performance - Very Large Fibonacci in Java -

i trying rapidly calculate large fibonacci numbers. here code. prohibitively slow numbers above 1 million, how can improved?

public static biginteger fib(biginteger n) {          int k = n.intvalue();         biginteger ans = null;          if(k == 0) {              ans = new biginteger("0");         } else if(math.abs(k) <= 2) {             ans = new biginteger("1");         } else {             biginteger km1 = new biginteger("1");             biginteger km2 = new biginteger("1");              for(int = 3; <= math.abs(k); ++i) {                 ans = km1.add(km2);                 km2 = km1;                 km1 = ans;             }         }         if(k<0 && k%2==0) { ans = ans.negate(); }        return ans;     }  

binet's worked well. guys!

one way calculate (n-1)th power of 2x2 matrix:

a = ((1, 1), (1, 0)) 

then have

fib(n) = a^(n-1)[0][0], n >= 1 

and power of matrix a can calculated efficiently using exponentiation squaring