How do you set a double value to a "non-value"

Question

I have two double data elements in an object.

Sometimes they are set with a proper value and sometimes not. When the form field from which they values are received is not filled I want to set them to some value that tells me, during the rest of the code that the form fields were left empty.

I can't set the values to null as that gives an error, is there some way I can make them 'Undefined'.

PS. Not only am I not sure that this is possible, it might not also make sense. But if there is some best practice for such a situation I would be keen to hear it.

Solution

Two obvious options:

• Use Double instead of double. You can then use null, but you've changed the memory patterns involved substantially.

• Use a "not a number" (NaN) value:

  double d = 5.5;
  System.out.println(Double.isNaN(d)); // false
  d = Double.NaN;
  System.out.println(Double.isNaN(d)); // true
  

  Note that some other operations on "normal" numbers could give you NaN values as well though (0 divided by 0 for example).

Comments

[Ankur]

Thanks ... NaN will solve my purposes. I won't be doing any arithmetic on these numbers so that will do it just nicely.

[Jesper]

Beware with NaN that using == with them always returns false, even Double.NaN == Double.NaN is false. You must use Double.isNaN(...) to check if a double is not-a-number.

[sleske]

I also commonly use the "Double=null" trick. I'd prefer it to the NaN option, both because NaN might occur as a value, and because "null for unset" is a common practice in Java.

[Jon Skeet]

@sleske: Yes - it depends on the situation though. It could be expensive in terms of heap usage if you ended up creating huge numbers of Double objects.

[sleske]

True. But I'd usually file that under "premature optimization" and worry about it later ;-). Still, good to consider the possibility.