What is the stream.collect() method in Java?

Overview

The stream.collect() method is used to perform a mutable reduction operation on the elements of a stream. It returns a new mutable object containing the results of the reduction operation.

This method can be used to perform several different types of reduction operations, such as:

Computing the sum of numeric values in a stream.
Finding the minimum or maximum value in a stream.
Constructing a new String by concatenating the contents of a stream.
Collecting elements into a new List or Set.

import java.util.*;
import java.util.stream.*;
import static java.util.stream.Collectors.*;
public class CollectExample {
   public static void main(String[] args) {
      Integer[] intArray = {1, 2, 3, 4, 5};  
      // Creating a List from an array of elements
      // using Arrays.asList() method
      List<Integer> list = Arrays.asList(intArray);
     
      // Collecting all elements of the list into a new 
      // list using collect() method 
      List<Integer> evenNumbersList = list.stream()
            .filter(i -> i%2 == 0)
            .collect(toList());
      System.out.println(evenNumbersList);
      // finding the sum of all the values 
      // in the stream
      Integer sum = list.stream()
         .collect(summingInt(i -> i));
      System.out.println(sum);
      // finding the maximum of all the values 
      // in the stream
      Integer max = list.stream()
         .collect(maxBy(Integer::compare)).get();
      System.out.println(max);
      // finding the minimum of all the values 
      // in the stream
      Integer min = list.stream()
         .collect(minBy(Integer::compare)).get();
      System.out.println(min);
      // counting the values in the stream
      Long count = list.stream()
         .collect(counting());
      System.out.println(count);
   }
}

Explanation

Line 7: We have a stream of integers, which are the elements of intArray.
Line 11: We use the Arrays.asList() method to get a List from an array of elements.
Lines 15–17: We use the stream() method to get a stream from the list. We filter the even elements and collect them into a new list using the collect() method.
Line 18: We use the System.out.println() method to print the filtered even numbers.
Lines 22–23: We use the collect() method summingInt(ToIntFunction) as an argument. The summingInt() method returns a collector that sums the integer values extracted from the stream elements by applying an int producing mapping function to each element.
Line 24: We use the System.out.println() method to print the sum of elements in the stream.
Lines 28–29: We use the collect() method with maxBy(Comparator) as an argument. The maxBy() accepts a Comparator and returns a collector that extracts the maximum element from the stream according to the given Comparator.
Line 30: We use the System.out.println() method to print the maximum value in the stream.
Lines 34–35: We use the collect() method with minBy(Comparator) as an argument. The minBy() accepts a Comparator and returns a collector that extracts the minimum element from the stream according to the given Comparator.
Lines 39–40: We use the collect() method with counting() as an argument, which returns a collector that counts the number of input elements.
Line 41: We use the System.out.println() method to print the number of elements in the stream.

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What is the stream.collect() method in Java?

Overview

Syntax

Parameters

Return value

Example

Explanation