Prime number calculation fun

Well I see a couple of quick optimizations that can be done. First you don't have to try each number up to half of the current number.

Instead you only have try up to the square root of the current number.

And the other optimization was what BP said with a twist: Instead of

int count = 0;
...
for (int i = 2; i < top; i++)
...
if (current == 2)
  current++;
else
  current += 2;

use

int count = 1;
...
for (int i = 3; i < top; i += 2)
...
current += 2;

This should speed things up quite a lot.

Edit:
More optimization courtesy of Joe Pineda:
Remove the variable "top".

int count = 1;
...
for (int i = 3; i*i <= current; i += 2)
...
current += 2;

If this optimization indeed increases speed is up to java.
Calculating the square root takes a lot of time compared to multiplying two numbers. However since we move the multiplication into the for loop this is done every single loop. So this COULD slow things down depending on how fast the square root algorithm in java is.

Since you're searching for them in ascending order, you could keep a list of the primes you've already found and only check for divisibility against them, since all non-prime numbers can be reduced to a list of lesser prime factors. Combine that with the previous tip about not checking for factors over the square root of the current number, and you'll have yourself a pretty darn efficient implementation.

That's a bit worse than my sieve did on a 8 Mhz 8088 in turbo pascal in 1986 or so. But that was after optimisations :)