Collectors: Aggregation Operations

In this lesson, we will learn about methods of the Collectors class, which are used for aggregation.

We'll cover the following

In this lesson, we will look at some of the methods of the Collectors class that help us aggregate the data in streams, e.g., sum, average, etc.

1) counting()

This function returns a Collector that counts the number of the input elements.

Suppose we have a list of employees, and we need the count of employees with an age more than 30.

In this case, we can use the counting() method as shown below.

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import java.util.ArrayList;

import java.util.List;

import java.util.stream.Collectors;



public class CollectorsDemo {



    public static void main(String args[]) {

        List<Employee> employeeList = new ArrayList<>();

        employeeList.add(new Employee("Alex", 23, 23000));

        employeeList.add(new Employee("Ben", 63, 25000));

        employeeList.add(new Employee("Dave", 34, 56000));

        employeeList.add(new Employee("Jodi", 43, 67000));

        employeeList.add(new Employee("Ryan", 53, 54000));



        long count = employeeList.stream()

                .filter(emp -> emp.getAge() > 30)

                .collect(Collectors.counting()); // Using the counting() method to get count of employees.



        System.out.println(count);

    }

}



class Employee {

    String name;

    int age;

    int salary;



    Employee(String name) {

        this.name = name;

    }



    Employee(String name, int age, int salary) {

        this.name = name;

        this.age = age;

        this.salary = salary;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    public int getSalary() {

        return salary;

    }



    @Override

    public String toString() {

        return "Employee{" +

                "name='" + name + '\'' +

                ", age=" + age +

                ", salary=" + salary +

                '}';

    }

}

2) Collectors.summingInt(ToIntFunction<? super T> mapper)

This method returns a Collector that produces the sum of an integer-valued function applied to the input elements.

This method takes a ToIntFunction as a parameter.

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import java.util.ArrayList;

import java.util.Collections;

import java.util.LinkedList;

import java.util.List;

import java.util.stream.Collectors;



public class CollectorsDemo {



    public static void main(String args[]) {

        List<Employee> employeeList = new ArrayList<>();

        employeeList.add(new Employee("Alex", 23, 23000));

        employeeList.add(new Employee("Ben", 63, 25000));

        employeeList.add(new Employee("Dave", 34, 56000));

        employeeList.add(new Employee("Jodi", 43, 67000));

        employeeList.add(new Employee("Ryan", 53, 54000));



        // Using summingInt() method to get the sum of salaries of all employees.

        int count = employeeList.stream()

                .collect(Collectors.summingInt(emp -> emp.getSalary()));



        System.out.println(count);

    }

}



class Employee {

    String name;

    int age;

    int salary;



    Employee(String name) {

        this.name = name;

    }



    Employee(String name, int age, int salary) {

        this.name = name;

        this.age = age;

        this.salary = salary;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    public int getSalary() {

        return salary;

    }



    @Override

    public String toString() {

        return "Employee{" +

                "name='" + name + '\'' +

                ", age=" + age +

                ", salary=" + salary +

                '}';

    }

}

There are similar functions for long and double as well, namely summingLong() and summingDouble(), respectively.

3) Collectors.averagingInt(ToIntFunction<? super T> mapper)

This method returns a Collector that produces the arithmetic mean of an integer-valued function applied to the input elements. If no elements are present, the result is 0.

This method takes a ToIntFunction as a parameter.

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import java.util.ArrayList;

import java.util.Collections;

import java.util.LinkedList;

import java.util.List;

import java.util.stream.Collectors;



public class CollectorsDemo {



    public static void main(String args[]) {

        List<Employee> employeeList = new ArrayList<>();

        employeeList.add(new Employee("Alex", 23, 23000));

        employeeList.add(new Employee("Ben", 63, 25000));

        employeeList.add(new Employee("Dave", 34, 56000));

        employeeList.add(new Employee("Jodi", 43, 67000));

        employeeList.add(new Employee("Ryan", 53, 54000));



        // Using averagingInt() method to get the average of salaries of all employees.

        double average = employeeList.stream()

                .collect(Collectors.averagingInt(emp -> emp.getSalary()));



        System.out.println(average);

    }

}



class Employee {

    String name;

    int age;

    int salary;



    Employee(String name) {

        this.name = name;

    }



    Employee(String name, int age, int salary) {

        this.name = name;

        this.age = age;

        this.salary = salary;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    public int getSalary() {

        return salary;

    }



    @Override

    public String toString() {

        return "Employee{" +

                "name='" + name + '\'' +

                ", age=" + age +

                ", salary=" + salary +

                '}';

    }

}

There are similar functions for long and double as well, namely averagingLong(), and averagingDouble() respectively.

3) minBy(Comparator<? super T> comparator)

It returns a Collector that returns the minimum element based on the given comparator.

Let’s say, we have an ArrayList of Employee objects and we need to find the Employee object with a minimum salary. In this case, we first need to create a Comparator that compares two Employee objects on the basis of salary.

Then we will use this Comparator in the minBy() method. The returned value is wrapped in an Optional instance. The reason for this is that, it is possible that the Employee list is empty.

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import java.util.*;

import java.util.stream.Collectors;



public class CollectorsDemo {



    public static void main(String args[]) {

        List<Employee> employeeList = new ArrayList<>();

        employeeList.add(new Employee("Alex", 23, 23000));

        employeeList.add(new Employee("Ben", 63, 25000));

        employeeList.add(new Employee("Dave", 34, 56000));

        employeeList.add(new Employee("Jodi", 43, 67000));

        employeeList.add(new Employee("Ryan", 53, 54000));



        //Using minBy() method to get the employee with min salary.

        Optional<Employee> employee = employeeList.stream()

                .collect(Collectors.minBy(Comparator.comparing(Employee::getSalary)));



        System.out.println(employee.get().getName());

    }

}



class Employee {

    String name;

    int age;

    int salary;



    Employee(String name) {

        this.name = name;

    }



    Employee(String name, int age, int salary) {

        this.name = name;

        this.age = age;

        this.salary = salary;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    public int getSalary() {

        return salary;

    }



    @Override

    public String toString() {

        return "Employee{" +

                "name='" + name + '\'' +

                ", age=" + age +

                ", salary=" + salary +

                '}';

    }

}

4) maxBy(Comparator<? super T> comparator)

It returns a Collector that returns the maximum element based on the given comparator.

The returned value is wrapped in an Optional instance.

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import java.util.*;

import java.util.stream.Collectors;



public class CollectorsDemo {



    public static void main(String args[]) {

        List<Employee> employeeList = new ArrayList<>();

        employeeList.add(new Employee("Alex", 23, 23000));

        employeeList.add(new Employee("Ben", 63, 25000));

        employeeList.add(new Employee("Dave", 34, 56000));

        employeeList.add(new Employee("Jodi", 43, 67000));

        employeeList.add(new Employee("Ryan", 53, 54000));



        //Using maxBy() method to get the employee with max salary.

        Optional<Employee> employee = employeeList.stream()

                .collect(Collectors.maxBy(Comparator.comparing(Employee::getSalary)));



        System.out.println(employee.get().getName());

    }

}



class Employee {

    String name;

    int age;

    int salary;



    Employee(String name) {

        this.name = name;

    }



    Employee(String name, int age, int salary) {

        this.name = name;

        this.age = age;

        this.salary = salary;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    public int getSalary() {

        return salary;

    }



    @Override

    public String toString() {

        return "Employee{" +

                "name='" + name + '\'' +

                ", age=" + age +

                ", salary=" + salary +

                '}';

    }

}

5) summarizingInt(ToIntFunction<? super T> mapper)

It returns a Collector that applies an int-producing mapping function to each input element and returns summary statistics for the resulting values.

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import java.util.*;

import java.util.stream.Collectors;

import java.util.stream.Stream;



public class CollectorsDemo {



    public static void main(String args[]) {



        IntSummaryStatistics summarizingInt = Stream.of("1", "2", "3")

                .collect(Collectors.summarizingInt(Integer::parseInt));

        System.out.println(summarizingInt);

    }

}

6) joining()

It returns a Collector that concatenates the input elements into a String, in the encounter order. It also has few overloaded versions which allow us to provide delimiters and prefix and suffix strings.

One very important use case of this method can be if we want to create a comma-separated String out of a given list.

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import java.util.*;

import java.util.stream.Collectors;

import java.util.stream.Stream;



public class CollectorsDemo {



    public static void main(String args[]) {

        // Joining all the strings.

        String joinedString = Stream.of("hello", "how", "are" , "you")

                .collect(Collectors.joining());

        System.out.println(joinedString);



        // Joining all the strings with space in between.

        joinedString = Stream.of("hello", "how", "are" , "you")

                .collect(Collectors.joining(" "));

        System.out.println(joinedString);



        // Joining all the strings with space in between and a prefix and suffix.

        joinedString = Stream.of("hello", "how", "are" , "you")

                .collect(Collectors.joining(" " , "prefix","suffix"));

        System.out.println(joinedString);

    }

}

In the next lesson, we will take a look at grouping operations using Collectors.

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