java实现两个大整数（不大于100位）的乘积

import org.junit.Test;

import java.math.BigInteger;

import java.util.Arrays;

import java.util.Scanner;

public class Test1 {

//方法一：

@Test

 public static void main(String[] args) {

Scanner in =new Scanner(System.in);

String str[] = in.nextLine().split(",");

char[] param1 =trim1(str[0]).toCharArray();

char[] param2 =trim1(str[1]).toCharArray();

if(param1.length >100){

System.out.println("若输入数据大于100位则只取前100位数字");

param1 = Arrays.copyOfRange(param1,0,100);

}

if(param2.length >100){

System.out.println("若输入数据大于100位则只取前100位数字");

param2 = Arrays.copyOfRange(param2,0,100);

}

String result =multiBigInt(param1, param2);

// 除去首位的0

char[] temp = result.toCharArray();

int i =0;

for (; i < temp.length; i++) {

if (temp[i] =='0')

continue;

else

                break;

}

System.out.println(result.substring(i, result.length()));

in.close();

}

//计算单个字符 乘 一个字符串

    public static String multiBigIntSingle(char[] a,char b) {

int pre =0;

String result ="";

for (int j = a.length -1; j >=0; j--) {

int temp = a[j] -'0';

int tempb = b -'0';

int sum = temp * tempb + pre;

pre = sum /10;

int left = sum %10;

result += left;

}

if (pre !=0)

result += pre;

char[] sb = result.toCharArray();

String value ="";

for (int j = sb.length -1; j >=0; j--)

value += sb[j];

return value;

}

public static String multiBigInt(char[] a,char[] b) {

String c ="";// 保存每一行相加后的结果

        int j =0;// 控制错位

        for (int i = b.length -1; i >=0; i--) {

c =addBigInt(c.toCharArray(),multiBigIntSingle(a, b[i]).toCharArray(), j++);

}

return c;

}

//将两个大整数相加 len用于控制错位相加

    public static String addBigInt(char[] a,char[] b,int len) {

int maxlen = a.length + b.length;

char[] revA =reverse(a);

char[] revB =reverse(b);

String sb ="";

int tempa =0;

int tempb =0;

int pre =0;

for (int i =0; i < maxlen; i++) {

tempa =0;

tempb =0;

if (i < revA.length)

tempa = revA[i] -'0';

// 第二行要先以为len的距离 ,错位相加

            if (i < revB.length + len && i >= len)

tempb = revB[i - len] -'0';

int sum = tempa + tempb + pre;

pre = sum /10;

int left = sum %10;

sb += left;

}

while (pre !=0) {

sb += pre %10;

pre /=10;

}

char[] result = sb.toCharArray();

String value ="";

for (int j = result.length -1; j >=0; j--)

value += result[j];

return value;

}

//将一个字符数组反转，便于整数相加

    public static char[] reverse(char[] a) {

char[] b =new char[a.length];

int i =0;

int j = a.length -1;

for (; j >=0; j--) {

b[i] = a[j];

i++;

}

return b;

}

//去掉字符串两端空格

    public static String trim1(String param){

param = param.trim();

//去全角空格

  while (param.startsWith("　")){

param = param.substring(1,param.length()).trim();

}

while (param.endsWith("　")){

param = param.substring(0,param.length() -1).trim();

}

return param;

}

//方法二：

BigInteger res1 = BigInteger.valueOf(Long.parseLong(String.valueOf(param1))).multiply(BigInteger.valueOf(Long.parseLong(String.valueOf(param2))));

System.out.println("res="+res1);

}