How to Count Occurrences of Multiple Elements in a List Using Java 8 Streams

Описание: Discover how to effectively `count occurrences` of multiple specific elements in a list using Java 8 Stream API for cleaner and more efficient code.
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Counting Occurrences of Multiple Elements in a List with Java 8

Have you ever found yourself in a situation where you need to tally the occurrences of specific items within a list in Java? Perhaps you are dealing with an ArrayList of strings and you want to know just how many times certain elements appear. This post will guide you through how to count occurrences of multiple elements using Java 8 Streams in a concise and efficient way.

Problem Statement

Suppose you have an ArrayList of animals that looks like this:

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In this list, you want to count the occurrences of only the "bat" and "owl" elements. Instead of iterating over the list multiple times or using traditional loops, we can leverage the Java 8 Stream API to achieve this goal.

Solution Overview

To count occurrences of specific elements in a list with Java 8 Stream API, you can follow these steps:

Create a Set of Elements to Filter - Define which elements you want to count.

Use Streams to Filter and Count - Stream the list, filter the items, and group them by their occurrence.

Step-by-Step Implementation

Step 1: Define the Elements to Filter

First, you need to create a Set containing the elements that you want to count (e.g., "bat" and "owl"). This will allow you to efficiently check if an item is among those you're interested in.

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Step 2: Stream and Count

Next, you can leverage the power of streams to filter the items based on our set and count their occurrences. Here’s how you could do it:

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Full Code Example

Here’s the complete code snippet which puts everything together:

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Output:

When you run the above code, it will print the count of occurrences for "bat" and "owl":

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Conclusion

Using the Java 8 Stream API to count occurrences of multiple elements in a list offers a clean and efficient way to handle this task in a single iteration. By creating a Set to filter elements and utilizing the groupingBy collector, you can quickly obtain the counts you need without resorting to cumbersome loops. This approach not only improves readability but also enhances performance when dealing with large datasets.

Now you're equipped with the knowledge to count occurrences in a more efficient manner. Happy coding!