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; } }