Скачать или смотреть Why the HashMap in Java Can't Distinguish Between Different NaN Values as Keys

Discover why Java's `HashMap` treats all `NaN` values as equal keys, affecting how you can store floating-point values.
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This video is based on the question https://stackoverflow.com/q/73647347/ asked by the user 'Vegard' ( https://stackoverflow.com/u/1926899/ ) and on the answer https://stackoverflow.com/a/73647403/ provided by the user 'Jon Skeet' ( https://stackoverflow.com/u/22656/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

When working with data structures in Java, especially the HashMap, developers might encounter unexpected behavior with floating-point numbers, specifically NaN (Not-a-Number) values. A common concern arises when you attempt to use different NaN values as keys in a HashMap, only to find that the structure returns a size of 1. In this guide, we'll explore why this happens and what it means for your programming.

The Problem

Consider the following code snippet:

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

In this case, you might expect the size of the HashMap to be 2 because you are trying to insert two different NaN float values. However, the output of the program is 1. This behavior raises the question: Why does the HashMap treat different NaN values as if they are the same?

The Mechanism Behind the Behavior

Understanding Float.equals

Let's take a closer look at how the Float class handles comparisons and equality:

The Float.equals(Object) method states that it compares the given Float object against another Float object.

For two Float values to be considered equal, they must not only be non-null but also represent the same floating-point value.

This explanation suggests that different NaN values should be treated as non-equal because NaN != NaN by definition. However, there is more to this than meets the eye.

The Role of floatToIntBits

The behavior of Float objects is closely tied to the floatToIntBits(float) method. According to its documentation:

If the argument is NaN, it always returns 0x7fc00000.

All NaN values are converted to a single canonical NaN value during this process.

This means that when two different NaN values are generated and you check their equality using Float.equals() or when calculating their hash codes using Float.hashCode(), the underlying mechanism treats them as equal due to their canonical representation.

Summary of Key Points

In Java HashMap, NaN values are treated as equal keys.

The Float.equals() method and Float.hashCode() both rely on the canonical representation of NaN, leading to this behavior.

Thus, when you attempt to insert different NaN values, the HashMap collapses them into a singular value, causing it to return a size of 1.

Conclusion

Understanding the behavior of HashMap with NaN values is critical for developers who use floating-point numbers as keys. It ensures that you write code that behaves as expected and avoid confusion in data storage. Always remember that NaNs are treated as equal in Java's HashMap due to inherent properties of floating-point representation, which can significantly influence your data structures.

If you have any experiences or additional questions about using HashMap with floating-point numbers, feel free to share in the comments below!