How to do it? in java [closed]

Question

please help me how to change decimal number in string like string 12.345+34.567=46.912

String i1 = "87157214442544356924697631849432716516515643245431231321543453484651321846121314111";
    String i2 = "1234879852154195616515131321579416268789454657874184546874196";
    String i3 = "211524115874134448864751174743815584515616511561516515167266484367746";
    String i4 = "112475647.221654313543153187463497744";
    String i5 = "112785415786434867646544467356546516847354134645161345919198646146581.2165468435465";


static String AddBigNumber(String s1,String s2) {
    //only int datatype is allow for use, no double, float of other mathematical datatype allo
    String s3 = " ";

    return s3;

and main is

String result1 = AddBigNumber(i1,i2);
    System.out.println(result1);
    result1 = AddBigNumber(i3,i4);
    System.out.println(result1);
    result1 = AddBigNumber(result1,i5);
    System.out.println(result1);