I need to solve some partial differential equations using java (using finite difference). In past I used to ignore this problem with double precision in Java. but now I am caught.

I have variable t which I increase 0.1 each time for example. Now when I print t:

Then I use the t in my calculations. In addition to the difference of above numbers to what I need for my calculations (which is small) these are not the numbers I need. I need to find the result for t=1.1 entry (by comparing t to 1.1 in an results array but there is no 1.1).

How can I deal with this? And generally how can I compare double variables (if it is possible at all).

But you have 1.1. Only it looks like 1.099999999999.

This is an unavoidable problem with floating-point arithmetic (see this FAQ page and look for no 20). You can try the BigDecimal class, but beware; it takes umbrage if it gets a recurring division and you haven't specified precision and rounding mode. Or you will have to live with the imprecision!

And don't try using == with floating point numbers; you can go wildly astray, for the same reason. You can't use == with BigDecimal; you have to use equals() or compareTo() == 0. The latter may be more useful.