Java8 中reduce的基本使用

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;    }}