答應我，下次開發就開始用 java Stream第四期

上期我們討論了java Stream的一些操作，這期我們繼續最後一期。

Collector 工具庫：Collectors

Student s1 = new Student("aa", "aa");

Student s2 = new Student("bb", "30");

Student s3 = new Student("cc","20");

List<student> list = Arrays.asList(s1, s2, s3);

//裝成list
 List<integer> ageList = list.stream().map(Student::getAge).collect(Collectors.toList()); // [10, 20, 10]

//轉成set
Set<integer> ageSet = list.stream().map(Student::getAge).collect(Collectors.toSet()); // [20, 10]

//轉成map,注:key不能相同，否則報錯
 Map<string> studentMap = list.stream().collect(Collectors.toMap(Student::getName, Student::getAge)); 
// {cc=10, bb=20, aa=10}

//字符串分隔符連接
String joinName = list.stream().map(Student::getName).collect(Collectors.joining(",", "(", ")"));
 // (aa,bb,cc)

聚合操作

//1.學生總數
Long count = list.stream().collect(Collectors.counting()); // 3

//2.最大年齡 (最小的minBy同理)
 Integer maxAge = list.stream().map(Student::getAge).collect(Collectors.maxBy(Integer::compare)).get(); 
// 20

//3.所有人的年齡
Integer sumAge = list.stream().collect(Collectors.summingInt(Student::getAge)); 
// 40

//4.平均年齡
 Double averageAge = list.stream().collect(Collectors.averagingDouble(Student::getAge)); 
// 13.333333333333334

// 帶上以上所有方法 

 DoubleSummaryStatistics statistics = list.stream().collect(Collectors.summarizingDouble(Student::getAge));

System.out.println("count:" + statistics.getCount() + ",max:" + statistics.getMax() + ",sum:" + statistics.getSum() + ",average:" + statistics.getAverage());

//分組
Map<integer>> ageMap = list.stream().collect(Collectors.groupingBy(Student::getAge));

//多重分組,先根據類型分再根據年齡分
 Map<integer>>> typeAgeMap = list.stream().collect(Collectors.groupingBy(Student::getType, Collectors.groupingBy(Student::getAge)));

//分區

//分成兩部分，一部分大於10歲，一部分小於等於10歲
 Map<boolean>> partMap = list.stream().collect(Collectors.partitioningBy(v -> v.getAge() > 10));

//規約
 Integer allAge = list.stream().map(Student::getAge).collect(Collectors.reducing(Integer::sum)).get(); //40
/<boolean>/<integer>/<integer>/<string>/<integer>/<integer>/<student>

好的，到這裡我的javaStream就寫完了，我以前也不會這些，現在基本上就是我看一遍，敲一遍，再放上來，下一期學習什麼我還沒用想好。