Collectors groupingBy() method in Java with Examples

https://www.geeksforgeeks.org/collectors-groupingby-method-in-java-with-examples/

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.
 

Syntax:  

 

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

 

// Java program to demonstrate

// Collectors groupingBy() method

 

import java.util.*;

import java.util.function.Function;

import java.util.stream.Collectors;

 

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

            = g.stream().collect(

                Collectors.groupingBy(

                    Function.identity(),

                    Collectors.counting()));

 

        // Print the result

        System.out.println(result);

    }

}

Output: 

{geeks=2, for=1}