Why does Math.round() return 0 for NaN arguments?

Question

I thought that result of any mathematical operation on a NaN should give me a NaN back, but Math.round(Float.NaN) == 0

What is the rationale for such behavior of Math.round()?

Curiously, C# behaves differently: http://msdn.microsoft.com/en-us/library/75ks3aby.aspx

Answer 1

Math.round() is defined as (long)Math.floor(a + 0.5d).

1. If a is NaN, then a+0.5d is NaN.
2. Math.floor() is delagated to StrictMath.floor() which returns NaN when passed in NaN.
3. When casting NaN to a long, it returns 0

So ultimately, it comes down to why casting NaN to a long returns 0. This issue has been thoroughly discussed in this question.