通过字符数组来存放计算后的结果，最后以字符串的形式输出。

char[] ch = new char[]{};
int length = ch.length;  //这里定义的length并不指ch数组的长度，只是一个初始值为0的int 变量
if( s1.length()>s2.length() ) {
  length = s1.length()+1;
}else {
  length = s2.length()+1;
}

这样写，最后没能对数组ch的长度进行设置,错错错，应改为:

int length = 0;
if( s1.length()>s2.length() ) {
  length = s1.length();  
}else {
  length = s2.length();
}
char[] ch = new char[ length ];

这样才是对数组长度进行了定义！！！！！！！！！！！

int 型的变量转变成char在存进char数组，需要更改asci码值。（+48）

ch[ i ] = ( char )( sum%10+48 );

源代码

package power;

import java.util.Scanner;

public class BigBigBigOOO {

    public static String computerBigNumber(String s1, String s2) {

        String s = "";
        
        if (s1.length() > s2.length()) {
            int length = 0;
            int abs = Math.abs(s1.length() - s2.length());// 1
            length = s1.length()+1;
            char[] ch = new char[length];
            int sum = 0;
            boolean flag1 = false;
            boolean flag2 = false;

            for (int i = s1.length() - 1; i >= abs; i--) {
                if (flag1 == false) {
                    sum = (s1.charAt(i) - '0') + (s2.charAt(i - abs) - '0');// 10
                    flag2 = false;
                } else {
                    sum = (s1.charAt(i) - '0') + (s2.charAt(i - abs) - '0') + 1;// 11
                    flag1 = false;// false
                    flag2 = true;// true
                }
                if (sum < 10) {
                    ch[i + 1] = (char) (sum + 48);
                } else {
                    ch[i + 1] = (char) (sum % 10 + 48);// 0
                    flag1 = true;// true true
                    flag2 = true;
                }
            }
            // --------------------------------------------------
            // 计算s2之后的数---------------------------------------------------
            for (int j = abs - 1; j >= 0; j--) {
                if (flag2 == false) {
                    ch[j + 1] = (char) ((s1.charAt(j) - '0') % 10 + 48);
                } else {
                    // ch[ j + 1 ] = ( char )((s1.charAt( j )-'0')+1+48);
                    ch[j + 1] = (char) (((s1.charAt(j) - '0') + 1) % 10 + 48);
                    if ((s1.charAt(j) - '0' + 1) < 10) {
                        flag2 = false;
                    } else {
                        flag2 = true;
                    }
                }
            }
            // -----------------------------------------------
            // 打印出ch2数组第一个数------------------------------------------------
            if ((ch[1] - '0') + (s1.charAt(0) - '0') < 10) {
                if (ch[1] - '0' == 0) {
                    ch[0] = '1';
                } else {
                    ch[0] = ' ';
                }
            } else {
                ch[0] = '1';
            }
            String s3 = new String(ch);
            s = s3;
            //==============================s1<s2的情况=====================================
        } else if( s1.length()<s2.length() ) {
            int length = 0;
            int abs = Math.abs(s1.length() - s2.length());// 1
            length = s2.length()+1;
            char[] ch = new char[length];
            int sum = 0;
            boolean flag1 = false;
            boolean flag2 = false;

            for (int i = s2.length() - 1; i >= abs; i--) {
                if (flag1 == false) {
                    sum = (s2.charAt(i) - '0') + (s1.charAt(i - abs) - '0');//10
                    flag2 = false;
                } else {
                    sum = (s2.charAt(i) - '0') + (s1.charAt(i - abs) - '0') + 1;//11
                    flag1 = false;//false
                    flag2 = true;//true
                }
                if (sum < 10) {
                    ch[i + 1] = (char) (sum + 48);
                } else {
                    ch[i + 1] = (char) (sum % 10 + 48);//0
                    flag1 = true;//true true
                    flag2 = true;
                }
            }
            //-------------------------------------------------- 计算s2之后的数---------------------------------------------------
            for (int j = abs-1; j >= 0; j--) {
                if( flag2 == false ) {
                    ch[j + 1] = ( char )((s2.charAt(j)-'0')%10+48);
                }else {
//                  ch[ j + 1 ] = ( char )((s1.charAt( j )-'0')+1+48);
                    ch[ j + 1 ] = ( char )( ((s2.charAt( j )-'0')+1)%10+48 );
                    if( (s2.charAt( j )-'0'+1) <10 ) {
                        flag2 = false;
                    }else {
                        flag2 = true;
                    }
                }
            }
            //----------------------------------------------- 打印出ch2数组第一个数------------------------------------------------
            if( (ch[ 1 ]-'0')+(s2.charAt( 0 )-'0') < 10 ) {
                if( ch[ 1 ]-'0' == 0 ) {
                    ch[ 0 ] = '1';
                }else {
                    ch[ 0 ] = ' ';
                }
            }else {
                ch[ 0 ] = '1';
            }
            String s3 = new String(ch);
            return s3;
        }
        //================================s1 = s2的情况=======================================
        else if( s1.length() == s2.length() ){
            int length = 0;
            int abs = Math.abs(s1.length() - s2.length());// 1
            length = s1.length()+1;
            char[] ch = new char[length];
            int sum = 0;
            boolean flag1 = false;
            boolean flag2 = false;

            for (int i = s1.length() - 1; i >= abs; i--) {
                if (flag1 == false) {
                    sum = (s1.charAt(i) - '0') + (s2.charAt(i - abs) - '0');
                    flag2 = false;
                } else {
                    sum = (s1.charAt(i) - '0') + (s2.charAt(i - abs) - '0') + 1;
                    flag1 = false;
                    flag2 = true;
                }
                if (sum < 10) {
                    ch[i + 1] = (char) (sum + 48);
                } else {
                    ch[i + 1] = (char) (sum % 10 + 48);
                    flag1 = true;
                    flag2 = true;
                }
            }
            //-------------------------------------------------- 计算s2之后的数---------------------------------------------------

            //----------------------------------------------- 打印出ch2数组第一个数------------------------------------------------
            if( (s2.charAt( 0 )-'0')+(s1.charAt( 0 )-'0')<10 ) {
                ch[ 0 ] = ' ';
            }else {
                ch[ 0 ] = '1';
            }
            String s3 = new String( ch );
            s = s3;
        }
        //返回String型的s
        return s;
    }
    public static void main(String[] args) {
//      String s1 = "999";
//      String s2 = "999";
        
        Scanner sc = new Scanner( System.in );
        System.out.println( "请输入一个正整数：" );
        String s1 = sc.nextLine();
        System.out.println( "请输入一个正整数：" );
        String s2 = sc.nextLine();

        System.out.println(computerBigNumber(s1, s2).toString());

    }

}