# NaN in Double class

Natalie Ap

Ranch Hand

Posts: 49

posted 8 years ago

Hello all!

I am so confused about the following outputs! In the api code for Double that i saw had the following comment before the declaration of NaN constant:

If this is so, how come the two values in LHS and RHS are unequal?? The type of NaN is a primitive double, not an object reference..

I am so confused about the following outputs! In the api code for Double that i saw had the following comment before the declaration of NaN constant:

If this is so, how come the two values in LHS and RHS are unequal?? The type of NaN is a primitive double, not an object reference..

Fiona

posted 8 years ago

I think this excerpt from the JLS answers your question

NaN is unordered, so the numerical comparison operators <, <=, >, and >= return false if either or both operands are NaN (§15.20.1). The equality operator == returns false if either operand is NaN, and the inequality operator != returns true if either operand is NaN (§15.21.1). In particular, x!=x is true if and only if x is NaN, and (x<y) == !(x>=y) will be false if x or y is NaN.

SCJP 6 | SCWCD 5 | Javaranch SCJP FAQ | SCWCD Links

Natalie Ap

Ranch Hand

Posts: 49

posted 8 years ago

Hi Ankit,

Thanks for your reply.

But did not quite understand the last part of the paragraph.

They say, (x<y) == !(x>=y) will be false if x or y is NaN.

But this also applies to the following:

Say,

x = 0;

y = 1;

(x<y) -> 0 < 1 -> true

(x>=y) -> 0 >= 1 -> false

hence the lhs and rhs are not equal and hence (x<y) == !(x>=y) is false even though neither of them are NaN

Thanks

Ankit Garg wrote:I think this excerpt from the JLS answers your question

NaN is unordered, so the numerical comparison operators <, <=, >, and >= return false if either or both operands are NaN (§15.20.1). The equality operator == returns false if either operand is NaN, and the inequality operator != returns true if either operand is NaN (§15.21.1). In particular, x!=x is true if and only if x is NaN, and (x<y) == !(x>=y) will be false if x or y is NaN.

Hi Ankit,

Thanks for your reply.

But did not quite understand the last part of the paragraph.

They say, (x<y) == !(x>=y) will be false if x or y is NaN.

But this also applies to the following:

Say,

x = 0;

y = 1;

(x<y) -> 0 < 1 -> true

(x>=y) -> 0 >= 1 -> false

hence the lhs and rhs are not equal and hence (x<y) == !(x>=y) is false even though neither of them are NaN

Thanks

Fiona

posted 8 years ago

Pats read the statement again. Its (x<y) ==

**!**(x>=y) and not (x<y) == (x>=y)SCJP 6 | SCWCD 5 | Javaranch SCJP FAQ | SCWCD Links

posted 8 years ago

Can you tell me from where did you get this type of question.is it a SCJP question ??

Please tell me from which book you got this question.

Can you give me an example with regards to positive and negative infinity. ??

--

Deepak Lal

Please tell me from which book you got this question.

Can you give me an example with regards to positive and negative infinity. ??

--

Deepak Lal

When The Going Gets Tougher,The Tougher gets Going

posted 8 years ago

The simple answer here is.... The IEEE floating point standard defines it as so -- NaN does not equal NaN. And Java is simply supporting the standard.

As a side note, the equals() method will report it as equal, if they are Double objects. This is in direct violation of the standard, and the JavaDoc states it as so. The reason this is necessary is because it will break the usability of the certain collections if NaN does not equal NaN.

Henry

If this is so, how come the two values in LHS and RHS are unequal?? The type of NaN is a primitive double, not an object reference..

The simple answer here is.... The IEEE floating point standard defines it as so -- NaN does not equal NaN. And Java is simply supporting the standard.

As a side note, the equals() method will report it as equal, if they are Double objects. This is in direct violation of the standard, and the JavaDoc states it as so. The reason this is necessary is because it will break the usability of the certain collections if NaN does not equal NaN.

Henry

Bert Bates

author

Sheriff

Sheriff

Posts: 8919

11

posted 8 years ago

No offense here -- but you are a SCJP candidate. It would take less than a minute to create a method, that creates two Double instances (assigned to NaN), and test the equals() method. What is it that is preventing you from writing this simple example?

Henry

Deepak Lal wrote:Hi Bert ,

Could you please give us an example atleast on NaN used with Double class.??

No offense here -- but you are a SCJP candidate. It would take less than a minute to create a method, that creates two Double instances (assigned to NaN), and test the equals() method. What is it that is preventing you from writing this simple example?

Henry