<\/span><BigDecimal,<\/span> Set<\/span><<\/span>String<\/span>>> result =<\/span><\/p>\n

items.stream().collect(<\/span><\/p>\n

Collectors.groupingBy(Item::getPrice,<\/span><\/p>\n

Collectors.mapping(Item::getName, Collectors.toSet())<\/span><\/p>\n

)<\/span><\/p>\n

);<\/span><\/p>\n

System.out.println(result);<\/span><\/p>\n

}<\/span><\/p>\n

}<\/span><\/p>\n

output<\/span><\/p>\n

{<\/span><\/p>\n

    19.99=[<\/span><\/p>\n

            Item{name='banana', qty=20, price=19.99},<\/span><\/p>\n

            Item{name='banana', qty=10, price=19.99}<\/span><\/p>\n

        ],<\/span><\/p>\n

    29.99=[<\/span><\/p>\n

            Item{name='orang', qty=10, price=29.99},<\/span><\/p>\n

            Item{name='watermelon', qty=10, price=29.99}<\/span><\/p>\n

        ],<\/span><\/p>\n

    9.99=[<\/span><\/p>\n

            Item{name='apple', qty=10, price=9.99},<\/span><\/p>\n

            Item{name='papaya', qty=20, price=9.99},<\/span><\/p>\n

            Item{name='apple', qty=10, price=9.99},<\/span><\/p>\n

            Item{name='apple', qty=20, price=9.99}<\/span><\/p>\n

        ]<\/span><\/p>\n

}<\/span><\/p>\n

\/\/group by + mapping to Set<\/span><\/p>\n

{<\/span><\/p>\n

    19.99=[banana],<\/span><\/p>\n

    29.99=[orang, watermelon],<\/span><\/p>\n

    9.99=[papaya, apple]<\/span><\/p>\n

}<\/span><\/p>\n

<\/span><\/p>\n

\u00a9 \u8457\u4f5c\u6743\u5f52\u4f5c\u8005\u6240\u6709<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

Stream Collectors groupingBy \u4f8b\u5b50 <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1],"tags":[18],"class_list":["post-181","post","type-post","status-publish","format-standard","hentry","category-life","tag-blog"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.espandy.com\/index.php?rest_route=\/wp\/v2\/posts\/181","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.espandy.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.espandy.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.espandy.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.espandy.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=181"}],"version-history":[{"count":0,"href":"https:\/\/www.espandy.com\/index.php?rest_route=\/wp\/v2\/posts\/181\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.espandy.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=181"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.espandy.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=181"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.espandy.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=181"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}