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() methodimport 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}
