Collectors groupingBy() method in Java

This post will discuss the groupingBy() method provided by the Collectors class in Java.

The groupingBy(classifier) returns a Collector implementing a “group by” operation on input elements, grouping elements according to a classification function, and returning the results in a map. Following are some examples demonstrating the usage of this function:

1. Split a list into two sublists

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

// Java program to split a list using `Collectors.groupingBy()` method

class Main

{

public static void main(String[] args)

{

List<Integer> inputList = Arrays.asList(1, 2, 3, 4, 5);

// get the mid-index for splitting the input list into two

int midIndex = (inputList.size() - 1) / 2;

Map<Boolean, List<Integer>> map = inputList.stream()

.collect(Collectors.groupingBy(s -> inputList.indexOf(s) > midIndex));

System.out.println(map);

List<List<Integer>> lists = new ArrayList<>(map.values());

System.out.println("The first sublist is " + lists.get(0));

System.out.println("The second sublist is " + lists.get(1));

}

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Output:

{false=[1, 2, 3], true=[4, 5]}
The first sublist is [1, 2, 3]
The second sublist is [4, 5]

This is equivalent to:

import java.util.*;

class Main

{

public static void main(String[] args)

{

List<Integer> inputList = Arrays.asList(1, 2, 3, 4, 5);

// get the mid-index for splitting the input list into two

int midIndex = (inputList.size() - 1) / 2;

Map<Boolean, List<Integer>> map = new HashMap<>();

for (Integer s : inputList) {

map.computeIfAbsent(inputList.indexOf(s) > midIndex, k -> new ArrayList<>()).add(s);

}

System.out.println(map);

List<List<Integer>> lists = new ArrayList<>(map.values());

System.out.println("The first sublist is " + lists.get(0));

System.out.println("The second sublist is " + lists.get(1));

}

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Output:

{false=[1, 2, 3], true=[4, 5]}
The first sublist is [1, 2, 3]
The second sublist is [4, 5]

2. Group students by grades

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

class Student

{

private String name;

private String grade;

private int marks;

Student(String name, String grade, Integer marks)

{

this.name = name;

this.grade = grade;

this.marks = marks;

}

public String getName() { return name; }

public String getGrade() { return grade; }

public int getMarks() { return marks; }

@Override

public String toString() { return getName(); }

}

// Java program to group students by grades using `Collectors.groupingBy()`

class Main

{

public static void main(String[] args)

{

List<Student> students = Arrays.asList(new Student("Tom", "A+", 90),

new Student("Lisa", "A+", 98),

new Student("John", "A", 85),

new Student("Joe", "A", 80),

new Student("Jason", "E", 35));

Map<String, List<Student>> studentsByGrade

= students.stream()

.collect(Collectors.groupingBy(Student::getGrade));

for (Map.Entry<String, List<Student>> entry: studentsByGrade.entrySet()) {

System.out.println("Students with " + entry.getKey() +

" grade are " + entry.getValue());

}

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Output:

Students with A grade are [John, Joe]
Students with E grade are [Jason]
Students with A+ grade are [Tom, Lisa]

The groupingBy(classifier, downstream) collector allows the collection of stream elements into a map by grouping elements according to a classifier method and then performing a reduction operation on the values associated with a given key using the specified downstream Collector. Following are some examples demonstrating the usage of this function:

1. Compute average marks of students with the same grades

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

class Student

{

private String name;

private String grade;

private int marks;

Student(String name, String grade, Integer marks)

{

this.name = name;

this.grade = grade;

this.marks = marks;

}

public String getName() { return name; }

public String getGrade() { return grade; }

public int getMarks() { return marks; }

@Override

public String toString() { return getName(); }

}

// Java program to compute average marks of students with the same grades

// using `Collectors.groupingBy()`

class Main

{

public static void main(String[] args)

{

List<Student> students = Arrays.asList(new Student("Tom", "A+", 90),

new Student("Lisa", "A+", 98),

new Student("John", "A", 85),

new Student("Joe", "A", 80),

new Student("Jason", "E", 35));

Map<String, Double> studentsByGrade =

students.stream()

.collect(Collectors.groupingBy(Student::getGrade,

Collectors.averagingInt(Student::getMarks)));

for (Map.Entry<String, Double> entry: studentsByGrade.entrySet()) {

System.out.println("Students with " + entry.getKey() +

" grade have average marks of " + entry.getValue());

}

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Output:

Students with A grade have average marks of 82.5
Students with E grade have average marks of 35.0
Students with A+ grade have average marks of 94.0

2. Count the occurrences of elements in a stream

We can also use the Collectors.groupingBy() method to count the frequency of elements present in a stream.

import java.util.Map;

import java.util.function.Function;

import java.util.stream.Collectors;

import java.util.stream.Stream;

// Java program to create a frequency map of elements in a stream

// using `Collectors.groupingBy()`

class Main

{

public static void main(String[] args)

{

Map<String, Long> freq = Stream.of("A", "B", "A", "C", "A", "C")

.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

System.out.println(freq);

}

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Output:

{A=3, B=1, C=2}

This is equivalent to:

import java.util.Arrays;

import java.util.HashMap;

import java.util.Map;

class Main

{

public static void main(String[] args)

{

Map<String, Long> freq = new HashMap<>();

for (String s : Arrays.asList("A", "B", "A", "C", "A", "C")) {

freq.merge(s, 1L, Long::sum);

}

System.out.println(freq);

}

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Output:

{A=3, B=1, C=2}

That’s all about the Collectors class groupingBy() method in Java.

Collectors groupingBy() method in Java

1. Split a list into two sublists

2. Group students by grades

1. Compute average marks of students with the same grades

2. Count the occurrences of elements in a stream

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