Java Collectors groupingByConcurrent() Method

The groupingByConcurrent() method in Java, part of the java.util.stream.Collectors class, is used to group elements of a stream concurrently by a classifier function. This method is useful when you need to organize elements into groups based on a common attribute and perform the grouping operation in a parallel or concurrent manner.

Table of Contents

1. Introduction
2. groupingByConcurrent() Method Syntax
3. Understanding groupingByConcurrent()
4. Examples
   • Basic Usage
   • Using groupingByConcurrent() with Downstream Collector
5. Real-World Use Case
6. Conclusion

Introduction

The groupingByConcurrent() method returns a Collector that groups the input elements according to a classifier function and returns a ConcurrentMap whose keys are the result of applying the classifier function to the input elements, and whose corresponding values are Lists of elements that map to the associated key.

groupingByConcurrent() Method Syntax

The syntax for the groupingByConcurrent() method is as follows:

public static <T, K> Collector<T, ?, ConcurrentMap<K, List<T>>> groupingByConcurrent(Function<? super T, ? extends K> classifier)

There are also overloaded versions that allow specifying a downstream collector and a map supplier:

public static <T, K, A, D> Collector<T, ?, ConcurrentMap<K, D>> groupingByConcurrent(Function<? super T, ? extends K> classifier, Collector<? super T, A, D> downstream)

public static <T, K, D, A, M extends ConcurrentMap<K, D>> Collector<T, ?, M> groupingByConcurrent(Function<? super T, ? extends K> classifier, Supplier<M> mapFactory, Collector<? super T, A, D> downstream)

Parameters:

• classifier: A function that classifies input elements.
• downstream (optional): A collector to apply to the values associated with a given key.
• mapFactory (optional): A function that provides a new empty concurrent map into which the results will be inserted.

Returns:

• A Collector that groups the input elements.

Throws:

• This method does not throw any exceptions.

Understanding groupingByConcurrent()

The groupingByConcurrent() method allows you to group the elements of a stream by a specified classifier function in a concurrent manner. This is useful for organizing data into categories based on a common attribute while taking advantage of parallel processing.

Examples

Basic Usage

To demonstrate the basic usage of groupingByConcurrent(), we will create a list of strings and group them by their length.

Example

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.ConcurrentMap;
import java.util.stream.Collectors;

public class GroupingByConcurrentExample {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("apple", "banana", "cherry", "date", "fig", "grape");

        // Group the words by their length using groupingByConcurrent
        ConcurrentMap<Integer, List<String>> groupedByLength = words.parallelStream()
                                                                    .collect(Collectors.groupingByConcurrent(String::length));

        System.out.println("Grouped by Length: " + groupedByLength);
    }
}

Output:

Grouped by Length: {3=[fig], 4=[date], 5=[grape, apple], 6=[cherry, banana]}

Using groupingByConcurrent() with Downstream Collector

This example shows how to use groupingByConcurrent() with a downstream collector to group elements and apply additional operations, such as counting the number of elements in each group.

Example

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.ConcurrentMap;
import java.util.stream.Collectors;

public class GroupingByConcurrentWithDownstreamExample {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("apple", "banana", "cherry", "date", "fig", "grape");

        // Group the words by their length and count the number of words in each group
        ConcurrentMap<Integer, Long> groupedByLengthWithCount = words.parallelStream()
                                                                     .collect(Collectors.groupingByConcurrent(
                                                                         String::length,
                                                                         Collectors.counting()
                                                                     ));

        System.out.println("Grouped by Length with Count: " + groupedByLengthWithCount);
    }
}

Output:

Grouped by Length with Count: {3=1, 4=1, 5=2, 6=2}

Real-World Use Case

Grouping Transactions by Type Concurrently

In real-world applications, the groupingByConcurrent() method can be used to group transactions by type concurrently.

Example

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.ConcurrentMap;
import java.util.stream.Collectors;

public class GroupingByTransactionTypeExample {
    static class Transaction {
        String type;
        double amount;

        Transaction(String type, double amount) {
            this.type = type;
            this.amount = amount;
        }

        String getType() {
            return type;
        }

        @Override
        public String toString() {
            return "Transaction{" +
                    "type='" + type + '\'' +
                    ", amount=" + amount +
                    '}';
        }
    }

    public static void main(String[] args) {
        List<Transaction> transactions = Arrays.asList(
            new Transaction("Food", 10.0),
            new Transaction("Electronics", 200.0),
            new Transaction("Food", 15.0),
            new Transaction("Clothing", 50.0),
            new Transaction("Electronics", 100.0)
        );

        // Group transactions by type concurrently
        ConcurrentMap<String, List<Transaction>> groupedByType = transactions.parallelStream()
                                                                             .collect(Collectors.groupingByConcurrent(Transaction::getType));

        System.out.println("Grouped by Type: " + groupedByType);
    }
}

Output:

Grouped by Type: {Clothing=[Transaction{type='Clothing', amount=50.0}], Electronics=[Transaction{type='Electronics', amount=100.0}, Transaction{type='Electronics', amount=200.0}], Food=[Transaction{type='Food', amount=15.0}, Transaction{type='Food', amount=10.0}]}

Conclusion

The Collectors.groupingByConcurrent() method is used to group elements of a stream by a classifier function in a concurrent manner. This method is particularly useful for organizing elements into categories based on a common attribute while taking advantage of parallel processing. By understanding and using this method, you can efficiently manage concurrent grouping operations in your Java applications.