Bigint

I need help finishing up this project. I need to make a big into class that can add to big int using arraylist so far i have the add method but i need help creating a dividie method,multiplication method, and finishing the subtract method.

import java.util.*;

public class BigInt {

//Instance variables for the class, a boolean for the sign and an ArrayList to store the input String

private boolean pos = true;

private ArrayList bigInt = new ArrayList();

public BigInt () {

this.pos = true;

bigInt.add(0);

}

//Constructor for big integers input as strings

public BigInt (String newBigInt) {

dealWithSign(newBigInt);

cleanUpNumber(newBigInt);

for(int i = newBigInt.length()-1; i >=0; i--) { //My Modified code is length()+1 to length()-1

bigInt.add(Integer.parseInt(newBigInt.substring(i, i+1)));

}

}

//Private method to deal with the sign of the incoming big integer

private void dealWithSign(String num) {

if(num.charAt(0) == '+' || num.charAt(0) == '-') {

if(num.length() == 1) {

System.out.println("Invalid value: sign only, no integer.");

System.exit(0);

}

if(num.charAt(0) == '-') {

this.pos = false;

}

else {

this.pos = true;

}

num = num.substring(1);

}

}

//Private method to clean up the number; remove leading zeros, check for other errors, etc

private void cleanUpNumber(String num) {

if(num.charAt(0) == ' ') {

System.out.println("Invalid value: leading blank space.");

System.exit(0);

}

while(num.charAt(0) == '0' && num.length() > 1) {

num = num.substring(1);

}

}

public BigInt addBigInts(BigInt one, BigInt two) {

BigInt result = new BigInt();

if (one.pos && two.pos && one.bigInt.size() == two.bigInt.size()) {

for(int i=one.bigInt.size()-1; i >=0; i--) { int v1=one.bigInt.get(i); int v2=two.bigInt.get(i); result.bigInt.add(v1+v2);

if (result.bigInt.get(i) > 9) {

result.bigInt.add(0);

result.bigInt.add(i+1, (result.bigInt.set(i+1,result.bigInt.get(i+1) + 1))); }

}

} return result;

}

//Public method that performs subtraction

public void subtract(BigInt one, BigInt two) {

}

//toString method

public String toString() {

String answer = "";

for(int i=bigInt.size()-1; i>=0;i--)

answer = answer + bigInt.get(i); return answer;

} }