How do you test to see if a double is equal to NaN?

You can check for NaN by using var != var. NaN does not equal NaN.

EDIT: This is probably by far the worst method. It's confusing, terrible for readability, and overall bad practice.

You might want to consider also checking if a value is finite via Double.isFinite(value). Since Java 8 there is a new method in Double class where you can check at once if a value is not NaN and infinity.

/**
 * Returns {@code true} if the argument is a finite floating-point
 * value; returns {@code false} otherwise (for NaN and infinity
 * arguments).
 *
 * @param d the {@code double} value to be tested
 * @return {@code true} if the argument is a finite
 * floating-point value, {@code false} otherwise.
 * @since 1.8
 */
public static boolean isFinite(double d)

Try Double.isNaN():

Returns true if this Double value is a Not-a-Number (NaN), false otherwise.

Note that [double.isNaN()] will not work, because unboxed doubles do not have methods associated with them.

Use the static Double.isNaN(double) method, or your Double's .isNaN() method.

// 1. static method
if (Double.isNaN(doubleValue)) {
    ...
}
// 2. object's method
if (doubleObject.isNaN()) {
    ...
}

Simply doing:

if (var == Double.NaN) {
    ...
}

is not sufficient due to how the IEEE standard for NaN and floating point numbers is defined.