Java 8 stream test

package com.mandi.sqlutil;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class streamTest {
    public static void main(String[] args) {
        Student student_a = new Student();
        student_a.setName("小明");
        student_a.setAge(20);
        student_a.setGroupName("象棋");
        student_a.setClassName("A班");
        Student student_b = new Student();
        student_b.setName("小花");
        student_b.setAge(30);
        student_b.setGroupName("象棋");
        student_b.setClassName("A班");
        Student student_c = new Student();
        student_c.setName("小黄");
        student_c.setAge(35);
        student_c.setGroupName("象棋");
        student_c.setClassName("B班");
        Student student_d = new Student();
        student_d.setName("小白");
        student_d.setAge(15);
        student_d.setGroupName("围棋");
        student_d.setClassName("A班");
        List<Student> students = new ArrayList<>();
        students.add(student_a);
        students.add(student_b);
        students.add(student_c);
        students.add(student_d);

        System.out.println("全部是大于25的学生 ："+students.stream().allMatch(student -> student.getAge()>25));
        System.out.println("有大于25的学生 ："+students.stream().anyMatch(student -> student.getAge()>25));
        System.out.println("没有大于25的学生 ："+students.stream().noneMatch(student -> student.getAge()>25));

        System.out.println("学生的年龄和 ："+students.stream().map(Student::getAge).reduce(0,Integer::sum));
        System.out.println("学生的年龄和 ："+students.stream().map(Student::getAge).reduce(Integer::sum).orElse(0));

        System.out.println("学生的年龄各种值（和，最大，最小等） ："+students.stream().collect(Collectors.summarizingInt(Student::getAge)));
        System.out.println("学生的年龄和 ："+students.stream().collect(Collectors.summingInt(Student::getAge)));
        System.out.println("学生的平均年龄 ："+students.stream().collect(Collectors.averagingInt(Student::getAge)));
        System.out.println("年龄最大的学生 ："+students.stream().collect(Collectors.maxBy(Comparator.comparing(Student::getAge))).get().getName());
        System.out.println("年龄最大的学生 ："+ students.stream().max(Comparator.comparingInt(Student::getAge)).get().getName());
        System.out.println("年龄最小的学生 ："+students.stream().collect(Collectors.minBy(Comparator.comparing(Student::getAge))).get().getName());
        System.out.println("年龄最小的学生 ："+ students.stream().min(Comparator.comparingInt(Student::getAge)).get().getName());

        System.out.println("学生名字展示 ："+ students.stream().map(Student::getName).collect(Collectors.joining(",")));


        //groupingBy
        //groupingBy的第二个参数可以传递任意Collector类型，
        //如果我们不添加第二个参数，则编译器会默认帮我们添加一个Collectors.toList()

        //groupingBy 分组 （按groupName分组）
        Map<String, List<Student>> groups = students.stream().
                collect(Collectors.groupingBy(Student::getGroupName));

        groups.forEach((k,v)->{ // map.foreach()
            System.out.println(k+"-爱好者：");
            // list.foreach()
            v.forEach(student -> System.out.println(student.getName()));
        });

        //groupingBy 分组 （按groupName和className分组 - groupingBy的第二个参数Collectors.groupingBy(Student::getClassName)）
        Map<String,Map<String, List<Student>>> groupsGroupAndClass = students.stream().
                collect(Collectors.groupingBy(Student::getGroupName,Collectors.groupingBy(Student::getClassName)));

        groupsGroupAndClass.forEach((k,v)-> {
            System.out.println(k+"-爱好组：");
            v.forEach((k2,v2)->{
                System.out.println("  "+k+"-爱好者"+k2);
                v2.forEach(student -> System.out.println("    "+student.getName()));
            });
        });

        //groupingBy 分组 （按groupName分组，计算每组人数 - groupingBy的第二个参数Collectors.counting()）
        Map<String, Long> groupsCount = students.stream().
                collect(Collectors.groupingBy(Student::getGroupName,Collectors.counting()));

        groupsCount.forEach((k,v)-> System.out.println(k+"-爱好者人数："+v));

    }
}

Java 8 stream test.jpg