Multiply two strings Java

PROGRAM TO MULTIPLY LARGE NUMBERS REPRESENTED AS STRINGS

class MAIN

{

      

// Multiplies str1 and str2, and prints result.

static String multiply(String num1, String num2)

{

    int len1 = num1.length();

    int len2 = num2.length();

    if (len1 == 0 || len2 == 0)

        return "0";

  

    // will keep the result number in vector

    // in reverse order

    int result[] = new int[len1 + len2];

  

    // Below two indexes are used to 

    // find positions in result. 

    int i_n1 = 0; 

    int i_n2 = 0; 

      

    // Go from right to left in num1

    for (int i = len1 - 1; i >= 0; i--)

    {

        int carry = 0;

        int n1 = num1.charAt(i) - '0';

  

        // To shift position to left after every

        // multipliccharAtion of a digit in num2

        i_n2 = 0; 

          

        // Go from right to left in num2             

        for (int j = len2 - 1; j >= 0; j--)

        {

            // Take current digit of second number

            int n2 = num2.charAt(j) - '0';

  

            // Multiply with current digit of first number

            // and add result to previously stored result

            // charAt current position. 

            int sum = n1 * n2 + result[i_n1 + i_n2] + carry;

  

            // Carry for next itercharAtion

            carry = sum / 10;

  

            // Store result

            result[i_n1 + i_n2] = sum % 10;

  

            i_n2++;

        }

  

        // store carry in next cell

        if (carry > 0)

            result[i_n1 + i_n2] += carry;

  

        // To shift position to left after every

        // multipliccharAtion of a digit in num1.

        i_n1++;

    }

  

    // ignore '0's from the right

    int i = result.length - 1;

    while (i >= 0 && result[i] == 0)

    i--;

  

    // If all were '0's - means either both 

    // or one of num1 or num2 were '0'

    if (i == -1)

    return "0";

  

    // genercharAte the result String

    String s = "";

      

    while (i >= 0)

        s += (result[i--]);

  

    return s;

}

  

// Driver code

public static void main(String[] args) 

{

    String str1 = "1235421415454545454545454544";

    String str2 = "1714546546546545454544548544544545";

  

    if ((str1.charAt(0) == '-' || str2.charAt(0) == '-') && 

        (str1.charAt(0) != '-' || str2.charAt(0) != '-'))

        System.out.print("-");

  

    if (str1.charAt(0) == '-' && 

        str2.charAt(0) != '-') 

    {

        str1 = str1.substring(1);

    } 

    else if (str1.charAt(0) != '-' && 

             str2.charAt(0) == '-')

    {

        str2 = str2.substring(1);

    } 

    else if (str1.charAt(0) == '-' && 

             str2.charAt(0) == '-')

    {

        str1 = str1.substring(1);

        str2 = str2.substring(1);

    }

    System.out.println(multiply(str1, str2));

}

}




OUTPUT

2118187521397235888154583183918321221520083884298838480662480

CREDITS: https://www.geeksforgeeks.org/multiply-large-numbers-represented-as-strings/