Java中List的Stream常见使用方法

1、基础类

package com.my.test;

public class Student {

    // 名称
    private String name;

    // 性别
    private String gender;

    // 年龄
    private Integer age;

    public Student() {
    }

    public Student(String name, String gender, Integer age) {
        this.name = name;
        this.gender = gender;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", gender='" + gender + '\'' +
                ", age=" + age +
                '}';
    }
}

2、测试类

package com.my.test;

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

import static java.util.stream.Collectors.averagingInt;
import static java.util.stream.Collectors.partitioningBy;

public class ListStream {
    public static void main(String[] args) {
        // 测试数据
        List<Student> list = new ArrayList<>();
        list.add(new Student("李明", "男", 18));
        list.add(new Student("李飞", "男", 17));
        list.add(new Student("张丽", "女", 17));
        list.add(new Student("张艳", "女", 20));
        list.add(new Student("王朋", "男", 18));
        list.add(new Student("陈晴", "女", 18));
        list.add(new Student("王菲", "女", 19));
        list.add(new Student("李雷", "男", 20));

        // 查找性别为男的学生
        List<Student> boys = list.stream().filter(s -> "男".equals(s.getGender())).collect(Collectors.toList());
        System.out.println("1、查找性别为男的学生：");
        System.out.println(boys.toString());

        // 查找性别为女的学生
        List<Student> girls = list.stream().filter(s -> "女".equals(s.getGender())).collect(Collectors.toList());
        System.out.println("2、查找性别为女的学生：");
        System.out.println(girls.toString());

        // map的key值true为男，false为女的集合
        Map<Boolean, List<Student>> map1 = list.stream().collect(partitioningBy(student -> "男".equals(student.getGender())));
        System.out.println("3、map的key值true为男，false为女的集合：");
        System.out.println(map1);

        // 求性别为男的学生总岁数
        int sum = list.stream().filter(s -> "男".equals(s.getGender())).mapToInt(Student::getAge).sum();
        System.out.println("4、求性别为男的学生总岁数：");
        System.out.println(sum);

        // 按性别进行分组统计人数
        Map<String, Integer> map2 = list.stream().collect(Collectors.groupingBy(Student::getGender, Collectors.summingInt(p -> 1)));
        System.out.println("5、按性别进行分组统计人数：");
        System.out.println(map2);

        // 按性别进行分组统计各组
        Map<String, List<Student>> map3 = list.stream().collect(Collectors.groupingBy(Student::getGender));
        System.out.println("6、按性别进行分组统计各组：");
        System.out.println(map3);

        // 判断是否有年龄大于25岁的学生
        boolean check = list.stream().anyMatch(student -> student.getAge() > 25);
        System.out.println("7、判断是否有年龄大于25岁的学生：");
        System.out.println(check);

        // 获取所有学生的姓名集合
        List<String> list2 = list.stream().map(Student::getName).collect(Collectors.toList());
        System.out.println("8、获取所有学生的姓名集合：");
        System.out.println(list2);

        // 求所有人的平均年龄
        double avg = list.stream().collect(averagingInt(Student::getAge));
        System.out.println("9、求所有人的平均年龄：");
        System.out.println(avg);

        // 求年龄最大的学生
        Student s = list.stream().reduce((student, student2) -> student.getAge() > student2.getAge() ? student : student2).get();
        System.out.println("10、求年龄最大的学生：");
        System.out.println(s);

        // 按照年龄从小到大排序
        List<Student> list3 = list.stream().sorted((s1, s2) -> s1.getAge().compareTo(s2.getAge())).collect(Collectors.toList());
        System.out.println("11、按照年龄从小到大排序：");
        System.out.println(list3);

        // 获取所有的名字，输出一个字符串
        String str = list.stream().map(Student::getName).collect(Collectors.joining(",", "[", "]"));
        System.out.println("12、获取所有的名字，输出一个字符串：");
        System.out.println(str);

        // 获取年龄的最大值、最小值、平均值、求和
        IntSummaryStatistics intSummaryStatistics = list.stream().mapToInt(Student::getAge).summaryStatistics();
        System.out.println("13、获取年龄的最大值、最小值、平均值、求和：");
        System.out.println(intSummaryStatistics);
    }
}