Double. isNaN(double var)

Double Medium NaN Indicates that a is not a number （Not a number）, Inquire about JAVA SE 18(Chapter 2. The Structure of the Java Virtual Machine) Specification document ,
Its official website JAVA SE 18 explain ：

Popular said ： NaN Used to express the result of illegal operation , such as 0/0. Depending on the floating point type , Contains two values Float.NaN and Double.Nan.

summary ： What will happen NaN?
(1) 0/0
(2) Square a negative number （ Such as usage Math.sqr（-1））
(3) Logarithm of negative numbers （ Such as Math.log10(-1)）.
(4) 3/0( The result after the test is positive infinity, It's a little strange. , Look at the official website and use it 0 And negative 0 It seems quite reasonable to say positive infinity and negative infinity , But with the previous definition （ The result of illegal operation is Nan） It's a bit contradictory ,)

NaN Features such as official website JAVA SE 18：

The general explanation is ：NaN Is chaotic , So it has the following properties ： 
(1) You and yourself are not equal in value , That's the equation NaN === NaN return false;
(2) NaN Nor is it equal to any number NaN == X(X Can be any number ) Equation return false;
(3) As long as one of the two sides of the inequality is NaN, Then the inequality will return true.

The practice result proves ： Notice that there is one on both sides of this final inequality Nan Will return as true The situation is limited to ！=; barring >, < Symbol .