list stream: reduce的使用
stream 中的 reduce 的主要作用就是stream中元素进行组合，组合的方式可以是加减乘除，也可以是拼接等，接下来我们就通过实例来看一下reduce的用法：
reduce 一共有三种实现
1、第一种
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T reduce(T identity, BinaryOperator accumulator); |
该实现有起始值 identity， 起始值的类型决定了返回结果的类型，通过 accumulator 操作最终得到 identity 类型的返回结果
2、第二种
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Optional<T> reduce(BinaryOperator accumulator); |
该实现只有一个参数 accumulator ， 由于没有办法确定具体的返回结果，所以该方法返回的是 Optional
3、第三种
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<U> U reduce(U identity, BiFunction<U, ? super T, U> accumulator, BinaryOperator<U> combiner); |
该方法有三个参数 identity 、 accumulator 、combiner ，该方法通过 identity 和 accumulator的处理得出最终结果，结果和第一个参数的类型相同
首先把我们下面操作的这个实体对象先放在这里
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pulbic class User { //ID private Long id; //年龄 private int age; //班级 private String classes; public Long getId() { return id; } public void setId(Long id) { this .id = id; } public int getAge() { return age; } public void setAge( int age) { this .age = age; } public String getClasses() { return classes; } public void setClasses(String classes) { this .classes = classes; } @Override public String toString() { return "User{" + "id=" + id + ", age=" + age + ", classes='" + classes + '\ '' + '}' ; } |
用来求和，如下所示是四种不同的方式来获取User对象中的age只和，其中两种是通过reduce来进行求和
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List<User> userList = new ArrayList<>(); User user1 = new User(); user1.setAge( 10 ); userList.add(user1); User user2 = new User(); user2.setAge( 20 ); userList.add(user2); User user3 = new User(); user3.setAge( 25 ); userList.add(user3); int ageSumThree = userList.stream().map(User::getAge).reduce( 0 , Integer::sum); System.out.println( "ageSumThree: " + ageSumThree); int ageSumFive = userList.stream().map(User::getAge).reduce(Integer::sum).orElse( 0 ); System.out.println( "ageSumFive: " + ageSumFive); int ageSumOne = userList.stream().collect(Collectors.summingInt(User::getAge)); System.out.println( "ageSumOne" + ageSumOne); int ageSumFour = userList.stream().mapToInt(User::getAge).sum(); System.out.println( "ageSumFour: " + ageSumFour); |
用来求最大最小值，如下所示是求User中age的最大最小值
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public static void main(String[] args) { List<User> userList = new ArrayList<>(); User user1 = new User(); user1.setAge( 10 ); userList.add(user1); User user2 = new User(); user2.setAge( 20 ); userList.add(user2); User user3 = new User(); user3.setAge( 25 ); userList.add(user3); User user4 = new User(); user4.setAge( 25 ); userList.add(user4); int min = userList.stream().map(User::getAge).reduce(Integer::min).orElse( 0 ); System.out.println( "min : " + min); int max = userList.stream().map(User::getAge).reduce(Integer::max).orElse( 0 ); System.out.println( "max : " + max); } |
用来拼接字符串，如下所示：
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public static void main(String[] args) { List<User> userList = new ArrayList<>(); User user1 = new User(); user1.setAge( 10 ); userList.add(user1); User user2 = new User(); user2.setAge( 20 ); userList.add(user2); User user3 = new User(); user3.setAge( 25 ); userList.add(user3); User user4 = new User(); user4.setAge( 25 ); userList.add(user4); String append = userList.stream().map(User::toString).reduce( "拼接字符串：" , String::concat); System.out.println( "append : " + append); } |
计算平均值：计算User对象中age字段的平均值
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public static void main(String[] args) { List<User> userList = new ArrayList<>(); User user1 = new User(); user1.setAge( 10 ); userList.add(user1); User user2 = new User(); user2.setAge( 20 ); userList.add(user2); User user3 = new User(); user3.setAge( 25 ); userList.add(user3); User user4 = new User(); user4.setAge( 25 ); userList.add(user4); double average = userList.stream().mapToInt(User::getAge).average().orElse( 0.0 ); System.out.println( "average : " + average); } |
reduce的基本用法
1、初识 reduce 的基本 api
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@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。
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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; } } |
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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; } } |
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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; } } |
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。
原文链接：https://zhangzehai.blog.csdn.net/article/details/106369542
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