Why does Double.NaN==Double.NaN return false?
It might not be a direct answer to the question.
But if you want to check if something is equal to Double.NaN
you should use this:
double d = Double.NaN
Double.isNaN(d);
This will return true
NaN means "Not a Number".
Java Language Specification (JLS) Third Edition says:
An operation that overflows produces a signed infinity, an operation that underflows produces a denormalized value or a signed zero, and an operation that has no mathematically definite result produces NaN. All numeric operations with NaN as an operand produce NaN as a result. As has already been described, NaN is unordered, so a numeric comparison operation involving one or two NaNs returns
false
and any!=
comparison involving NaN returnstrue
, includingx!=x
whenx
is NaN.
NaN is by definition not equal to any number including NaN. This is part of the IEEE 754 standard and implemented by the CPU/FPU. It is not something the JVM has to add any logic to support.
http://en.wikipedia.org/wiki/NaN
A comparison with a NaN always returns an unordered result even when comparing with itself. ... The equality and inequality predicates are non-signaling so x = x returning false can be used to test if x is a quiet NaN.
Java treats all NaN as quiet NaN.
Why that logic
NaN
means Not a Number
. What is not a number? Anything. You can have anything in one side and anything in the other side, so nothing guarantees that both are equals. NaN
is calculated with Double.longBitsToDouble(0x7ff8000000000000L)
and as you can see in the documentation of longBitsToDouble
:
If the argument is any value in the range
0x7ff0000000000001L
through0x7fffffffffffffffL
or in the range0xfff0000000000001L
through0xffffffffffffffffL
, the result is aNaN
.
Also, NaN
is logically treated inside the API.
Documentation
/**
* A constant holding a Not-a-Number (NaN) value of type
* {@code double}. It is equivalent to the value returned by
* {@code Double.longBitsToDouble(0x7ff8000000000000L)}.
*/
public static final double NaN = 0.0d / 0.0;
By the way, NaN
is tested as your code sample:
/**
* Returns {@code true} if the specified number is a
* Not-a-Number (NaN) value, {@code false} otherwise.
*
* @param v the value to be tested.
* @return {@code true} if the value of the argument is NaN;
* {@code false} otherwise.
*/
static public boolean isNaN(double v) {
return (v != v);
}
Solution
What you can do is use compare
/compareTo
:
Double.NaN
is considered by this method to be equal to itself and greater than all otherdouble
values (includingDouble.POSITIVE_INFINITY
).
Double.compare(Double.NaN, Double.NaN);
Double.NaN.compareTo(Double.NaN);
Or, equals
:
If
this
andargument
both representDouble.NaN
, then theequals
method returnstrue
, even thoughDouble.NaN==Double.NaN
has the valuefalse
.
Double.NaN.equals(Double.NaN);