【Java 8】Stream常见场景使用，看这一篇就够啦

【Java 8】Stream常见场景使用，看这一篇就够啦

学习Stream API如何在常见场景下使用

Student2.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Random;
import java.util.UUID;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class Student2 {private String id;private String name;private double score;private String teacher;private int classId;public Student2(String name, double score, String teacher, int classId) {this.id = UUID.randomUUID().toString().substring(0,8);this.name = name;this.score = acher = teacher;this.classId = classId;}public static List<Student2> getStudent(){Student2  a = new Student2("王冬冬1",88.8,"晓东老师1",1);Student2  b = new Student2("王冬冬5",60,"晓东老师1",1);Student2  c = new Student2("王冬冬3",70,"晓东老师1",1);Student2  d = new Student2("王冬冬4",95,"晓东老师1",1);System.out.println("说明 sum1(4): "+(88.8+95+70+60) + "--" +(88.8+95+70+60)/4);Student2  f = new Student2("王冬冬6",55,"晓东老师3",3);Student2  e = new Student2("王冬冬5",100,"晓东老师2",2);Student2  g = new Student2("王冬冬7",45,"晓东老师1",2);Student2  h = new Student2("王冬冬8",90,"晓东老师2",2);Student2  j = new Student2("王冬冬9",75,"晓东老师2",2);Student2  k = new Student2("王冬冬10",88.8,"晓东老师2",2);System.out.println("说明 sum2(5): "+(88.8+100+45+90+75) + "--" +(88.8+100+45+90+75)/5);return Stream.of(a, b, c, d, e, f, g, h, j, k).List());}public String getId() {return id;}public void setId(String id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getScore() {return score;}public void setScore(double score) {this.score = score;}public String getTeacher() {return teacher;}public void setTeacher(String teacher) {acher = teacher;}public int getClassId() {return classId;}public void setClassId(int classId) {this.classId = classId;}@Overridepublic String toString() {return "nStudent2{" +"id='" + id + ''' +", name='" + name + ''' +", score=" + score +", teacher='" + teacher + ''' +", classId='" + classId + ''' +"}n";}
}

练习题

1 查询出成绩不及格的集合
2 查询出晓东老师1所带班级的总分
3 查询出class==1，班级总人数
4 查询班级1的平均分
5 查询各班的总分
6 查询各班的平均分
7 按照班级进行分组，再通过name进行分组
8 获取各班的人数
9 获取学生老师的名字，不重复
10 获取学生老师的数量
11 获取每个老师带的学生个数
12 按照学生分数排名，获取列表
13 获取成绩前3的学生
-----解析字符串-----
1 “a,b;c;d;e,f;” 解析成Pair集合 List<Pair<Integer,Long>>
2 将 "a,b" 解析成Pair对象
-----数组操作-------
1 计算数组总和
2 计算数组平均值
3 给数组*2 计算总和
4 求和二维数组的个数
5 计算int[][] arr_2 的数组总和(求和二维数组)

Demo

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;public class Review {public static void main(String[] args) {List<Student2> list = Student();// 1 查询出成绩不及格的集合List<Student2> score_min_60 = list.stream().filter(student2 -> Score() < 60).List());System.out.println("1 查询出成绩不及格的集合n"+score_min_60);// 2 查询出晓东老师1所带班级的总分double sum_1 = list.stream().filter(student2 -> "晓东老师1".Teacher())).mapToDouble(Student2::getScore).sum();System.out.println("2 查询出晓东老师1所带班级的总分t"+sum_1);// 3 查询出class==1，班级总人数long count = list.stream().filter(student2 -> ClassId() == 1).count();System.out.println("3 查询出class==1，班级总人数t"+count);// 4 查询班级1的平均分// firstlong count1 = list.stream().filter(student2 -> ClassId() == 1).count();double sum = list.stream().filter(student2 -> ClassId() == 1).mapToDouble(Student2::getScore).sum();// seconddouble avg = list.stream().filter(student2 -> ClassId() == 1).mapToDouble(Student2::getScore).average().orElse(0.0);System.out.println("4 查询班级1的平均分t"+(sum/count1));System.out.println("4.1 (orElse)查询班级1的平均分t"+ avg);// 5 查询各班的总分Map<Integer, Double> score_1_sum = list.stream().upingBy(Student2::getClassId, Collectors.summingDouble(Student2::getScore)));System.out.println("5 查询各班的总分");priMap(score_1_sum);// 6 查询各班的平均分Map<Integer, Double> score_1_avg = list.stream().upingBy(Student2::getClassId, Collectors.averagingDouble(Student2::getScore)));System.out.println("6 查询各班的平均分");priMap(score_1_avg);// 7 按照班级进行分组，再通过name进行分组Map<Integer, Map<String, Student2>> map_class_name = list.stream().upingBy(Student2::getClassId, Map(Student2::getName, student2 -> student2)));System.out.println("7 按照班级进行分组，再通过name进行分组");Student2 wdd1 = map_(1).get("王冬冬1");System.out.println("7.1 获取class1_王冬冬1   wdd1t"+wdd1);priMap(map_class_name);// 8 获取各班的人数Map<Integer, Integer> map_count = list.stream().upingBy(Student2::getClassId, Collectors.summingInt(student -> 1)));System.out.println("8 获取各班的人数");priMap(map_count);// 9 获取学生老师的名字，不重复Set<String> teachers = list.stream().map(Student2::getTeacher).Set());System.out.println("9 获取学生老师的名字，不重复t" + teachers);// 10 获取学生老师的数量long teacher_count = list.stream().map(Student2::getTeacher).distinct().count();System.out.println("10 获取学生老师的数量 t" + teacher_count);// 11 获取每个老师带的学生个数Map<String, Integer> teacher_student_cnt = list.stream().upingBy(Student2::getTeacher, Collectors.summingInt(Student2 -> 1)));System.out.println("11 获取每个老师带的学生个数");priMap(teacher_student_cnt);// 12 按照学生分数排名，获取列表List<Student2> sorted_stu = list.stream().sorted((a,b)->Score(),a.getScore())).List());System.out.println("12 按照学生分数排名，获取列表 t"+ sorted_stu);// 13 获取成绩前3的学生List<Student2> top_3 = list.stream().sorted((a, b) -> Score(), a.getScore())).limit(3).List());System.out.println("13 获取成绩前3的学生t"+top_3);System.out.println("--------------解析字符串-------------------");// a 将 “a,b;c;d;e,f;” 解析成P对象的集合 List<Pair<Integer,Long>>String pairs = "100,2;101,2;102,3;103,4";List<Pair<Integer, Long>> str_pairs = Arrays.stream(pairs.split(";")).map(str -> str.split(",")).map(arr -> new Pair<Integer, Long>(Integer.parseInt(arr[0]), Long.parseLong(arr[1]))).List());System.out.println("a 将 “a,b;c;d;e,f;” 解析成P对象的集合 List<Pair<Integer,Long>>t"+str_pairs);// b 将 "a,b" 解析成P对象String pair = "101,2";// firList<Pair<Integer, Long>> pairs1 = Stream.of(pair).map(str -> {String[] sp = str.split(",");if(sp.length<2) return  null;return new Pair<Integer, Long>(Integer.parseInt(sp[0]), Long.parseLong(sp[1]));}).List());// secondPair<Integer, Long> fir = Stream.of(pair).map(str -> {String[] sp = str.split(",");if (sp.length < 2) return null;return new Pair<Integer, Long>(Integer.parseInt(sp[0]), Long.parseLong(sp[1]));}).findFirst().get();System.out.println("b1 将 "a,b" 解析成P对象" + (0));System.out.println("b2 将 "a,b" 解析成P对象" + fir);// arr1 计算数组总和int[] arr = {1,2,3};int arr_sum = Arrays.stream(arr).sum();System.out.println("arr1 计算数组总和t"+arr_sum);// arr2 计算数组平均值double avg_arr = Arrays.stream(arr).average().orElse(0.0);System.out.println("arr2 计算数组平均值t"+ avg_arr);// arr3 给数组*2 计算总和int sum_2 = Arrays.stream(arr).map(num -> num * 2).sum();System.out.println("arr3 给数组*2 计算总和t"+sum_2);// arr4 计算int[][] arr_2 的数组总和(求和二维数组)int[][] arr_2 = {{1,2,3},{2,3}};int sum_2_21 = Stream.of(arr_2).mapToInt(ar -> Arrays.stream(ar).sum()).sum();System.out.println("arr4 求和二维数组t" + sum_2_21);// arr5 求和二维数组的个数int avg_2_arr = Stream.of(arr_2).mapToInt(ar -> ar.length).sum();System.out.println("arr5 求和二维数组的个数t"+avg_2_arr);// arr6 获取数组中的最大值int max = Arrays.stream(arr).max().orElseGet(() -> 0);System.out.println("arr6 获取数组中的最大值t"+max);// arr7 获取数组中的最小值int min = Arrays.stream(arr).min().orElseGet(() -> 0);System.out.println("arr7 获取数组中的最小值t"+min);// arr8 判断元素是否在数组中(ar==1)boolean is_one_in = Arrays.stream(arr).anyMatch(ar -> ar == 1);System.out.println("arr8 判断元素是否在数组中(ar==1)t"+is_one_in);// arr9 判断元素是否不在数组中(ar==10)boolean is_not_in = Arrays.stream(arr).noneMatch(ar -> ar == 10);System.out.println("arr9 判断元素是否不在数组中(ar==10)t"+is_not_in);// arr10 获取最大的元素}public static void priMap(Map map){Set().forEach(System.out::println);}static class Pair<T,V>{public T key;public V value;public Pair() {}public Pair(T key, V value) {this.key = key;this.value = value;}@Overridepublic String toString() {return "MyPair{" +"key=" + key +", value=" + value +'}';}}
}