java - 以函数和自定义对象收集器为参数进行分组

Question

我有以下要求，这可能很简单，但作为初学者，我想在这里寻求帮助。

class Employee{
  String department;
  int age;
  int salary;
}

class Foo{
  double averagingAge;
  double averagingSalary;
}

我想按部门分组使用groupingBy(department, collector(Foo)). 任何人都可以帮忙吗？

Answer 1

custom collector is the solution in java8: 

class Foo{
  private double ageSum;
  private double salarySum;
  private long ageCount;
  private long salaryCount;

    public double getAgeAvg(){
        return ageCount != 0 ? ageSum/ageCount : 0.0;
    }

    public double getSalaryAvg(){
        return salaryCount != 0 ? salarySum/salaryCount : 0.0;
    }
}

自定义收集器：

Map<Department, Foo> groupByResult = empList.stream().collect(groupingBy(department,
                Collector.of(
                        () -> {
                            return new Foo();
                        },
                        (result, emp) -> {
                            result.setAgeSum(result.getAgeSum() + emp.getAge());
                            result.setSalarySum(result.getSalarySum() + emp.getSalarySum());
                            result.setSalaryCount((result.getSalaryCount()) + 1);
                            result.setAgeCount(result.getAgeCount() + 1);
                        },
                        (result1, result2) -> {
                            result1.setSalarySum(result1.getSalarySum() + result2.getSalarySum());
                            result1.setAgeSum(result1.getAgeSum() + result2.getAgeSum());
                            result1.setAgeCount(result1.getAgeCount() + result2.getAgeCount());
                            result1.setSalaryCount(result1.getSalaryCount() + result2.getSalaryCount());
                            return result1;
                        }
        ));