1、初识 reduce 的基本 api

    @Test
    public void testReduce() {
        Stream<Integer> stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7, 8});

        //求集合元素只和
        Integer result = stream.reduce(0, Integer::sum);
        System.out.println(result);

        stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7});

        //求和
        stream.reduce((i, j) -> i + j).ifPresent(System.out::println); stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7}); //求最大值 stream.reduce(Integer::max).ifPresent(System.out::println); stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7}); //求最小值 stream.reduce(Integer::min).ifPresent(System.out::println); stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7}); //做逻辑 stream.reduce((i, j) -> i > j ? j : i).ifPresent(System.out::println); stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7}); //求逻辑求乘机 int result2 = stream.filter(i -> i % 2 == 0).reduce(1, (i, j) -> i * j); Optional.of(result2).ifPresent(System.out::println); } 

2、应用场景测试

求所有学生的成绩之和。

package com.jd;
import com.jd.bean.Score;
import com.jd.bean.Student;
import org.junit.Test;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Stream;

/**
 * @author: wangyingjie1
 * @version: 1.0
 * @createdate: 2017-09-26 09:35
 */
public class ReduceTest {

    @Test
    public void reduceList() {

        List<Student> list = getStudents();

        //使用Reduce 将所有的所有的成绩进行加和
        Optional<Score> totalScore = list.stream()
                .map(Student::getScore)
                .reduce((x, y) -> x.add(y)); System.out.println(totalScore.get().getPoint()); } @Test public void reduceList2() { List<Student> list = getStudents(); Student student = getStudent(); //使用Reduce 求 list 、student 的总成绩之和 Score scoreSum = list.stream() .map(Student::getScore) //相当于加了一个初始值 .reduce(student.getScore(), (x, y) -> x.add(y)); System.out.println(scoreSum.getPoint()); } private Student getStudent() { Student student = new Student(); student.setId(4); Score score = new Score(); score.setPoint(100); student.setScore(score); return student; } private List<Student> getStudents() { List<Student> list = new ArrayList<>(); for (int i = 0; i < 3; i++) { Student stu = new Student(); Score score = new Score(); score.setPoint(80); score.setCourseName("English"); stu.setId(i); stu.setScore(score); list.add(stu); } return list; } }

package com.jd.bean;
//学生
public class Student {

    private Integer id;

    //课程分数
    private Score score;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Score getScore() {
        return score;
    }

    public void setScore(Score score) {
        this.score = score;
    }
}

package com.jd.bean;
//课程分数
public class Score {

    //分数
    private Integer point;

    //课程名称
    private String courseName;

    public Integer getPoint() {
        return point;
    }

    public Score add(Score other) {

        this.point += other.getPoint();

        return this;
    }

    public void setPoint(Integer point) {
        this.point = point;
    }

    public String getCourseName() {
        return courseName;
    }

    public void setCourseName(String courseName) {
        this.courseName = courseName;
    }
}