Why doesn't Integer represent NaN in Java?

问题:

When I write something like

double a = 0.0;
double b = 0.0;
double c = a/b;

The result is Double.NaN, but when I try the same for integers, it produces an ArithmeticException. So, why isn't there a Integer.NaN?

回答1:

The answer has very little to do with Java. Infinity or undefined numbers are not a part of the integer set, so they are excluded from Integer, whereas floating point types represent real numbers as well as complex numbers, so to deal with these, NaN has been included with floating point types.

回答2:

For the same reason that there is no integer NaN in any other language.

Modern computers use 2's complement binary representation for integers, and that representation doesn't have a NaN value. (All values in the domain of the representation type represent definite integers.)

It follows that computer integer arithmetic hardware does not recognize any NaN representation.

In theory, someone could invent an alternative representation for integers that includes NaN (or INF, or some other exotic value). However, arithmetic using such a representation would not be supported by the hardware. While it would be possible to implement it in software, it would be prohibitively expensive¹... and undesirable in other respects too to include this support in the Java language.

^{1 - It is of course relative, but I'd anticipate that a software implementation of NaNs would be (at least) an order of magnitude slower than hardware. If you actually, really, needed this, then that would be acceptable. But the vast majority of integer arithmetic codes don't need this. In most cases throwing an exception for "divide by zero" is just fine, and an order of magnitude slow down in all integer arithmetic operations is ... not acceptable.}

---

By contrast:

• the "unused" values in the representation space already exist
• NaN and INF values are part of the IEE floating point standard, and
• they are (typically) implemented by the native hardware implementation of floating point arithmetic

回答3:

As noted in other comments, it's largely because NaN is a standard value for floating point numbers. You can read about the reasons NaN would be returned on Wikipedia here:

http://en.wikipedia.org/wiki/NaN

Notice that only one of these reasons exists for integer numbers (divide by zero). There is also both a positive and a negative infinity value for floating point numbers that integers don't have and is closely linked to NaN in the floating point specification.