Скачать или смотреть How to Collect a 2D Array in Java 8 from a List of Objects

Learn how to effectively collect a `2D array` in Java 8 from a list of objects using streams and arrays. This guide offers a step-by-step approach and alternative methods for efficient implementation.
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Collecting a 2D Array in Java 8 from a List of Objects

When dealing with a list of objects in Java 8, you may find yourself needing to convert it into a more structured format, like a 2D array. This can be particularly useful in scenarios where you want to group properties from the objects into arrays. In this guide, we will explore how to efficiently collect a 2D array from a list of Pair objects, where each pair contains two integers: start and end. Let’s dive into the solution step by step.

Understanding the Problem

Suppose you have a simple class definition for Pair as follows:

[[See Video to Reveal this Text or Code Snippet]]

Given a list of Pair objects, your task is to convert this list into a 2D array where each sub-array corresponds to the start and end values of each Pair object. Thus, the resulting 2D array will have each row containing two values.

Solution Approach

Using Java Streams

Java 8 introduced a powerful feature called Streams that allows you to perform complex operations on collections with ease. For our case, we can utilize the map() function of the Stream API to transform each Pair into a 2-element array and then collect everything into a 2D array.

Here’s how you can achieve this:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code

Stream Creation: listOfPair.stream() initiates a stream from the list of pairs.

Mapping to Array: The map(x -> new int[]{x.start, x.end}) takes each Pair object and translates it into a new integer array containing start and end values.

Collecting to List: The result of the mapping is collected into a List<int[]> using Collectors.toList(), which is a temporary storage before conversion to a 2D array.

Conversion to 2D Array: Finally, toArray(new int[0][0]) converts the list of arrays into a new 2D array of the desired type.

Optimized Memory Allocation

While the above method works perfectly, it performs two memory allocations: one for the list and one for the final array. If you know the size of the 2D array beforehand, you can streamline the process with a single memory allocation as shown below:

[[See Video to Reveal this Text or Code Snippet]]

Key Advantages of This Method:

One Memory Allocation: By pre-allocating the 2D array and populating it in a single pass, you enhance performance especially for large lists.

Clear Iteration: The for-each loop provides simple and clear code while avoiding the overhead of additional collections.

Conclusion

Converting a list of objects into a 2D array in Java 8 can be accomplished easily using streams, but optimizing this process can lead to better performance in memory usage. By understanding both methods presented, you now have the tools to choose the best fit for your use case. Whether you prefer the elegance of streams or the efficiency of creating a pre-sized array, you are well-equipped to tackle similar challenges in your Java programming.