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Collectors groupingBy() method in Java with Examples

  • Last Updated : 29 Jul, 2021

The groupingBy() method of Collectors class in Java are used for grouping objects by some property and storing results in a Map instance. In order to use it, we always need to specify a property by which the grouping would be performed. This method provides similar functionality to SQL’s GROUP BY clause.


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public static Collector<T, ?, Map<K, List>> groupingBy(Function classifier) 

Type Parameter: This method takes two type parameters: 

  • T- It is the type of the input elements.
  • K- It is the type the input elements to be converted.

Parameters: This method accepts two mandatory parameters:  

  • Function- It is the property which is to be applied to the input elements.
  • Classifier- It is used to map input elements into the destination map.

Return value: It returns a collector as a map.

Below is the program implementation of groupingBy() method:
Program 1: 


// Java program to demonstrate
// Collectors groupingBy() method
import java.util.*;
import java.util.function.Function;
public class GFG {
    public static void main(String[] args)
        // Get the List
        List<String> g
            = Arrays.asList("geeks", "for", "geeks");
        // Collect the list as map
        // by groupingBy() method
        Map<String, Long> result
        // Print the result
{geeks=2, for=1}



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