Java 8 | Collectors counting() with Examples

Collectors counting() method is used to count the number of elements passed in the stream as the parameter. It returns a Collector accepting elements of type T that counts the number of input elements. If no elements are present, the result is 0. It is a terminal operation i.e, it may traverse the stream to produce a result or a side-effect. It returns the total count of elements in the stream which reach the collect() method after undergoing various pipelined stream operations such as filtering, reduction etc.

Syntax:

public static <T> Collector<T, ?, Long> counting()

where the mentioned terms are as follows:

• Interface Collector<T, A, R>: A mutable reduction operation that accumulates input elements into a mutable result container, optionally transforming the accumulated result into a final representation after all input elements have been processed. Reduction operations can be performed either sequentially or in parallel.
  • T: The type of input elements to the reduction operation.
  • A: The mutable accumulation type of the reduction operation.
  • R: The result type of the reduction operation.
• Long: The Long class wraps a value of the primitive type long in an object. An object of type Long contains a single field whose type is long. In addition, this class provides several methods for converting a long to a String and a String to a long, as well as other constants and methods useful when dealing with a long.
• T: The type of the input elements.

Parameters: This method does not take any parameter.

Return Value: A Collector that counts the input elements. The count is returned as Long object.

Below are examples to illustrate counting() method:

Program 1:

// Java code to show the implementation of
// Collectors counting() method

import java.util.stream.Collectors;
import java.util.stream.Stream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {
        // creating a stream of strings
        Stream<String> s = Stream.of("1", "2", "3", "4");

        // using Collectors counting() method to
        // count the number of input elements
        long ans = s.collect(Collectors.counting());

        // displaying the required count
        System.out.println(ans);
    }
}

Output:

4

Program 2: When no element is passed as input element.

// Java code to show the implementation of
// Collectors counting() method

import java.util.stream.Collectors;
import java.util.stream.Stream;

class GFG {

    // Driver code
    public static void main(String[] args)
    {
        // creating a stream of strings
        Stream<String> s = Stream.of();

        // using Collectors counting() method to
        // count the number of input elements
        long ans = s.collect(Collectors.counting());

        // displaying the required count
        System.out.println(ans);
    }
}

Output:

0