先声明一个学生实体类方便使用
package com.ddtech.business.influencing_factors.test;
public class Student {
private Integer no;
private String name;
private Integer age;
private Double mathScore;
private Double chineseScore;
public Student(Integer no, String name, Integer age, Double mathScore, Double chineseScore) {
this.no = no;
this.name = name;
this.age = age;
this.mathScore = mathScore;
this.chineseScore = chineseScore;
}
public Integer getNo() {
return no;
}
public void setNo(Integer no) {
this.no = no;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
public Double getMathScore() {
return mathScore;
}
public void setMathScore(Double mathScore) {
this.mathScore = mathScore;
}
public Double getChineseScore() {
return chineseScore;
}
public void setChineseScore(Double chineseScore) {
this.chineseScore = chineseScore;
}
@Override
public String toString() {
return "Student{" +
"no=" + no +
", name='" + name + '\'' +
", age=" + age +
", mathScore=" + mathScore +
", chineseScore=" + chineseScore +
'}';
}
}
封装一个数组，伪造数据
package com.ddtech.business.influencing_factors.test;
import java.util.ArrayList;
import java.util.List;
public class StudentUtil2 {
/**
* 生成指定的学生类的列表，用于测试
*
* @return List<Student>
*/
public static List<Student> createStudentList() {
List<Student> studentList = new ArrayList<>();
studentList.add(new Student(1, "林高禄", 20, 90.5, 90.5));
studentList.add(new Student(11, "林高禄", 20, 90.5, 90.5));
studentList.add(new Student(1, "1林高禄", 20, 90.5, 90.5));
studentList.add(new Student(2, "林高禄", 10, 80.0, 90.0));
studentList.add(new Student(1, "林高禄", 30, 90.5, 90.0));
studentList.add(new Student(1, "陈文文", 10, 100.0, 90.0));
studentList.add(new Student(2, "陈文文", 20, 90.0, 70.0));
studentList.add(new Student(1, "蔡金鑫", 30, 80.0, 90.0));
return studentList;
}
}
开始使用
一、求相同姓名的学生的年龄之和（姓名组合）
package com.ddtech.business.influencing_factors.test;
import java.util.*;
import java.util.stream.Collectors;
public class Demo2 {
public static void main(String[] args) {
// 求相同姓名的学生的年龄之和（姓名组合）
List<Student> studentList = StudentUtil2.createStudentList();
Map<String, List<Student>> listMap = studentList.stream().collect(Collectors.groupingBy(Student::getName, Collectors.toList()));
System.out.println("分组后的数据：");
listMap.forEach((k,v)->{
System.out.println("key："+k);
v.forEach(System.out::println);
});
// 求和
System.out.println("分组后求和：");
listMap.forEach((k,v)->{
System.out.print("key："+k);
System.out.println("--年龄求和:"+v.stream().mapToInt(Student::getAge).sum());
});
}
}
二、求相同姓名和相同编号的学生的年龄之和（姓名——编号组合）
package com.ddtech.business.influencing_factors.test;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Demo3 {
public static void main(String[] args) {
List<Student> studentList = StudentUtil2.createStudentList();
// 通过combination返回的字符串分组，为key,相同的学生为列表
Map<String, List<Student>> collect = studentList.stream().collect(Collectors.groupingBy(Demo3::combination, Collectors.toList()));
// 分组后的年龄和为
System.out.println("分组后的年龄和为:");
collect.forEach((key,list)-> System.out.println("key:"+key+"，年龄和"+list.stream().mapToInt(Student::getAge).sum()));
}
// 编号+姓名
private static String combination(Student s){
return s.getNo()+s.getName()+s.getName();
}
}
三、 按一个字段(key)分组排序，两种排序的方法
package com.ddtech.business.influencing_factors.test;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Demo4 {
public static void main(String[] args) {
// 按一个字段分组排序
List<Student> studentList = StudentUtil2.createStudentList();
Map<Integer, List<Student>> collect =studentList.stream().collect(Collectors.groupingBy(Student::getAge));
ArrayList<Integer> keys = new ArrayList<>();
collect.forEach((k,v)->{
keys.add(k);
});
keys.sort(Comparator.comparing(Integer::intValue));
for (Integer key : keys) {
System.out.println("key"+key);
List<Student> students = collect.get(key);
for (Student student : students) {
System.out.println(student);
}
}
// 例子：学生按编号排序，再按年龄排序
System.out.println("第一种：直接排序");
studentList.sort(Comparator.comparing(Student::getNo).reversed().thenComparing(Comparator.comparing(Student::getAge).reversed()));
studentList.forEach(System.out::println);
System.out.println("第二种：用Stream排序");
studentList.stream()
.sorted(Comparator.comparing(Student::getNo).reversed().thenComparing(Comparator.comparing(Student::getAge).reversed()))
.forEach(System.out::println);
}
}
四、求出学生数学分数的各个值（最大，最小，平均，总数，个数）
package com.ddtech.business.influencing_factors.test;
import java.util.DoubleSummaryStatistics;
import java.util.List;
import java.util.stream.Collectors;
public class Demo5 {
public static void main(String[] args) {
//例子：求出学生数学分数的各个值（最大，最小，平均，总数，个数）
List<Student> studentList = StudentUtil2.createStudentList();
DoubleSummaryStatistics collect = studentList.stream().collect(Collectors.summarizingDouble(Student::getMathScore));
System.out.println(collect);
System.out.println("总分为："+collect.getSum());
}
}
五、综合例子：求相同年龄学生的语数总分的排名，倒序
package com.ddtech.business.influencing_factors.test;
import java.security.Key;
import java.util.*;
import java.util.stream.Collectors;
public class Demo6 {
public static void main(String[] args) {
// 综合例子：求相同年龄学生的语数总分的排名，倒序
List<Student> studentList = StudentUtil2.createStudentList();
// 先按年龄分组求出极值对象
Map<Integer, DoubleSummaryStatistics> collect1 = studentList.stream().collect(Collectors.groupingBy(Student::getAge, Collectors.summarizingDouble(s -> s.getMathScore() + s.getChineseScore())));
collect1.forEach((k,v)->{
System.out.println("key:"+ k+"--v:"+v.toString());
});
List<DoubleSummaryStatistics> doubleSummaryStatisticsList = new ArrayList<>();
// 遍历map把极值对象放入集合
collect1.forEach((key, doubleSummaryStatistics) -> doubleSummaryStatisticsList.add(doubleSummaryStatistics));
// 极值对象排序输出
doubleSummaryStatisticsList.stream().sorted(Comparator.comparing(DoubleSummaryStatistics::getSum).reversed()).forEach(System.out::println);
System.out.println("------------");
// 极值对象排序输出总分
doubleSummaryStatisticsList.stream().sorted(Comparator.comparing(DoubleSummaryStatistics::getSum).reversed()).forEach(s -> System.out.println(s.getSum()));
}
}