Java8 Stream：2萬(wàn)字20個(gè)實(shí)例，玩轉(zhuǎn)集合的篩選、歸約、分組、聚合

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來(lái)源：

https://blog.csdn.net/mu_wind/article/details/109516995

1. 從員工集合中篩選出 salary 大于 8000 的員工，并放置到新的集合里。
2. 統(tǒng)計(jì)員工的最高薪資、平均薪資、薪資之和。
3. 將員工按薪資從高到低排序，同樣薪資者年齡小者在前。
4. 將員工按性別分類，將員工按性別和地區(qū)分類，將員工按薪資是否高于 8000 分為兩部分。

1. Stream 概述

Stream將要處理的元素集合看作一種流，在流的過(guò)程中，借助Stream API對(duì)流中的元素進(jìn)行操作，比如：篩選、排序、聚合等。

1. 中間操作，每次返回一個(gè)新的流，可以有多個(gè)。
2. 終端操作，每個(gè)流只能進(jìn)行一次終端操作，終端操作結(jié)束后流無(wú)法再次使用。終端操作會(huì)產(chǎn)生一個(gè)新的集合或值。

1. stream 不存儲(chǔ)數(shù)據(jù)，而是按照特定的規(guī)則對(duì)數(shù)據(jù)進(jìn)行計(jì)算，一般會(huì)輸出結(jié)果。
2. stream 不會(huì)改變數(shù)據(jù)源，通常情況下會(huì)產(chǎn)生一個(gè)新的集合或一個(gè)值。
3. stream 具有延遲執(zhí)行特性，只有調(diào)用終端操作時(shí)，中間操作才會(huì)執(zhí)行。
   
   

2. Stream 的創(chuàng)建

List?list?=?Arrays.asList("a",?"b",?"c");
//?創(chuàng)建一個(gè)順序流
Stream?stream?=?list.stream();
//?創(chuàng)建一個(gè)并行流
Stream?parallelStream?=?list.parallelStream();

int[]?array={1,3,5,6,8};
IntStream?stream?=?Arrays.stream(array);

Stream?stream?=?Stream.of(1,?2,?3,?4,?5,?6);

Stream?stream2?=?Stream.iterate(0,?(x)?->?x?+?3).limit(4);
stream2.forEach(System.out::println);

Stream?stream3?=?Stream.generate(Math::random).limit(3);
stream3.forEach(System.out::println);

0 3 6 90.67961569092719940.19143142088542830.8116932592396652

Optional?findFirst?=?list.stream().parallel().filter(x->x>6).findFirst();

3. Stream 的使用

Optional類是一個(gè)可以為null的容器對(duì)象。如果值存在則isPresent()方法會(huì)返回true，調(diào)用get()方法會(huì)返回該對(duì)象。更詳細(xì)說(shuō)明請(qǐng)見(jiàn)：Java 8 Optional 類詳解（http://itsoku.com/article/200）

3.1. 案例使用的員工類

List?personList?=?new?ArrayList();
personList.add(new?Person("Tom",?8900,?"male",?"New?York"));
personList.add(new?Person("Jack",?7000,?"male",?"Washington"));
personList.add(new?Person("Lily",?7800,?"female",?"Washington"));
personList.add(new?Person("Anni",?8200,?"female",?"New?York"));
personList.add(new?Person("Owen",?9500,?"male",?"New?York"));
personList.add(new?Person("Alisa",?7900,?"female",?"New?York"));

class?Person?{
????private?String?name;??//?姓名
????private?int?salary;?//?薪資
????private?int?age;?//?年齡
????private?String?sex;?//性別
????private?String?area;??//?地區(qū)

????//?構(gòu)造方法
????public?Person(String?name,?int?salary,?int?age,String?sex,String?area)?{
????????this.name?=?name;
????????this.salary?=?salary;
????????this.age?=?age;
????????this.sex?=?sex;
????????this.area?=?area;
????}
????//?省略了get和set，請(qǐng)自行添加
}

3.2. 遍歷/匹配（foreach/find/match）

//?import已省略，請(qǐng)自行添加，后面代碼亦是
public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList(7,?6,?9,?3,?8,?2,?1);

????????//?遍歷輸出符合條件的元素
????????list.stream().filter(x?->?x?>?6).forEach(System.out::println);
????????//?匹配第一個(gè)
????????Optional?findFirst?=?list.stream().filter(x?->?x?>?6).findFirst();
????????//?匹配任意（適用于并行流）
????????Optional?findAny?=?list.parallelStream().filter(x?->?x?>?6).findAny();
????????//?是否包含符合特定條件的元素
????????boolean?anyMatch?=?list.stream().anyMatch(x?->?x?>?6);
????????System.out.println("匹配第一個(gè)值："?+?findFirst.get());
????????System.out.println("匹配任意一個(gè)值："?+?findAny.get());
????????System.out.println("是否存在大于6的值："?+?anyMatch);
????}
}

3.3. 篩選（filter）

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList(6,?7,?3,?8,?1,?2,?9);
????????Stream?stream?=?list.stream();
????????stream.filter(x?->?x?>?7).forEach(System.out::println);
????}
}

8 9

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));
????????personList.add(new?Person("Anni",?8200,?24,?"female",?"New?York"));
????????personList.add(new?Person("Owen",?9500,?25,?"male",?"New?York"));
????????personList.add(new?Person("Alisa",?7900,?26,?"female",?"New?York"));

????????List?fiterList?=?personList.stream().filter(x?->?x.getSalary()?>?8000).map(Person::getName)
????????????.collect(Collectors.toList());
????????System.out.print("高于8000的員工姓名："?+?fiterList);
????}
}

高于 8000 的員工姓名：[Tom, Anni, Owen]

3.4. 聚合（max/min/count)

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList("adnm",?"admmt",?"pot",?"xbangd",?"weoujgsd");

????????Optional?max?=?list.stream().max(Comparator.comparing(String::length));
????????System.out.println("最長(zhǎng)的字符串："?+?max.get());
????}
}

最長(zhǎng)的字符串：weoujgsd

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList(7,?6,?9,?4,?11,?6);

????????//?自然排序
????????Optional?max?=?list.stream().max(Integer::compareTo);
????????//?自定義排序
????????Optional?max2?=?list.stream().max(new?Comparator()?{
????????????@Override
????????????public?int?compare(Integer?o1,?Integer?o2)?{
????????????????return?o1.compareTo(o2);
????????????}
????????});
????????System.out.println("自然排序的最大值："?+?max.get());
????????System.out.println("自定義排序的最大值："?+?max2.get());
????}
}

自然排序的最大值：11

自定義排序的最大值：11

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));
????????personList.add(new?Person("Anni",?8200,?24,?"female",?"New?York"));
????????personList.add(new?Person("Owen",?9500,?25,?"male",?"New?York"));
????????personList.add(new?Person("Alisa",?7900,?26,?"female",?"New?York"));

????????Optional?max?=?personList.stream().max(Comparator.comparingInt(Person::getSalary));
????????System.out.println("員工工資最大值："?+?max.get().getSalary());
????}
}

員工工資最大值：9500

import?java.util.Arrays;
import?java.util.List;

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList(7,?6,?4,?8,?2,?11,?9);

????????long?count?=?list.stream().filter(x?->?x?>?6).count();
????????System.out.println("list中大于6的元素個(gè)數(shù)："?+?count);
????}
}

list 中大于 6 的元素個(gè)數(shù)：4

3.5. 映射(map/flatMap)

• map：接收一個(gè)函數(shù)作為參數(shù)，該函數(shù)會(huì)被應(yīng)用到每個(gè)元素上，并將其映射成一個(gè)新的元素。
• flatMap：接收一個(gè)函數(shù)作為參數(shù)，將流中的每個(gè)值都換成另一個(gè)流，然后把所有流連接成一個(gè)流。

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????String[]?strArr?=?{?"abcd",?"bcdd",?"defde",?"fTr"?};
????????List?strList?=?Arrays.stream(strArr).map(String::toUpperCase).collect(Collectors.toList());

????????List?intList?=?Arrays.asList(1,?3,?5,?7,?9,?11);
????????List?intListNew?=?intList.stream().map(x?->?x?+?3).collect(Collectors.toList());

????????System.out.println("每個(gè)元素大寫(xiě)："?+?strList);
????????System.out.println("每個(gè)元素+3："?+?intListNew);
????}
}

每個(gè)元素大寫(xiě)：[ABCD, BCDD, DEFDE, FTR]?

每個(gè)元素+3：[4, 6, 8, 10, 12, 14]

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));
????????personList.add(new?Person("Anni",?8200,?24,?"female",?"New?York"));
????????personList.add(new?Person("Owen",?9500,?25,?"male",?"New?York"));
????????personList.add(new?Person("Alisa",?7900,?26,?"female",?"New?York"));

????????//?不改變?cè)瓉?lái)員工集合的方式
????????List?personListNew?=?personList.stream().map(person?->?{
????????????Person?personNew?=?new?Person(person.getName(),?0,?0,?null,?null);
????????????personNew.setSalary(person.getSalary()?+?10000);
????????????return?personNew;
????????}).collect(Collectors.toList());
????????System.out.println("一次改動(dòng)前："?+?personList.get(0).getName()?+?"-->"?+?personList.get(0).getSalary());
????????System.out.println("一次改動(dòng)后："?+?personListNew.get(0).getName()?+?"-->"?+?personListNew.get(0).getSalary());

????????//?改變?cè)瓉?lái)員工集合的方式
????????List?personListNew2?=?personList.stream().map(person?->?{
????????????person.setSalary(person.getSalary()?+?10000);
????????????return?person;
????????}).collect(Collectors.toList());
????????System.out.println("二次改動(dòng)前："?+?personList.get(0).getName()?+?"-->"?+?personListNew.get(0).getSalary());
????????System.out.println("二次改動(dòng)后："?+?personListNew2.get(0).getName()?+?"-->"?+?personListNew.get(0).getSalary());
????}
}

一次改動(dòng)前：Tom–>8900?

一次改動(dòng)后：Tom–>18900?

二次改動(dòng)前：Tom–>18900?

二次改動(dòng)后：Tom–>18900

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList("m,k,l,a",?"1,3,5,7");
????????List?listNew?=?list.stream().flatMap(s?->?{
????????????//?將每個(gè)元素轉(zhuǎn)換成一個(gè)stream
????????????String[]?split?=?s.split(",");
????????????Stream?s2?=?Arrays.stream(split);
????????????return?s2;
????????}).collect(Collectors.toList());

????????System.out.println("處理前的集合："?+?list);
????????System.out.println("處理后的集合："?+?listNew);
????}
}

處理前的集合：[m-k-l-a, 1-3-5]?

處理后的集合：[m, k, l, a, 1, 3, 5]

3.6. 歸約(reduce)

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList(1,?3,?2,?8,?11,?4);
????????//?求和方式1
????????Optional?sum?=?list.stream().reduce((x,?y)?->?x?+?y);
????????//?求和方式2
????????Optional?sum2?=?list.stream().reduce(Integer::sum);
????????//?求和方式3
????????Integer?sum3?=?list.stream().reduce(0,?Integer::sum);

????????//?求乘積
????????Optional?product?=?list.stream().reduce((x,?y)?->?x?*?y);

????????//?求最大值方式1
????????Optional?max?=?list.stream().reduce((x,?y)?->?x?>?y???x?:?y);
????????//?求最大值寫(xiě)法2
????????Integer?max2?=?list.stream().reduce(1,?Integer::max);

????????System.out.println("list求和："?+?sum.get()?+?","?+?sum2.get()?+?","?+?sum3);
????????System.out.println("list求積："?+?product.get());
????????System.out.println("list求和："?+?max.get()?+?","?+?max2);
????}
}

list 求和：29,29,29

list?求積：2112list

求和：11,11

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));
????????personList.add(new?Person("Anni",?8200,?24,?"female",?"New?York"));
????????personList.add(new?Person("Owen",?9500,?25,?"male",?"New?York"));
????????personList.add(new?Person("Alisa",?7900,?26,?"female",?"New?York"));

????????//?求工資之和方式1：
????????Optional?sumSalary?=?personList.stream().map(Person::getSalary).reduce(Integer::sum);
????????//?求工資之和方式2：
????????Integer?sumSalary2?=?personList.stream().reduce(0,?(sum,?p)?->?sum?+=?p.getSalary(),
????????????????????????????????????????????????????????(sum1,?sum2)?->?sum1?+?sum2);
????????//?求工資之和方式3：
????????Integer?sumSalary3?=?personList.stream().reduce(0,?(sum,?p)?->?sum?+=?p.getSalary(),?Integer::sum);

????????//?求最高工資方式1：
????????Integer?maxSalary?=?personList.stream().reduce(0,?(max,?p)?->?max?>?p.getSalary()???max?:?p.getSalary(),
???????????????????????????????????????????????????????Integer::max);
????????//?求最高工資方式2：
????????Integer?maxSalary2?=?personList.stream().reduce(0,?(max,?p)?->?max?>?p.getSalary()???max?:?p.getSalary(),
????????????????????????????????????????????????????????(max1,?max2)?->?max1?>?max2???max1?:?max2);

????????System.out.println("工資之和："?+?sumSalary.get()?+?","?+?sumSalary2?+?","?+?sumSalary3);
????????System.out.println("最高工資："?+?maxSalary?+?","?+?maxSalary2);
????}
}

工資之和：49300,49300,49300?

最高工資：9500,9500

3.7. 收集(collect)

collect主要依賴java.util.stream.Collectors類內(nèi)置的靜態(tài)方法。

3.7.1 歸集(toList/toSet/toMap)

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?list?=?Arrays.asList(1,?6,?3,?4,?6,?7,?9,?6,?20);
????????List?listNew?=?list.stream().filter(x?->?x?%?2?==?0).collect(Collectors.toList());
????????Set?set?=?list.stream().filter(x?->?x?%?2?==?0).collect(Collectors.toSet());

????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));
????????personList.add(new?Person("Anni",?8200,?24,?"female",?"New?York"));

????????Map?map?=?personList.stream().filter(p?->?p.getSalary()?>?8000)
????????????.collect(Collectors.toMap(Person::getName,?p?->?p));
????????System.out.println("toList:"?+?listNew);
????????System.out.println("toSet:"?+?set);
????????System.out.println("toMap:"?+?map);
????}
}

toList：[6, 4, 6, 6, 20]?

toSet：[4, 20, 6]?

toMap：{Tom=mutest.Person@5fd0d5ae, Anni=mutest.Person@2d98a335}

3.7.2 統(tǒng)計(jì)(count/averaging)

• 計(jì)數(shù)：count
• 平均值：averagingInt、averagingLong、averagingDouble
• 最值：maxBy、minBy
• 求和：summingInt、summingLong、summingDouble
• 統(tǒng)計(jì)以上所有：summarizingInt、summarizingLong、summarizingDouble

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));

????????//?求總數(shù)
????????Long?count?=?personList.stream().collect(Collectors.counting());
????????//?求平均工資
????????Double?average?=?personList.stream().collect(Collectors.averagingDouble(Person::getSalary));
????????//?求最高工資
????????Optional?max?=?personList.stream().map(Person::getSalary).collect(Collectors.maxBy(Integer::compare));
????????//?求工資之和
????????Integer?sum?=?personList.stream().collect(Collectors.summingInt(Person::getSalary));
????????//?一次性統(tǒng)計(jì)所有信息
????????DoubleSummaryStatistics?collect?=?personList.stream().collect(Collectors.summarizingDouble(Person::getSalary));

????????System.out.println("員工總數(shù)："?+?count);
????????System.out.println("員工平均工資："?+?average);
????????System.out.println("員工工資總和："?+?sum);
????????System.out.println("員工工資所有統(tǒng)計(jì)："?+?collect);
????}
}

員工總數(shù)：3

員工平均工資：7900.0?

員工工資總和：23700?

員工工資所有統(tǒng)計(jì)：DoubleSummaryStatistics{count=3, sum=23700.000000,min=7000.000000, average=7900.000000, max=8900.000000}

3.7.3 分組(partitioningBy/groupingBy)

• 分區(qū)：將stream按條件分為兩個(gè)Map，比如員工按薪資是否高于 8000 分為兩部分。
• 分組：將集合分為多個(gè) Map，比如員工按性別分組。有單級(jí)分組和多級(jí)分組。

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?"female",?"Washington"));
????????personList.add(new?Person("Anni",?8200,?"female",?"New?York"));
????????personList.add(new?Person("Owen",?9500,?"male",?"New?York"));
????????personList.add(new?Person("Alisa",?7900,?"female",?"New?York"));

????????//?將員工按薪資是否高于8000分組
????????Map>?part?=?personList.stream().collect(Collectors.partitioningBy(x?->?x.getSalary()?>?8000));
????????//?將員工按性別分組
????????Map>?group?=?personList.stream().collect(Collectors.groupingBy(Person::getSex));
????????//?將員工先按性別分組，再按地區(qū)分組
????????Map>>?group2?=?personList.stream().collect(Collectors.groupingBy(Person::getSex,?Collectors.groupingBy(Person::getArea)));
????????System.out.println("員工按薪資是否大于8000分組情況："?+?part);
????????System.out.println("員工按性別分組情況："?+?group);
????????System.out.println("員工按性別、地區(qū)："?+?group2);
????}
}

員工按薪資是否大于8000分組情況：{false=[mutest.Person@2d98a335,?mutest.Person@16b98e56,?mutest.Person@7ef20235],?true=[mutest.Person@27d6c5e0,?mutest.Person@4f3f5b24,?mutest.Person@15aeb7ab]}
員工按性別分組情況：{female=[mutest.Person@16b98e56,?mutest.Person@4f3f5b24,?mutest.Person@7ef20235],?male=[mutest.Person@27d6c5e0,?mutest.Person@2d98a335,?mutest.Person@15aeb7ab]}
員工按性別、地區(qū)：{female={New York=[mutest.Person@4f3f5b24,?mutest.Person@7ef20235],?Washington=[mutest.Person@16b98e56]},?male={New?York=[mutest.Person@27d6c5e0,?mutest.Person@15aeb7ab],?Washington=[mutest.Person@2d98a335]}}

3.7.4 接合(joining)

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));

????????String?names?=?personList.stream().map(p?->?p.getName()).collect(Collectors.joining(","));
????????System.out.println("所有員工的姓名："?+?names);
????????List?list?=?Arrays.asList("A",?"B",?"C");
????????String?string?=?list.stream().collect(Collectors.joining("-"));
????????System.out.println("拼接后的字符串："?+?string);
????}
}

所有員工的姓名：Tom,Jack,Lily

拼接后的字符串：A-B-C

3.7.5 歸約(reducing)

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();
????????personList.add(new?Person("Tom",?8900,?23,?"male",?"New?York"));
????????personList.add(new?Person("Jack",?7000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?7800,?21,?"female",?"Washington"));

????????//?每個(gè)員工減去起征點(diǎn)后的薪資之和（這個(gè)例子并不嚴(yán)謹(jǐn)，但一時(shí)沒(méi)想到好的例子）
????????Integer?sum?=?personList.stream().collect(Collectors.reducing(0,?Person::getSalary,?(i,?j)?->?(i?+?j?-?5000)));
????????System.out.println("員工扣稅薪資總和："?+?sum);

????????//?stream的reduce
????????Optional?sum2?=?personList.stream().map(Person::getSalary).reduce(Integer::sum);
????????System.out.println("員工薪資總和："?+?sum2.get());
????}
}

員工扣稅薪資總和：8700?

員工薪資總和：23700

3.8. 排序(sorted)

• sorted()：自然排序，流中元素需實(shí)現(xiàn) Comparable 接口
• sorted(Comparator com)：Comparator 排序器自定義排序

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????List?personList?=?new?ArrayList();

????????personList.add(new?Person("Sherry",?9000,?24,?"female",?"New?York"));
????????personList.add(new?Person("Tom",?8900,?22,?"male",?"Washington"));
????????personList.add(new?Person("Jack",?9000,?25,?"male",?"Washington"));
????????personList.add(new?Person("Lily",?8800,?26,?"male",?"New?York"));
????????personList.add(new?Person("Alisa",?9000,?26,?"female",?"New?York"));

????????//?按工資升序排序（自然排序）
????????List?newList?=?personList.stream().sorted(Comparator.comparing(Person::getSalary)).map(Person::getName)
????????????.collect(Collectors.toList());
????????//?按工資倒序排序
????????List?newList2?=?personList.stream().sorted(Comparator.comparing(Person::getSalary).reversed())
????????????.map(Person::getName).collect(Collectors.toList());
????????//?先按工資再按年齡升序排序
????????List?newList3?=?personList.stream()
????????????.sorted(Comparator.comparing(Person::getSalary).thenComparing(Person::getAge)).map(Person::getName)
????????????.collect(Collectors.toList());
????????//?先按工資再按年齡自定義排序（降序）
????????List?newList4?=?personList.stream().sorted((p1,?p2)?->?{
????????????if?(p1.getSalary()?==?p2.getSalary())?{
????????????????return?p2.getAge()?-?p1.getAge();
????????????}?else?{
????????????????return?p2.getSalary()?-?p1.getSalary();
????????????}
????????}).map(Person::getName).collect(Collectors.toList());

????????System.out.println("按工資升序排序："?+?newList);
????????System.out.println("按工資降序排序："?+?newList2);
????????System.out.println("先按工資再按年齡升序排序："?+?newList3);
????????System.out.println("先按工資再按年齡自定義降序排序："?+?newList4);
????}
}

按工資升序排序：[Lily, Tom, Sherry, Jack, Alisa]?

按工資降序排序：[Sherry, Jack, Alisa, Tom, Lily]?

先按工資再按年齡升序排序：[Lily, Tom, Sherry, Jack, Alisa]?

先按工資再按年齡自定義降序排序：[Alisa, Jack, Sherry, Tom, Lily]

3.9. 提取/組合

public?class?StreamTest?{
????public?static?void?main(String[]?args)?{
????????String[]?arr1?=?{?"a",?"b",?"c",?"d"?};
????????String[]?arr2?=?{?"d",?"e",?"f",?"g"?};

????????Stream?stream1?=?Stream.of(arr1);
????????Stream?stream2?=?Stream.of(arr2);
????????// concat:合并兩個(gè)流 distinct：去重
????????List?newList?=?Stream.concat(stream1,?stream2).distinct().collect(Collectors.toList());
????????// limit：限制從流中獲得前n個(gè)數(shù)據(jù)
????????List?collect?=?Stream.iterate(1,?x?->?x?+?2).limit(10).collect(Collectors.toList());
????????// skip：跳過(guò)前n個(gè)數(shù)據(jù)
????????List?collect2?=?Stream.iterate(1,?x?->?x?+?2).skip(1).limit(5).collect(Collectors.toList());

????????System.out.println("流合并："?+?newList);
????????System.out.println("limit："?+?collect);
????????System.out.println("skip："?+?collect2);
????}
}

流合并：[a, b, c, d, e, f, g]?

limit：[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]?

skip：[3, 5, 7, 9, 11]

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