How can I use Java 8 Streams to efficiently count occurrences of duplicate elements?

Counting Occurrences with Java 8 Streams Using groupBy

In Java 8 streams, the groupBy method allows you to group elements based on a particular property or value. When combined with another collector, such as counting, you can efficiently count the occurrences of duplicate elements in a stream.

Let's consider an example:

List<String> list = Arrays.asList("Hello", "Hello", "World");
Map<String, Long> wordToFrequency = list.stream()
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

In this example, we use the collect method with an overload that takes a Collector as an argument. Inside the collector, we use groupingBy to group the items based on their string value (the identity function). We then combine this with Collectors.counting() to count the occurrences of each unique string.

As a result, we obtain a map where each key is a unique string and the corresponding value is the number of occurrences in the list. In this case, the map would contain:

{Hello=2, World=1}

This approach is highly efficient and easy to use, making it suitable for various scenarios where you need to count occurrences of elements in a stream based on a specific property. Remember that you can also leverage groupingByConcurrent for improved performance when appropriate.

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