Java随笔7-stream api

常见用法

public class Student {
    private Integer id;
    private Integer age;
    private String name;
    
    //getter and setter
}

public class Clbum {
    private List students;

    //getter and setter
}

List clbums = new ArrayList<>();

//1、降维
List students = clbums.stream().flatMap(item->item.getStudents().stream()).collect(Collectors.toList());

//2、分组
Map> groupByAge = students.stream().collect(Collectors.groupingBy(Student::getAge));
Map countByAge = students.stream().collect(Collectors.groupingBy(Student::getAge, Collectors.counting()));
//两层分组
Map> groupByAgeAndName = students.stream()
        .collect(Collectors.groupingBy(Student::getAge, Collectors.toMap(item -> item.getName(), item -> item)));

//3、list转map
//键不会重复
Map id2Student = students.stream().collect(Collectors.toMap(Student::getId, item -> item));
//键可能重复
Map age2Student = students.stream().collect(Collectors.toMap(Student::getId, item -> item, (k1, k2) -> k1));

//4、求和
List list = Arrays.asList(1, 2, 3, 4, 5, 6);
Integer sum2 = list.stream().reduce(0, (x, y) -> x + y);
Integer sum3 = list.stream().mapToInt((x) -> x).sum();