本文主要是介绍java8 lambada表达式Collectors.groupingBy分组顺序无序，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
代码：
package com.java;import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;import com.cc.MyMapImpl;public class Test4 {static int m=0;
public static void main(String[] args) {List<Student>list=new ArrayList<>();for(int index=0;index<15;index++) {if(index<5) {list.add(new Student("张三",String.valueOf(index)));continue;}else if(index<10) {list.add(new Student("李四",String.valueOf(index)));continue;}list.add(new Student("王五",String.valueOf(index)));}Map<String,List<Student>>result2=list.stream().collect(Collectors.groupingBy(Student::getName));System.out.println(result2.toString());System.out.println(list.toString());Map<String,Long>result=list.stream().collect(Collectors.groupingBy(Student::getName,Collectors.counting()));System.out.println(result.toString());String str= list.stream().collect(Collectors.groupingBy(Student::getName,Collectors.counting())).entrySet().stream().max(Map.Entry.comparingByValue()).map(Map.Entry::getKey).get();
System.out.println(str);}
}
class Student{private String name;private String age;public Student(String name,String age ) {this.name=name;this.age=age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getAge() {return age;}public void setAge(String age) {this.age = age;}@Overridepublic String toString() {return "Student [name=" + name + ", age=" + age + "]";}}
预期效果应该是：
{张三=[Student [name=张三, age=5], Student [name=张三, age=6], Student [name=张三, age=7], Student [name=张三, age=8], Student [name=张三, age=9]], 李四=[Student [name=李四, age=0], Student [name=李四, age=1], Student [name=李四, age=2], Student [name=李四, age=3], Student [name=李四, age=4]], 王五=[Student [name=王五, age=10], Student [name=王五, age=11], Student [name=王五, age=12], Student [name=王五, age=13], Student [name=王五, age=14]]}
[Student [name=张三, age=5], Student [name=张三, age=6], Student [name=张三, age=7], Student [name=张三, age=8], Student [name=张三, age=9]], Student [name=李四, age=0], Student [name=李四, age=1], Student [name=李四, age=2], Student [name=李四, age=3], Student [name=李四, age=4], Student [name=王五, age=10], Student [name=王五, age=11], Student [name=王五, age=12], Student [name=王五, age=13], Student [name=王五, age=14]]
{张三=5, 李四=5, 王五=5}
张三
实际结果：
{李四=[Student [name=李四, age=5], Student [name=李四, age=6], Student [name=李四, age=7], Student [name=李四, age=8], Student [name=李四, age=9]], 张三=[Student [name=张三, age=0], Student [name=张三, age=1], Student [name=张三, age=2], Student [name=张三, age=3], Student [name=张三, age=4]], 王五=[Student [name=王五, age=10], Student [name=王五, age=11], Student [name=王五, age=12], Student [name=王五, age=13], Student [name=王五, age=14]]}
[Student [name=张三, age=0], Student [name=张三, age=1], Student [name=张三, age=2], Student [name=张三, age=3], Student [name=张三, age=4], Student [name=李四, age=5], Student [name=李四, age=6], Student [name=李四, age=7], Student [name=李四, age=8], Student [name=李四, age=9], Student [name=王五, age=10], Student [name=王五, age=11], Student [name=王五, age=12], Student [name=王五, age=13], Student [name=王五, age=14]]
{李四=5, 张三=5, 王五=5}
李四
分组之后数据变为了无序，预期的张三在第一个而实际却放在了第二个输出。
第一个参数：分组按照什么分类
第二个参数：分组最后用什么容器保存返回
第三个参数：按照第一个参数分类后，对应的分类的结果如何收集
其实一个参数的Collectors.groupingBy方法的 ，第二个参数默认是HashMap::new， 第三个参数收集器其实默认是Collectors.toList
所以HashMap是无序的大家都是知道的，所以原因找到了。
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