How to Efficiently Count Occurrences of Strings in Java Using Streams?

Counting Occurrences with Java Stream's groupBy

In Java, you can utilize streams to efficiently aggregate data. One common operation is grouping items by their values and counting their occurrences.

Example

Suppose you have a list of strings and want to count the frequency of each unique string. The goal is to create a map where the key is a string, and the value is its occurrence count.

groupBy with a Counting Collector

To achieve this, we can use the groupBy method in conjunction with a counting collector. The groupBy method organizes elements into groups based on the specified key. In this case, we want to group strings by their identity.

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

In this code, Function.identity() obtains the string itself as the grouping key, essentially preserving the original elements. Collectors.counting() provides a collector that returns the count of elements in each group.

Result

After grouping and counting, we obtain a map with the desired counts:

{Hello=2, World=1}

This map contains the count of occurrences for each unique string in the original list.

Efficiency Considerations

For large datasets, using Collectors.groupingByConcurrent may offer improved efficiency. It works in a parallel fashion, enhancing performance in multi-core environments. However, this approach may be unsuitable in scenarios where the order of elements or thread safety is crucial.

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