Write multi-digit multiplication with Java

A friend encountered a problem during the interview, write programs in Java, calculate two 20-digit multiplication. Listen, it's funny, just try to make it.

The idea is to use code to simulate the process of multiplying the hand, that is, the multiplication of a bit, the result remains on the corresponding bit, and the rounding problem is solved.

Ask if the other side is solved, the other side told me that Java has a Java.math.BigInteger package, there is a BigInteger s1=new BigInteger (a); Receive string type data, you can use S1.multiply ( S2) is multiplied by a large number of digits.

Here's My Code:

ImportJava.math.BigInteger;ImportJava.util.Map;ImportJava.util.Scanner;ImportJava.util.TreeMap; Public  class chengfa {    StaticMap<integer, integer> map1 =NewTreemap<integer, integer> ();//Store the first number entered by the user, the key is the number of digits, 123 in turn represents a 1000, and so on, the value is the number of that bit.     StaticMap<integer, integer> map2 =NewTreemap<integer, integer> ();//Store The second number entered by the user, meaning the same as map1.     StaticMap<integer, integer> map3 =NewTreemap<integer, integer> ();//Store The calculated results, the keys and values have the same meaning as MPA1.     Static intCalcnumber=0;//used to count the number of times     Public Static void Main(string[] args) {System.out.println ("Please enter the first number:"); Scanner in =NewScanner (system.in); String a = In.next ();//Receive the first number entered by the user, saved as a string typeSystem.out.println ("Please enter a second number:"); String B = In.next ();//Receive a second number entered by the user, saved as String typeMap1=savetomap (a);//The number in string A is stored in the Map1 collection by BitsMap2=savetomap (b);//The number in string b is stored in the Map1 collection by BitsMap3=calc (Map1, MAP2);        String Result=getresult (MAP3); System.out.println (The answer to the product is: "+ result); System.out.println ("The total number of operations is:"+calcnumber); BigInteger s1=NewBigInteger (a); BigInteger s2=NewBigInteger (b); System.out.println ("The answer with Java.math.BigInteger:"+s1.multiply (S2)); }/** * The string entered by the user is stored in the map collection by bits * @param number passed in the parameter for the user input string * @return Returns the map collection of the saved numbers */
          Public StaticMap<integer, integer>Savetomap(String number) {Map<integer, integer> maptemp =NewTreemap<integer, integer> ();intLength = Number.length ();//The length of the passed string is taken out so that in the following loop, the number flashback is placed in the map collection         for(intm =1; M <= number.length (); m++) {//Converts the first string entered into a single number and places it in Map1Maptemp.put (length--, integer.parseint (number.substring (M-1, m)));//Put the digits in the string flashback into the map collection}returnMaptemp; }/** * Nesting of 2 for loops, realizing that each number in the MAP1 is multiplied by each number in the MAP2, and the result is saved in the result set in a bitwise * @param map1 * @param map2 * @return * *     Public StaticMap<integer, integer>Calc(Map<integer, Integer> map1, Map<integer, integer> map2) {Map<integer, integer> mapTemp1 =NewTreemap<integer, integer> (); for(intTotal =1; Total <= map1.size () + map2.size (); total++) {//Initialize data in MAPTEMP1 to avoid null pointer problems when saving numbersMaptemp1.put (Total,0); } for(inti =1; I <= map1.size (); i++) {//First layer cycle, when 1 take map1 in the bit, when 2 take map1 10 bits, according to this sliding scale             for(intK =1; K <= Map2.size (); k++) {///second layer loop, when 1 take map2 in the bit, when 2 take MAP2 10 bits, according to this sliding scale                intRESULT1 = Map1.get (i) * Map2.get (k);//The number of I in the map1 is multiplied with each position in the MAP2, RESULT1 is the productcalcnumber++;intCurrentnum = k + I-1;if(RESULT1/Ten==0) {//When the result of the product is single digit, enter this code block, only need to put the bits inAddnumber (Currentnum, RESULT1,MAPTEMP1); }Else{//When the product is a two-digit number, enter this code block, first put the bits in, and then put the ten inAddnumber (currentnum, result1%Ten, MAPTEMP1);//Place the position of the Currentnum in the Map3 collectionAddnumber (Currentnum +1, RESULT1/Ten, MAPTEMP1);//Put ten to the next position of Currentnum in the Map3 collection}            }        }returnMAPTEMP1; }/** * The operation method that saves the result of the operation to this bit, mainly to solve the rounding problem that is greater than 9 after the addition of the number * @param where the currentnum result should be placed (number of digits) * @param Res Ult the value that needs to be put in this position * /     Public Static void Addnumber(intCurrentnum,intResult,map<integer, integer> Resultmap) {intResulttem = Resultmap.get (currentnum) + result;if(Resulttem/Ten==0) {//If the Resulttem is single digit, add it directly to the location, no need to consider rounding issuesResultmap.put (Currentnum, Resulttem);        calcnumber++; }Else{//If the Resulttem is a two-digit number, you need to consider rounding issuesResultmap.put (currentnum, Resulttem%Ten);//Add single digit to this locationAddnumber (Currentnum +1, Resulttem/Ten, Resultmap);        calcnumber++; }    }/** * method to convert the results inside map3 to a string type for final display to the user * @param resultmap * @return * /     Public StaticStringGetResult(Map<integer, integer> resultmap) {String result=""; for(intg = Resultmap.size (); G >=1; g--) {if(Resultmap.get (g) = =0&& Result.equals ("")) {//If Map3.get (g) is 0 and there is no number in result, it proves to be 0 omitted before, do not do any action}Else{result + = Resultmap.get (g) +""; }        }returnResult }}

Write multi-digit multiplication with Java