implementing multiplication algorithm in java -

i have following multiplication algorithm, trying implement using java:

m: number of digits

n: b number of digits

β: base

this java function implments algorithm:

public biginteger prodbigbig(biginteger a, biginteger b, integer base ){          arraylist<integer> lista = new arraylist<integer>();         arraylist<integer> listb  = new arraylist<integer>();         arraylist<integer> listc = new arraylist<integer>();         int m = a.tostring().length();         int n = b.tostring().length();         integer carry, temp;         biginteger result = biginteger.zero;          for(int = 0; < a.tostring().length(); i++){             lista.add(character.getnumericvalue( a.tostring().charat(i)));         }         for(int = 0; < b.tostring().length(); i++){             listb.add((character.getnumericvalue( b.tostring().charat(i))));         }         for(int i= 0; <= m - 1; i++){             listc.add(0);         }          for(int k = 0; k <= n - 1 ; k++) {             carry = 0;             for(int  = 0; <= m - 1; i++){                 temp = (lista.get(i) * listb.get(k)) + listc.get(i + k) + carry;                 listc.add(i+k,temp % base);                 carry = temp / base;             }             listc.add(k + m,carry);         }         for(int = 0; < m + n; i++) {             result = result.add(biginteger.valueof((long) (listc.get(i)*(math.pow(base, i)))));         }          return result;     } 

but don't know why not getting correct result, until not able detect fails.

listc.add(i+k,temp % base); 

that should be

listc.set(i+k,temp % base); 

and final transformation biginteger overflow sufficiently large numbers. rid of arraylists altogether , use arrays of int, , convert byte[] @ end , feed directly constructor biginteger.