Java Float.isNaN Double.isNaN方法工作原理

From Java Language Specification:


Floating-point equality testing is performed in accordance with the rules of the IEEE 754 standard:


If either operand is NaN, then the result of == is false but the result of != is true. Indeed, the test x!=x is true if and only if the value of x is NaN. (The methods Float.isNaN and Double.isNaN may also be used to test whether a value is NaN.)
Positive zero and negative zero are considered equal. Therefore, -0.0==0.0 is true, for example.

Otherwise, two distinct floating-point values are considered unequal by the equality operators. In particular, there is one value representing positive infinity and one value representing negative infinity; each compares equal only to itself, and each compares unequal to all other values.

Floating-point equality testing is performed in accordance with the rules of the IEEE 754 standard:

• If either operand is NaN, then the result of == is false but the result of != is true. Indeed, the test x!=x is true if and only if the value of x is NaN. (The methods Float.isNaN and Double.isNaN may also be used to test whether a value is NaN.)

• Positive zero and negative zero are considered equal. Therefore, -0.0==0.0 is true, for example.

• Otherwise, two distinct floating-point values are considered unequal by the equality operators. In particular, there is one value representing positive infinity and one value representing negative infinity; each compares equal only to itself, and each compares unequal to all other values.

That method can return true for certain operations, for example:

System.out.println(Float.isNaN(0.0f / 0.0f));
System.out.println(Double.isNaN(Math.sqrt(-1)));

Basically, NaN represents an undefined value. The value of 0.0 / 0.0 is NaN, and Nan != NaN. It may seem logical because Math.sqrt(-1) also gives you NaN.

See the javadoc of Double.NaN:

It is equivalent to the value returned by Double.longBitsToDouble(0x7ff8000000000000L)

And then Double.longBitsToDouble():

If the argument is any value in the range 0x7ff0000000000001L through 0x7fffffffffffffffL or in the range 0xfff0000000000001L through 0xffffffffffffffffL, the result is a NaN. No IEEE 754 floating-point operation provided by Java can distinguish between two NaN values of the same type with different bit patterns.

From Java Language Specification:


Floating-point equality testing is performed in accordance with the rules of the IEEE 754 standard:


If either operand is NaN, then the result of == is false but the result of != is true. Indeed, the test x!=x is true if and only if the value of x is NaN. (The methods Float.isNaN and Double.isNaN may also be used to test whether a value is NaN.)
Positive zero and negative zero are considered equal. Therefore, -0.0==0.0 is true, for example.
Otherwise, two distinct floating-point values are considered unequal by the equality operators. In particular, there is one value representing positive infinity and one value representing negative infinity; each compares equal only to itself, and each compares unequal to all other values.