How to group by range of values in Java 8 using streams

Question

Here is a sample scenario:

Imagine we have employee records like:

name, age, salary (in 1000 dollars)
   a,  20,     50
   b,  22,     53
   c,  34,         


        

Answer 1

The below code should give you what you are looking for. The key is "Collectors" class which support grouping.

Map<Double,Integer> ageGroup= employees.stream().collect(Collectors.groupingBy(e->Math.ceil(e.age/10.0),Collectors.summingInt(e->e.salary)));

The illustration assuming the salary is integer but easy to switch to double

The complete program looks like

public static void main(String[] args) {
    // TODO Auto-generated method stub

    List<Employee> employees = new ArrayList<>();
    employees.add(new Employee("a",20,100));
    employees.add(new Employee("a",21,100));
    employees.add(new Employee("a",35,100));
    employees.add(new Employee("a",32,100));


    Map<Double,Integer> ageGroup= employees.stream().collect(Collectors.groupingBy(e->Math.ceil(e.age/10.0),Collectors.summingInt(e->e.salary)));
    System.out.println(ageGroup);
}

public static class Employee {
    public Employee(String name, int age, int salary) {
        super();
        this.name = name;
        this.age = age;
        this.salary = salary;
    }
    public String name;
    public int age;
    public int salary;

}

The output is

{4.0=200, 2.0=100, 3.0=100}

Answer 2

Yes, you could define an AgeGroup interface or even an enum like this (assuming Employee definition):

enum AgeGroup {
    TWENTIES,
    THIRTIES,
    FORTIES,
    FIFTIES;
    .....
}
Function<Employee, AgeGroup> employee2Group = e -> {
    if(e.age >= 20 && e.getAge() < 30)
        return AgeGroup.TWENTIES;
    ....
    return null;
};

Map<AgeGroup, Double> avgByAgeGroup = employees.stream()
    .collect(Collectors.groupingBy(employee2Group, Collectors.averagingInt(Employee::getSalary)));

avgByAgeGroup.get(AgeGroup.TWENTIES)