大数相乘算法实现：（Java版，使用BigInteger）（含有大数的加、减、乘、除）

package com.bigInteger;import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;/******************************************************************** 功能描述：* 创建信息：jtm 2016-4-1* 修改信息：********************************************************************/public class BigIntegerFunc {public static void main(String[] args) throws IOException {	//获取用户的输入信息	BufferedReader br=new BufferedReader(new InputStreamReader(System.in));		String str1=null;String str2=null;System.out.print("第一个乘数是:");str1 =br.readLine();		System.out.print("第二个乘数是:");str2 =br.readLine();BigNumMutiply(str1, str2);}private static void BigNumMutiply(String str1, String str2) {// TODO Auto-generated method stubBigInteger bigNum1 =new BigInteger(str1);BigInteger bigNum2 =new BigInteger(str2);BigInteger sumNum =bigNum1.add(bigNum2);//大数相加BigInteger subNum =bigNum1.subtract(bigNum2);//大数相减BigInteger mutiNum =bigNum1.multiply(bigNum2);//大数相乘BigInteger divNum =bigNum1.divide(bigNum2);//大数相除BigInteger remNum =bigNum1.remainder(bigNum2);//大数取余System.out.println("加"+sumNum+"减"+subNum+"乘积"+mutiNum+"除"+divNum+"余"+remNum);}}

大数相乘算法实现2-----不是用BigInteger

package com.bigInteger;import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
/**** 大数相乘* ******************************************************************/public class bigInteger {public static void main(String[] args) throws IOException{BufferedReader br=new BufferedReader(new InputStreamReader(System.in));		String str1=null;String str2=null;System.out.print("第一个乘数是:");str1 =br.readLine();		System.out.print("第二个乘数是:");str2 =br.readLine();	BigNumMutiply(str1, str2);}public static void BigNumMutiply(String str1,String str2){System.out.print("大数相乘：\n" + str1 + "*" + str2 + "="); int temp=0;//用于记录输入大数的正负号个数if(str1.charAt(0)=='-'){//判断str1代表的数值的正负，如果负，删除负号，temp++StringBuffer sb1=new StringBuffer(str1);sb1.delete(0, 1);str1=sb1.toString();temp++;}if(str2.charAt(0)=='-'){//判断str2代表的数值的正负，如果负，删除负号，temp++StringBuffer sb2=new StringBuffer(str2);sb2.delete(0, 1);str2=sb2.toString();temp++;}//将字符串转为字符型数组，并反转char[] a=new StringBuffer(str1).reverse().toString().toCharArray(); char[] b=new StringBuffer(str2).reverse().toString().toCharArray();		int lenA =a.length;int lenB =b.length;//常识：一般两个数相乘，结果长度不大于两个乘数的长度之和int len =lenA+lenB;//初始化乘数结果int[] result =new int[len];// 计算结果集合 for (int i = 0; i < a.length; i++) {  for (int j = 0; j < b.length; j++) {  result[i + j] += (int) (a[i] - '0') * (int) (b[j] - '0');  }  } //处理结果集合，如果是大于10的就向前一位进位，本身进行除10取余for (int i = 0; i < result.length; i++) {  if (result[i] > 10) {  result[i + 1] += result[i] / 10;  result[i] %= 10;  }  }         StringBuffer sb = new StringBuffer();  //判断有没有前置的0，有的话跳过for (int i = len - 1; i >= 0; i--) {  if (result[i] == 0) {  continue;  } sb.append(result[i]);  } if(temp==0||temp==2){//判断最后大数相乘的正负     	System.out.println(sb.toString());}else{sb.insert(0,'-');System.out.println(sb.toString());}}
}