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Collectors and reduction in java

In the Java programming language Streams API, the Collectors and Reduction methods are used to merge or combine elements of array, list, and set data types in Java and display the final result. Java users use the Collectors and Reduction-supported function methods to perform multiple Java stream operations.

Collectors Method with Java Streams.

To use the Collectors method in Java programs, Java provides a built-in utility class called Collectors, java.util.stream, which helps users collect stream data type elements and return results from lists, sets, maps, strings, and other collection data types in a Java program.

Import these required class package libraries.

import java.util.stream.Collectors;

collect(toList()) Streams function method with Java.

The collect(toList()) function method in a Java Streams program collects and displays the elements of the Collection Array data type in a list.

Example of the collect(toList()) Streams function.

List<Integer> integers =

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

public class Main {

public static void main(String[] args) {

List<Integer> integers = Arrays.asList(1, 4, 7, 10, 17, 21, 29, 39, 5);

List<Integer> output = integers.stream()

.filter(integer -> integer > 10)

.collect(Collectors.toList());

System.out.println(output);

}

}

It’s output.

[17, 21, 29, 39]

collect(toSet()) Streams function method with Java.

The collect(toSet()) function in a Java Streams program removes any duplicate values ​​from a collection array data element and creates a new list of Set data type elements.

Example of the collect(toSet()) Streams function.

List<Integer> integers =

import java.util.Arrays;

import java.util.Set;

import java.util.stream.Collectors;

public class Main {

public static void main(String[] args) {

var integers = Arrays.asList(1, 4, 1, 7, 9, 7, 10, 4, 21, 77);

Set<Integer> output = integers.stream()

.collect(Collectors.toSet());

System.out.println(output);

}

}

It’s output.

[1, 4, 21, 7, 9, 10, 77]

joining() Streams function method with Java.

The joining() function is used in Java Streams programs to group or combine collection array data element strings into a single string.

Example of the joining() Streams function.

List<String> city =

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;

public class Main {

public static void main(String[] args) {

List<String> city = Arrays.asList(“Mumbai”, “Delhi”, “Bengaluru”, “Hyderabad”, “Ahmedabad”);

String output = city.stream()

.collect(Collectors.joining(“: “));

System.out.println(output);

}

}

It’s output.

Mumbai: Delhi: Bengaluru: Hyderabad: Ahmedabad

groupingBy() Streams function method with Java.

The groupingBy() function in a Java Streams program displays collection array data type elements grouped by a particular array property.

Example of the groupingBy() Streams function.

List<String> country =

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

public class Main {

public static void main(String[] args) {

List<String> country = Arrays.asList(“India”, “United States”, “China”, “Canada”, “Brazil”, “Australia”, “Argentina”);

Map<Integer, List<String>> output = country.stream()

.collect(Collectors.groupingBy(String::length));

System.out.println(output);

}

}

Its output.

{5=[India, China], 6=[Canada, Brazil], 9=[Australia, Argentina], 13=[United States]}

This function displays country names grouped by their length.

counting() Streams function method with Java.

The counting() function counts the elements of the Collection array data type in Java streams programs. It is mostly used with the groupingBy() function method in Java streams.

Example of the counting() Streams function.

Map<Integer, Long> output =

import java.util.Arrays;

import java.util.List;

import java.util.Map;

import java.util.stream.Collectors;

public class Main {

public static void main(String[] args) {

List<String> programming = Arrays.asList(“Rust”, “Pearl”, “Matlab”, “Mysql”, “Jquery”, “Html”);

Map<Integer, Long> output = programming.stream()

.collect(Collectors.groupingBy(

String::length,

Collectors.counting()

));

System.out.println(output);

}

}

Its output.

{4=2, 5=2, 6=2}

Reduction Method with Java Streams.

The concept of stream reduction in a Java program means grouping multiple available stream collection data type element values ​​into a single result and displaying them.

Common reduction operations in a Java program include.

• sum() reduction function.
• count() reduction function.
• min() reduction function.
• max() reduction function.
• reduce() reduction function.

The reduce() Streams function method with Java.

The reduce() function method in a Java program displays the collection data type elements by adding them step by step.

Example of the reduce() Streams function.

List<Integer> integers =

import java.util.Arrays;

import java.util.List;

public class Main {

public static void main(String[] args) {

List<Integer> integers = Arrays.asList(1, 3, 7, 11, 17, 24, 39, 12);

int sum = integers.stream()

.reduce(0, (p, q) -> p + q);

System.out.println(sum);

}

}

Its output.

114

Its calculation is as follows.

0 + 1 = 1

1 + 3 = 4

4 + 7 = 11

11 + 11 = 22

22 + 17 = 39

39 + 24 = 63

63 + 39 = 102

102 + 12 = 114

Finding Maximum Streams function method with Java.

In Java programs, the max() function method is used to find the largest maximum value element from a collection data type.

Example of the max() Streams function.

int max =

import java.util.Arrays;

import java.util.List;

public class Main {

public static void main(String[] args) {

List<Integer> integers = Arrays.asList(1, 3, 9, 2, 7, 9, 5, 6, 11);

int max = integers.stream()

.reduce(Integer.MIN_VALUE, Integer::max);

System.out.println(max);

}

}

Its output.

11

Searching Products Streams function method with Java.

In Java programs, the product() function method is used to find the product or multiplication value of a collection data type element.

Example of the product Streams function.

int product =

import java.util.Arrays;

import java.util.List;

public class Main {

public static void main(String[] args) {

List<Integer> integers = Arrays.asList(3, 3, 2, 7, 5);

int product = integers.stream()

.reduce(1, (p, q) -> p * q);

System.out.println(product);

}

}

Its output.

630

Its calculation is as follows.

[3, x 3, x 2, x 7, x 5] = 630

Main difference between Collectors vs Reduction in java stream

Conclusion of Collector vs. Reduction method in Java streams.

Collector vs. Here’s an easy way to remember the Reduction method:

• Collector stream method → ​​This user-defined Java program “collects the results” of a collection data type.
• Reduction stream method → ​​This user-defined Java program “groups the results” of a collection data type into a single result.

Collector vs. Reduction stream method concept example.

integers.stream()

.collect(Collectors.toList());

This means collecting the elements of the collection data into a list.

Whereas.

integers.stream()

.reduce(0, Integer::sum);

This means combining the elements of the collection data into a single value in a list.