写个算法实现大数的乘法

懒惰是最大的敌人！笔都不愿意动，怎么能行

package com.example.myapplication.ui;

import java.util.Arrays;

public class Main {
    //    写一个算法。2个大int整数相乘，结果会超过int的范围，写个算法实现大数的乘法。
    //    可以用string来实现
    //            不能用long等

    public static void main(String[] args) {
        int a = 9999;
        int b = 9999;
        int c = a * b;
        int[] aR = new int[]{0,0,9,9,9,9};
        int[] bR = new int[]{0,0,9,9,9,9};
        System.out.println(c);
        System.out.println(Arrays.toString(aR));
        System.out.println(Arrays.toString(bR));
        int[] lRTemp = null;
        for (int i = bR.length - 1; i >= 0; i--) {
            int[] aRTemp = new int[aR.length + (bR.length - 1)];
            for (int j = aR.length - 1; j >= 0; j--) {
                aRTemp[j + i] = bR[i] * aR[j];
            }
            for (int j = aRTemp.length - 1; j >= 0; j--) {
                if (aRTemp[j] > 9) {
                    aRTemp[j - 1] = aRTemp[j - 1] + aRTemp[j] / 10;
                    aRTemp[j] = aRTemp[j] % 10;
                }
            }
            if (lRTemp != null) {
                for (int j = aRTemp.length - 1; j >= 0; j--) {
                    aRTemp[j] = aRTemp[j] + lRTemp[j];
                    if (aRTemp[j] > 9) {
                        aRTemp[j - 1] = aRTemp[j - 1] + aRTemp[j] / 10;
                        aRTemp[j] = aRTemp[j] % 10;
                    }
                }
            }
            lRTemp = aRTemp;
        }
        System.out.println(toString(lRTemp));
    }

    public static String toString(int[] a) {
        if (a == null)
            return "null";
        int iMax = a.length - 1;
        if (iMax == -1)
            return "[]";

        StringBuilder b = new StringBuilder();
        b.append('[');
        for (int i = 0; ; i++) {
            b.append(a[i] >= 10 ? a[i] : "" + a[i]);
            if (i == iMax)
                return b.append(']').toString();
            b.append(", ");
        }
    }
}