How do I count occurrences of elements in a Java Stream using `groupBy()`?

Counting Occurrences with GroupBy in Java Streams

Java Stream API provides powerful mechanisms for manipulating and aggregating data efficiently. One common task is to count occurrences of elements within a stream. Achieving this with the groupBy() method requires leveraging an additional collector.

Consider the following code snippet, which aims to collect elements in a stream into a map where keys represent unique elements and values represent their occurrences:

List<String> list = Arrays.asList("Hello", "Hello", "World");

// Desired output:
// {Hello=2, World=1}

To accomplish this, we can employ the groupBy() method with a Collectors.counting() collector to perform the counting:

import java.util.*;
import java.util.stream.*;

class Test {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();

        list.add("Hello");
        list.add("Hello");
        list.add("World");

        Map<String, Long> counted = list.stream()
            .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

        System.out.println(counted);
    }
}

Explanation:

• Collectors.groupingBy(Function.identity()): Groups elements based on their identity, effectively treating each unique element as a key.
• Collectors.counting(): Computes the number of occurrences for each group.

The resulting counted map will have keys as elements and values as their respective counts:

{Hello=2, World=1}

Alternatively, for efficiency considerations in certain scenarios, you can explore using Collectors.groupingByConcurrent. Remember to assess if your specific use case aligns with the safety requirements of concurrent grouping.

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