1.题目

给定两个正整数（不含前导 0），计算它们的和。

输入格式
共两行，每行包含一个整数。

输出格式
共一行，包含所求的和。

数据范围
$1\le 整数长度\le 100000$

输入样例：

12
23

输出样例：

35

2.基本思想

Java中有两个类可以来处理高精度的计算
分别是处理整数的BigInteger和处理小数的BigDecimal

3.代码实现

import java.util.Scanner;public class Main {public static void main(String[] args) {Scanner sc = new Scanner(System.in);String a = sc.next(), b = sc.next();char[] A = new char[a.length()], B = new char[b.length()];for (int i = a.length() - 1; i >= 0; i--)A[a.length() - 1 - i] = a.charAt(i);//将大整数a 的数字 按照 各位在前排序 a:【6 5 4】——>A:【4 5 6】for (int i = b.length() - 1; i >= 0; i--) B[b.length() - 1 - i] = b.charAt(i);String s = add(A, B);System.out.println(s);}private static String add(char[] A, char[] B) {StringBuffer sb = new StringBuffer();int t = 0;if (A.length < B.length) return add(B, A);for (int i = 0; i < A.length; i++) {t += A[i] - '0';//转数字if (i < B.length) t += B[i] - '0';sb.append(t % 10);t /= 10;}//判断最后的tif (t != 0) sb.append(1);return sb.reverse().toString();}
}

调用 函数 实现

import java.io.*;
import java.math.BigInteger;public class Main{public static void main(String args[]) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigInteger a = new BigInteger(reader.readLine());BigInteger b = new BigInteger(reader.readLine());System.out.println(a.add(b));reader.close();}
}

4.总结

BigInteger 只可用于整数

构造方法

BigInteger(byte[] val)
将包含BigInteger的二进制补码二进制表达式的字节数组转换为BigInteger
BigInteger(int numBits, Random rnd)
构造一个随机生成的BigInteger，均匀分布在0到（2 numBits - 1）的范围内。
BigInteger(String val)
将BigInteger的十进制字符串表示形式转换为BigInteger。

加法 add( )

import java.math.BigInteger;
import java.io.*;public class Main {public static void main(String[] args) throws IOException{BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigInteger a = new BigInteger(reader.readLine());BigInteger b = new BigInteger(reader.readLine());System.out.println(a.add(b));reader.close();}
}

减法 subtract( )

import java.io.*;
import java.math.BigInteger;public class Main {public static void main(String[] args) throws IOException{BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigInteger a = new BigInteger(reader.readLine());BigInteger b = new BigInteger(reader.readLine());System.out.println(a.subtract(b));reader.close();}
}

乘法 multiply( )

import java.io.*;
import java.math.BigInteger;public class Main {public static void main(String[] args) throws IOException{BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigInteger a = new BigInteger(reader.readLine());BigInteger b = new BigInteger(reader.readLine());System.out.println(a.multiply(b));reader.close();}
}

除法 divideAndRemainder( )

import java.io.*;
import java.math.BigInteger;public class Main {public static void main(String[] args) throws IOException{BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigInteger a = new BigInteger(reader.readLine());BigInteger b = new BigInteger(reader.readLine());//divide 返回值为 a/bBigInteger[] c = a.divideAndRemainder(b); //返回值为数组，分别为a/b和a%bSystem.out.println(c[0]);System.out.println(c[1]);reader.close();}
}

取余 mod( )

import java.io.*;
import java.math.BigInteger;public class Main {public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigInteger a = new BigInteger(reader.readLine());BigInteger b = new BigInteger(reader.readLine());System.out.println(a.mod(b));reader.close();}
}

BigDecimal 处理浮点数运算

构造方法

BigDecimal(char[] in) 
一个转换的字符数组表示 BigDecimal成 BigDecimal ，接受字符作为的相同序列 BigDecimal(String)构造。  
BigDecimal(char[] in, int offset, int len) 
一个转换的字符数组表示 BigDecimal成 BigDecimal ，接受字符作为的相同序列 BigDecimal(String)构造，同时允许一个子阵列被指定。    
BigDecimal(double val) 
将 double转换为 BigDecimal ，这是 double的二进制浮点值的精确十进制表示 
BigDecimal(int val)
将 int成 BigDecimal
BigDecimal(long val) 
将 long成 BigDecimal
BigDecimal(String val) 

加法 add( )

import java.io.*;
import java.math.BigDecimal;public class Main {public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigDecimal a = new BigDecimal(reader.readLine());BigDecimal b = new BigDecimal(reader.readLine());System.out.println(a.add(b));reader.close();}
}

取余 remainder( )

import java.io.*;
import java.math.BigDecimal;public class Main {public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigDecimal a = new BigDecimal(reader.readLine());BigDecimal b = new BigDecimal(reader.readLine());System.out.println(a.remainder(b));reader.close();}
}

除法 divide( )

import java.io.*;
import java.math.BigDecimal;public class Main {public static void main(String[] args) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));BigDecimal a = new BigDecimal(reader.readLine());BigDecimal b = new BigDecimal(reader.readLine());System.out.println(a.divide(b));reader.close();}
}