HashMap<Object, Double> returns what instead of null?

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aaronfude@gmail.com - 09 May 2006 03:30 GMT

Hi,

What is get(Object) supposed to return when called on an object of type
HashMap<Object, Double> as in:

double x = hashMap.get(key);

when the key is not available?

Thanks!

Aaron Fude

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Rhino - 09 May 2006 04:46 GMT

> Hi,
>
[quoted text clipped - 4 lines]
>
> when the key is not available?

Without actually trying it, I'm guessing that you'd get a null if you did a
get() on a key that didn't exist. But why guess? If I were you, I'd just
write a few lines of code then try it in your debugger and see what actually
happens. An actual experiment is always better than an educated guess if the
experiment isn't too hard to conduct.

--
Rhino

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Simon - 09 May 2006 09:19 GMT

>>What is get(Object) supposed to return when called on an object of type
>>HashMap<Object, Double> as in:
[quoted text clipped - 5 lines]
> Without actually trying it, I'm guessing that you'd get a null if you did a
> get() on a key that didn't exist.

The problem is that a double cannot be null. Think of the above line of code as

double x = hashMap.get(key).doubleValue();

This will throw a NPE as will the original code. The same happens for

Double d = null;
double x = d;

Cheers,
Simon

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Thomas Hawtin - 09 May 2006 09:35 GMT

> The problem is that a double cannot be null. Think of the above line of code as
>
[quoted text clipped - 4 lines]
> Double d = null;
> double x = d;

The outrageous dangers of auto*un*boxing...

If you do want a "null" for double, don't use a very small number.
Double.NaN makes a suitable substitute. Note, to check for NaNs use
Double.isNan. IEEE 754 demands Double.NaN != Double.NaN...

Tom Hawtin

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http://jroller.com/page/tackline/

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Chris Uppal - 09 May 2006 10:48 GMT

> What is get(Object) supposed to return when called on an object of type
> HashMap<Object, Double> as in:
>
> double x = hashMap.get(key);
>
> when the key is not available?

Exactly the same as if you tried to unbox null. I.e. the same as if you'd
executed:

Double d = null;
double x = d;

or, the exact equivalent to the above two lines:

Double d = null;
double x = d.doubleValue();

Why should you expect that to do anything except throw a NullPointerException ?

-- chris

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HashMap<Object, Double> returns what instead of null?

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