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Learn how to efficiently find the largest image size in Kotlin when working with Camera2 API, resolving the 'Immutable Collection Treated as Mutable' error.
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Resolving Immutable Collection Treated as Mutable Error in Kotlin's Collections.max Function

When working with the Camera2 API in Android, one common requirement is to determine the largest available image size. This task can be straightforward when handled in Java, but many developers encounter obstacles when converting the same logic to Kotlin, particularly with the Collections.max method. In this post, we’ll examine the common pitfalls and offer solutions to effectively retrieve the maximum image size using Kotlin.

The Problem: Immutable vs Mutable Collections

As developers attempt to use Collections.max in Kotlin, they often run into an error message indicating that the correct collection type cannot be applied. This is due to a misunderstanding of how collections work in Kotlin compared to Java. Here’s what typically happens:

The Java Approach

In Java, the following code snippet works seamlessly:

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In this code:

Arrays.asList() effectively converts an array of sizes into a List<Size>, which can easily be processed by Collections.max.

The Kotlin Conversion Error

However, when translating this into Kotlin, developers might write:

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This leads to the following error:

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The issue arises from the fact that listOf() takes a single item (the array of sizes), creating a List<Array<Size>>, rather than a List<Size> which Collections.max expects.

Solution: Correcting the Collection Type

To resolve this issue, there are several approaches you can take to successfully obtain the largest available image size.

1. Using asList() Method

The direct Kotlin equivalent of Arrays.asList() can be achieved with .asList() on the array:

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This approach ensures that you are working with a List<Size> as intended, and Collections.max will function without raising an error.

2. Purely Kotlin Approach: Utilizing maxBy

If you prefer a more idiomatic Kotlin solution, you can bypass Collections.max entirely. The function maxBy can be utilized to find the maximum value based on criteria. In this case, you would evaluate the size based on width and height:

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This not only simplifies your code but also leverages Kotlin's powerful features for a cleaner syntax.

Conclusion

Working with collections can be tricky when transitioning between Java and Kotlin, especially in the context of the Camera2 API. By understanding the difference between mutable and immutable collections and how they are created in both languages, you can avoid common pitfalls. Whether you choose to use the asList() method or opt for Kotlin's straightforward maxBy, you can confidently retrieve the largest image size without any headaches.

By carefully analyzing collection types and leveraging Kotlin's idiomatic features, you can streamline your code and enhance your projects effectively.