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众所周知，使用stream流可以让我们的代码看上去很简洁，现在我们实战使用一下stream的分组与分区。
准备用到的数据类public class Student{ //年级 private String grade; //班级 private String classNumber; //姓名 private String name; //年龄 private int age; //地址 private String address; //数学成绩 private int mathScores; //语文成绩 private int chainessScores; }添加数据 Student student1 = new Student("701","张三",16,"北京",78,90); Student student2 = new Student("700","李四",17,"北京",78,90); Student student3 = new Student("703","王五",16,"上海",78,90); Student student4 = new Student("701","赵六",16,"上海",78,90); Student student5 = new Student("700","钱七",18,"",78,90); Student student6 = new Student("701","老八",17,"",78,90);//这是一个高二年级的成绩单 List students = Stream.of(student1,student2,student3,student4,student5,student6).collect(Collectors.toList()); 按照班级分组
Map > collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber));System.out.println(JSON.toJSONString(collect));//{"700":[{"age":17,"chainessScores":90,"classNumber":"700","mathScores":78,"name":"李四"},{"age":18,"chainessScores":90,"classNumber":"700","mathScores":78,"name":"钱七"}],//"701":[{"age":16,"chainessScores":90,"classNumber":"701","mathScores":78,"name":"张三"},{"age":16,"chainessScores":90,"classNumber":"701","mathScores":78,"name":"赵六"},{"age":17,"chainessScores":90,"classNumber":"701","mathScores":78,"name":"老八"}],//"703":[{"age":16,"chainessScores":90,"classNumber":"703","mathScores":78,"name":"王五"}]}
按照班级分组得到每个班级的同学姓名
Map > collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.mapping(Student::getName, Collectors.toList())));System.out.println(JSON.toJSONString(collect));//{"700":["李四","钱七"],"701":["张三","赵六","老八"],"703":["王五"]}
统计每个班级人数
Map collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.counting()));System.out.println(JSON.toJSONString(collect));//{"700":2,"701":3,"703":1}
求每个班级的数学平均成绩
Map collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.averagingDouble(Student::getMathScores)));System.out.println(JSON.toJSONString(collect));//{"700":65.0,"701":61.0,"703":82.0}
按班级分组求每个同学的总成绩
Map > collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.toMap(Student::getName, student -> student.getMathScores() + student.getChainessScores())));System.out.println(JSON.toJSONString(collect));//{"700":{"钱七":150,"李四":160},"701":{"张三":168,"老八":148,"赵六":137},"703":{"王五":172}}
嵌套分组，先按班级分组，再按年龄分组
Map >> collect = students.stream().collect(Collectors.groupingBy(Student::getClassNumber, Collectors.groupingBy(Student::getAge)));
分组后得到一个线程安全的ConcurrentMap
ConcurrentMap > collect = students.stream().collect(Collectors.groupingByConcurrent(Student::getClassNumber));
加上排序来一波
根据年龄分组并小到大排序,TreeMap默认为按照key升序
TreeMap > collect = students.stream().collect(Collectors.groupingBy(Student::getAge, TreeMap::new, Collectors.mapping(Student::getName, Collectors.toList())));System.out.println(JSON.toJSONString(collect)); //{16:["张三","王五","赵六"],17:["李四","老八"],18:["钱七"]}
根据年龄分组并大到小排序,因为TreeMap默认为按照key升序，所以排完顺序再反转一下就OK了
TreeMap > collect = students.stream().collect(Collectors.groupingBy(Student::getAge, TreeMap::new, Collectors.mapping(Student::getName, Collectors.toList())));Map > collect2 = collect.descendingMap();System.out.println(JSON.toJSONString(collect2));//{18:["钱七"],17:["李四","老八"],16:["张三","王五","赵六"]}
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