java8 lambada表达式Collectors.groupingBy分组顺序无序

 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) {

	Listlist=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>result2=list.stream()
			.collect(Collectors.groupingBy(Student::getName));
	System.out.println(result2.toString());
	
	System.out.println(list.toString());
	Mapresult=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;
	}
	@Override
	public 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。