Trusted answers to developer questions
Collect results of groupingBy() into different map implementation
abhilash
Overview
In Java, the groupingBy() method collects the results to HashMap implementation of the Map interface by default. In this shot, we’ll learn how to collect the results in a different implementation of Map such as TreeMap.
Note: Refer to this shot about grouping() to learn more about the method.
mapFactory parameter
The groupingBy() method optionally accepts the mapFactory parameter, which provides a new empty map into which the results of the group by operation will be stored. This parameter is an implementation of the Supplier interface that provides an empty map.
Syntax
public static <T, K, D, A, M extends Map<K, D>> Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier, Supplier<M> mapFactory, Collector<? super T, A, D> downstream)
Parameters
Function<? super T, ? extends K> classifier: This is the classification function.Collector<? super T, A, D> downstream: This is the collector implementing downstream reduction operation.Supplier<M> mapFactory: This is the supplier that provides amapinto which results are inserted.
Example
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collectors;
public class Main {
static class Student{
String firstName;
String lastName;
public Student(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
@Override
public String toString() {
return "Student{" +
"firstName='" + firstName + '\'' +
", lastName='" + lastName + '\'' +
'}';
}
}
public static void main(String[] args){
List<Student> studentList = Arrays.asList(new Student("sam", "collins"),
new Student("lilly", "collins"),
new Student("john", "adkins"),
new Student("jasmine", "adkins"),
new Student("kite", "mason"));
Function<Student, String> classificationFunction = student -> student.lastName;
Supplier<Map<String, List<Student>>> mapSupplier = TreeMap::new;
Map<String, List<Student>> groupedStudents = studentList.stream().collect(Collectors.groupingBy(classificationFunction, mapSupplier, Collectors.toList()));
System.out.println("Number of students in each group grouped according to their last name - \n" + groupedStudents);
}
}"mapFactory" parameter
Explanation
- Lines 1–7: We import the relevant classes.
- Lines 11–27: We define a
Studentclass that consists of thefirstNameandlastNameas fields. The class has a constructor that initializes the value of the fields and atoString()method implementation. - Lines 30–34: We create a list of
studentobjects. - Line 36: We define the
classificationFunctionbecause it returns thelastNamevalue for everyStudentobject passed. - Line 37: We create a supplier that provides a
mapinto which results are inserted. - Line 38: We create
groupedStudentsand get respected results into thismap. - Line 39: We print the results.
RELATED TAGS
java
communitycreator
CONTRIBUTOR
abhilash
RELATED COURSES
View all Courses
Keep Exploring
Related Courses
