Java中List的Stream常見使用方法

1、基礎類

package com.my.test;

public class Student {

    // 名稱
    private String name;

    // 性別
    private String gender;

    // 年齡
    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、測試類

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) {
        // 測試數據
        List<Student> list = new ArrayList<>();
        list.add(new Student("李明", "男", 18));
        list.add(new Student("李飛", "男", 17));
        list.add(new Student("張麗", "女", 17));
        list.add(new Student("張艷", "女", 20));
        list.add(new Student("王朋", "男", 18));
        list.add(new Student("陳晴", "女", 18));
        list.add(new Student("王菲", "女", 19));
        list.add(new Student("李雷", "男", 20));

        // 查找性別為男的學生
        List<Student> boys = list.stream().filter(s -> "男".equals(s.getGender())).collect(Collectors.toList());
        System.out.println("1、查找性別為男的學生：");
        System.out.println(boys.toString());

        // 查找性別為女的學生
        List<Student> girls = list.stream().filter(s -> "女".equals(s.getGender())).collect(Collectors.toList());
        System.out.println("2、查找性別為女的學生：");
        System.out.println(girls.toString());

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

        // 求性別為男的學生總歲數
        int sum = list.stream().filter(s -> "男".equals(s.getGender())).mapToInt(Student::getAge).sum();
        System.out.println("4、求性別為男的學生總歲數：");
        System.out.println(sum);

        // 按性別進行分組統計人數
        Map<String, Integer> map2 = list.stream().collect(Collectors.groupingBy(Student::getGender, Collectors.summingInt(p -> 1)));
        System.out.println("5、按性別進行分組統計人數：");
        System.out.println(map2);

        // 按性別進行分組統計各組
        Map<String, List<Student>> map3 = list.stream().collect(Collectors.groupingBy(Student::getGender));
        System.out.println("6、按性別進行分組統計各組：");
        System.out.println(map3);

        // 判斷是否有年齡大於25歲的學生
        boolean check = list.stream().anyMatch(student -> student.getAge() > 25);
        System.out.println("7、判斷是否有年齡大於25歲的學生：");
        System.out.println(check);

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

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

        // 求年齡最大的學生
        Student s = list.stream().reduce((student, student2) -> student.getAge() > student2.getAge() ? student : student2).get();
        System.out.println("10、求年齡最大的學生：");
        System.out.println(s);

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

        // 獲取所有的名字，輸出一個字符串
        String str = list.stream().map(Student::getName).collect(Collectors.joining(",", "[", "]"));
        System.out.println("12、獲取所有的名字，輸出一個字符串：");
        System.out.println(str);

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