大整数相乘------java实现

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 stub
BigInteger 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);
}

}

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());
}
}
}