Do not use the BigInteger class to implement large number additions (Java)

 Packageadd;ImportJava.math.BigInteger; Public classBigadd { Public Staticstring Bigadd (String n1, string n2) {//string buffers: Improving the efficiency of string manipulationStringBuffer result =NewStringBuffer (); //Invert stringN1 =reverse (N1); N2=reverse (N2); //Maximum length        intLen1 =n1.length (); intLen2 =n2.length (); //Maximum length        intMaxLen =Max (len1, len2); //If rounding is required        Booleanover =false; //the high position of the short string is 0 paddedcover (N1, N2, Len1, len2);  for(inti = 0; i < MaxLen; i++) {            //add each bit            intNSum = Changetoint (n1, i) +Changetoint (n2, i); if(NSum < 9) {                //last have carry                if(over =true) {                    if(NSum = = 9) {result.append (0); } Else{result.append (nSum+ 1); //Cancel Roundingover =false; }                } Else{result.append (nSum); }            } Else {                if(Over = =true) {result.append (NSum-10 + 1); } Else{result.append (nSum-10); over=true; }            }        }        //is there a final carry?        if(Over = =true) {result.append (1); }        returnresult.reverse (). toString (); }     Public Staticstring Reverse (string n) {n=NewStringBuffer (n). Reverse (). toString (); returnN; }     Public Static intMaxintAintb) {returna > B?a:b; }     Public Static intChangetoint (String N,inti) {//add "" to convert character to string        returnInteger.parseint (N.charat (i) + ""); }     Public Static voidCover (string n1, string n2,intLen1,intlen2) {        if(Len1 <len2) {             for(inti = len1; i < len2; i++) {N1+ = "0"; }        } Else if(Len1 >len2) {             for(inti = len2; i < len1; i++) {n2+ = "0"; }        }    }    //Test     Public Static voidMain (string[] args) {String N1= "666666666666666666"; String N2= "696969696969696969"; String N=Bigadd (N1, N2);        SYSTEM.OUT.PRINTLN (n); //Verify with BigInteger classBigInteger B1 =NewBigInteger (N1); BigInteger B2=NewBigInteger (n2);    System.out.println (B1.add (B2)); }}

Do not use the BigInteger class to implement large number additions (Java)