double math in Java

Jeremy — Sun, 14 Mar 2010 22:51:01 +0000

I have been programming extensively using doubles and Doubles in Java. Often, I have run into problems where simple mathematical operators will return 0.0. For example:

ArrayList a, b;
double temp = (a.get(0) + b.get(0)) * (a.get(0) + b.get(0));

After executing this code, temp would be equal to 0 no matter what values a.get(0) and b.get(0) had. This seemed very annoying and backwards and the only way I had to solve it was thusly:

double temp = (double)a.get(0) + b.get(0);
double result = temp * temp;

Then, result would actually have a result in it. Go figure.

Integer to int null pointer exception?

Jeremy — Sat, 21 Mar 2009 01:19:04 +0000

As I was coding up some stuff for class today I was using your friendly HashMap in Java. I created the hash map so it would use an Object and an Integer. I would have rather done an int but you can’t template ints. Anyway, at one point I was pulling the Integer out of the map with something like this:
int parent = map.get(parentObj);
There happened to be a case where the parent object wasn’t in the map and that line kept throwing a null pointer exception. After a few minutes, I realized that is because the conversion from Integer to int implies an implicit method call. And when the value returned from .get was null, there was a null pointer exception on the Integer.toInt() method. So beware of implicit calls.

Jeremyopolis » bugs

double math in Java

Integer to int null pointer exception?