groupingBy()是Stream API中最强大的收集器Collector之一，提供与SQL的GROUP BY子句类似的功能。

使用stream流通过传递lambda表达式，可以让代码看上去很简洁。

此外，还可以用Java Stream和Collectors轻松完成字段的聚合。比如：相加，取平均数，或最大/最小值。更好的帮助我们分析数据。

目录

数据准备

员工类

员工list 

分组

按部门进行分组

分组得到ConcurrentMap

分组得到每个部门的员工姓名

嵌套分组

计数count 

计算平均值

求和

排序

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数据准备

 员工类

// 员工类
public class Employee{
	//部门
	private String deptId;
	//姓名
	private String name;
	//年龄
	private int age;
	//性别
	private String sex;
	//薪资
	private int salary;

    // 构造方法
	public Employee(String deptId, String name, int age, String sex, int salary) {
		this.deptId = deptId;
		this.name = name;
		this.age = age;
		this.sex = sex;
		this.salary = salary;
	}
	// 省略了get和set，请自行添加
}
// 实例化对象
Employee emp1 = new Employee("1201","张三",26,"男",7000);
Employee emp2 = new Employee("1202","李四",27,"男",7500);
Employee emp3 = new Employee("1203","王五",26,"女",7800);
Employee emp4 = new Employee("1201","赵六",24,"女",7000);
Employee emp5 = new Employee("1202","钱七",28,"男",5000);
Employee emp6 = new Employee("1201","老八",34,"男",8000);

员工list 

// 利用stream，转为员工清单list
List<Employee> emps = Stream.of(emp1,emp2,emp3,emp4,emp5,emp6).collect(Collectors.toList());

分组

 按部门进行分组

// 按照部门进行分组
Map<String, List<Employee>> collect = emps.stream().collect(Collectors.groupingBy(Employee::getDeptId));
System.out.println(collect);

 分组得到ConcurrentMap

// 分组后得到一个线程安全的ConcurrentMap
ConcurrentMap<String, List<Employee>> collect = emps.stream().collect(Collectors.groupingByConcurrent(Employee::getDeptId));
System.out.println(collect);

 分组得到每个部门的员工姓名

// 按照部门分组，得到每个部门的员工姓名
Map<String, List<String>> collect = emps.stream().collect(Collectors.groupingBy(Employee::getDeptId, Collectors.mapping(Employee::getName, Collectors.toList())));
System.out.println(collect);

嵌套分组

// 先按部门分组，再按性别分组
Map<String, Map<Integer, List<Employee>>> collect = emps.stream().collect(Collectors.groupingBy(Employee::getDeptId, Collectors.groupingBy(Employee::getSex)));
System.out.println(collect);

计数count 

// 统计每个部门的人数，得到map集合
Map<String, Long> collect = emps.stream().collect(Collectors.groupingBy(Employee::getDeptId, Collectors.counting()));
System.out.println(collect);

计算平均值

// 计算每个部门的平均薪水
Map<String, Double> collect = emps.stream().collect(Collectors.groupingBy(Employee::getDeptId, Collectors.averagingDouble(Employee::getSalary)));
System.out.println(collect);

求和

// 计算每个部门的整体薪水
Map<String, Double> collect = emps.stream().collect(Collectors.groupingBy(Employee::getDeptId, Collectors.summingInt(Employee::getSalary)));
System.out.println(collect);

排序

// 根据薪水分组并小到大排序,TreeMap默认为按照key升序
TreeMap<Integer, List<String>> collect = emps.stream().collect(Collectors.groupingBy(Employee::getSalary, TreeMap::new, Collectors.mapping(Employee::getName, Collectors.toList())));
System.out.println(collect);