Why is using `==` for float equality comparison in Java unreliable and what's the correct approach?

Pitfalls of Float Equality Comparison with == in Java

Java provides the == operator for equality comparisons, including floating-point numbers. However, using == to compare floats can lead to surprising and unreliable results.

Contrary to the information provided on the Java.sun page, using == for floating-point comparison can trigger static analysis warnings like "JAVA0078 Floating point values compared with ==" in IDEs. This is because floating-point numbers in Java are represented using IEEE 754 floating-point format, which introduces inherent inaccuracies.

Due to limitations in floating-point precision, floating-point values are stored as approximations. Operations like addition, subtraction, and comparison introduce rounding errors that accumulate over time. As a result, direct comparisons using == may return false even when the numbers are essentially equal.

Correct Approach: Epsilon-Based Comparison

To reliably compare floating-point values in Java, the recommended approach is to use an epsilon-based comparison. Instead of testing for exact equality using ==, we check if the absolute difference between the two numbers is less than a small threshold value called epsilon.

The code snippet below demonstrates the correct way to compare floats:

if (Math.abs(sectionID - currentSectionID) < epsilon)

Here, epsilon is a predetermined very small number (e.g., 0.00000001) that defines the acceptable range of deviation for equality. This approach effectively allows for a small margin of error due to floating-point inaccuracies, ensuring reliable equality checks.

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