In Java 8 and later versions, the Collectors class in the java.util.stream package provides various utility methods to perform reduction operations on streams. One of the powerful features of the Collectors class is the ability to group and count elements in a list or stream.
To group and count elements in a list, you can use the Collectors.groupingBy() and Collectors.counting() methods in combination. The groupingBy() method is used to create a Map where the elements are grouped based on a specific classifier, and the counting() method is used to count the occurrences of each group.
How to use the Collectors class in Java 8 to group and count elements in a list
How to use the Collectors class in Java 8 to group and count elements in a list? Click To Tweet
Here’s an example of the usage of Collectors class to group and count elements in a list:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | /** * Use of Collectors class to count and group elements in a List in Java 8 * @author Deepak Verma * */ import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class Collectors_Class_To_Count_and_Group_Elements_In_a_List_In_Java8 { public static void main(String[] args) { List<String> programmingWebsites = Arrays.asList("Techndeck", "MkYong", "Baeldung", "JavaTPoint", "Java67", "MkYong", "Techndeck", "Java67", "Java67", "Java67", "Techndeck", "Baeldung", "Baeldung"); // Group and count elements in the list Map<String, Long> websitesCount_Map = programmingWebsites.stream() .collect(Collectors.groupingBy(website -> website, Collectors.counting())); // Print out the result for (Map.Entry<String, Long> entry : websitesCount_Map.entrySet()) { System.out.println(entry.getKey() + " = " + entry.getValue()); } } } |
Output:
1 2 3 4 5 | Baeldung = 3 Java67 = 4 Techndeck = 3 JavaTPoint = 1 MkYong = 2 |
In the example above, we have a list of websites and our objective is to count the occurrences of each website present in the list. First, we will use the stream() method to convert the list to a stream, and then we use the collect() method along with Collectors.groupingBy() to group the elements by the website name. The second parameter to groupingBy() is Collectors.counting(), which will count the occurrences of each group. The websiteCount_map contains the result, where each website name is associated with its count in the original list. The result shows the count of each website in the list. So, what we have learnt is that by using the Collectors.groupingBy() and Collectors.counting() methods, we can easily perform grouping and counting operations on lists in Java 8 or any other later versions.
Learn to use Collectors() class to group and count elements in a List in Java 8..!!! Click To Tweet
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