java 计算两个大数之间的乘积(非BigDecimal版)

public class JFQ_Number {


/**
* @param args
*/
public static void main(String[] args) {
System.out.println("I'm sorry, you have to input a number:");
BufferedReader br1 = new BufferedReader(new InputStreamReader(System.in));
String str1 = "";
try {
str1 = br1.readLine().trim();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} 
if("".equals(str1)) {
System.out.println("Do You want to kill me? The params have to be not empty!");
System.exit(0);
}
System.out.println("I'm sorry, you have to input a number again:");
BufferedReader br2 = new BufferedReader(new InputStreamReader(System.in));

String str2 = "";
try {
str2 = br2.readLine().trim();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
if(br2 != null) {
try {
br2.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
if("".equals(str2)) {
System.out.println("Do You want to kill me? The params have to be not empty!");
System.exit(0);
}

Map<String, String> map = new HashMap<String, String>();
char[] ch1 = str1.toCharArray();
char[] ch2 = str2.toCharArray();
int length1 = ch1.length;
int length2 = ch2.length;
for(int i = 0; i < length1; i++) {
StringBuffer zero1 = new StringBuffer();
for(int k = 0; k < (length1 - 1 - i); k++) {
zero1.append("0");
}
int n1 = Integer.valueOf(String.valueOf(ch1[i]));
for(int j = 0; j < ch2.length; j++) {
int n2 = Integer.valueOf(String.valueOf(ch2[j]));
StringBuffer zero2 = new StringBuffer();
for(int k2 = 0; k2 < (length2 - 1 - j); k2++) {
zero2.append("0");
}
//System.out.println(n1 + ", " + n2 + ", " + zero1.toString() + ", " + zero2.toString());
map.put((length1 - 1 - i) + "_" + (length2  -1 - j), n1*n2+zero2.append(zero1).toString());
}
}
StringBuffer sb = new StringBuffer();
for(Entry<String, String> entry : map.entrySet()) {
String value = entry.getValue();
if(sb.length() > 0) {
char[] chh1 = sb.toString().toCharArray();
char[] chh2 = value.toCharArray();
StringBuffer temp = new StringBuffer();
int tempCount = 0;
if(chh1.length >= chh2.length) {
for(int i = chh2.length - 1; i >= 0; i--) {  //旧值
char c2 = chh2[i];
int nn2 = Integer.valueOf(String.valueOf(c2));
char c1 = chh1[i + chh1.length - chh2.length];
int nn1 = Integer.valueOf(String.valueOf(c1));
int count = nn1 + nn2 + tempCount;
if(count >= 10) {
tempCount = 1;
count = count - 10;
} else {
tempCount = 0;
}
temp.append(count);
if(i == 0) {
for(int j = (chh1.length - chh2.length - 1); j >= 0; j--) {
int tt = Integer.valueOf(String.valueOf(chh1[j]));
int ct = tt + tempCount;
if(ct >= 10) {
ct = ct - 10;
tempCount = 1;
} else {
tempCount = 0;
}
temp.append(ct);
if(j == 0 && tempCount != 0) {
temp.append(tempCount);
}
}
}
}
sb = temp.reverse();
} else {
for(int i = chh1.length - 1; i >= 0; i--) {  
char c1 = chh1[i];
int nn1 = Integer.valueOf(String.valueOf(c1));
char c2 = chh2[i + chh2.length - chh1.length];
int nn2 = Integer.valueOf(String.valueOf(c2));
int count = nn1 + nn2 + tempCount;
if(count >= 10) {
tempCount = 1;
count = count - 10;
} else {
tempCount = 0;
}
temp.append(count);

if(i == 0) {
for(int j = (chh2.length - chh1.length - 1); j >= 0; j--) {
int tt = Integer.valueOf(String.valueOf(chh2[j]));
int ct = tt + tempCount;
if(ct >= 10) {
ct = ct - 10;
tempCount = 1;
} else {
tempCount = 0;
}
temp.append(ct);
if(j == 0 && tempCount != 0) {
temp.append(tempCount);
}
}
}
}
sb = temp.reverse();
}
} else {
sb.append(value);
}
}

System.out.println(sb.toString());


}


}