Java面试题总汇-阿里云开发者社区

6位短信验证码(不以0开头)

String random = String.valueOf(Math.random());
        boolean start = false;
        StringBuilder sb = new StringBuilder();
        for (int i = 2,k = 0; k < 6 && i < random.length(); i++) {
        if (!start && random.charAt(i) == '0')
            continue;
        else {
            start = true;
            sb.append(random.charAt(i));
            k++;
        }
    }
    System.out.println(sb.toString());

求从10到100中能被3或5整除的数的和

private static final int faceQuestions1() {
        int sum = 0;
        for (int i = 10; i <= 100; i++) {
            if (i % 3 == 0 || i % 5 == 0)
                sum += i;
        }
        return sum;
    }

将一个字符串逆序，不要使用反转函数

private static final String faceQuestions2(String originalStr) {
        StringBuilder sb = new StringBuilder();
        for (int i = originalStr.length() - 1; i >= 0; i--)
            sb.append(originalStr.charAt(i));
        return sb.toString();
    }

反转一个栈

private static final void faceQuestions3() {
        Stack<String> items = new Stack<>();
        items.push("my");
        items.push("name");
        items.push("is");
        items.push("benjamin");
        revertStack(items);
        System.out.println(items);
    }

    private static final <E> void revertStack(Stack<E> stack) {
        Queue<E> queue = new LinkedList<>();
        while (stack.size() > 0)
            queue.offer(stack.pop());
        while (queue.size() > 0)
            stack.push(queue.poll());
    }

Date转换字符串

给定一个java.util.Date对象，如何转化为”2007-3-22 20:23:22”格式的字符串
我这里用一种简单的方式，不使用SimpleDateFormat

1
2
3

private static void formatDate(Date date) {
    System.out.printf("%TF %<TT", date);
}

如何判断一个数是否是素数

private static void checkPrimeNumber(int num) {
    for (int i = 2; i < num/2; ++i) {
        if (num % i == 0) {
            System.out.println("不是素数");
            break;
        }
        if (i == num/2 - 1) {
            System.out.println(num + "是素数");
        }
    }
}

如何得到一个数的阶乘

阶乘举个例子吧：6的阶乘是 6 5 4 3 2 * 1

private static int factorial(int num) {
        if (num == 1) {
            return 1;
        }
        return num * factorial(num - 1);
    }

二分法查找

写一个方法，用二分查找法判断任意整数在任意整数数组里面是否存在，若存在就返回它在数组中的索引位置，不存在返回-1

/**
     * 二分法查找，使用递归
     * @param originArr
     * @param num
     * @param startIndex
     * @param endIndex
     * @return
     */
    private static int binarySearch(int[] originArr, int num, int startIndex, int endIndex) {
        int index = (startIndex + endIndex) / 2;
        if (startIndex < 0 || endIndex > originArr.length || num < originArr[startIndex] || num > originArr[endIndex]) {
            throw new IllegalArgumentException("输入的参数不规范，请重新输入");
        }
        if (num < originArr[index]) {
            return binarySearch(originArr, num, startIndex, index);
        } else if (num > originArr[index]) {
            return binarySearch(originArr, num, index, endIndex);
        } else {
            return index;
        }
    }

    /**
     * 二分法查找，不适用递归
     * @param originArr
     * @param num
     * @return
     */
    private static int binarySearch(int[] originArr, int num) {
        int index = -1;
        int startIndex = 0;
        int endIndex = originArr.length - 1;
        while (startIndex <= endIndex) {
            index = (startIndex + endIndex) / 2;
            if (num < originArr[index]) {
                endIndex = index;
            } else if (num > originArr[index]) {
                startIndex = index;
            } else {
                break;
            }
        }
        return index;
    }

数组移动

定义一个int型的一维数组，包含10个元素，然后将数组中的元素都向前移一个位置，即，a[0]=a[1],a[1]=a[2],…最后一个元素的值是原来第一个元素的值，然后输出这个数组。

private static void moveArray(int a[]) {
    int[] b = new int[a.length];
    System.arraycopy(a, 1, b, 0, a.length-1);
    b[b.length-1] = a[0];
}

杨辉三角

private static void yanghuiTriangle(int row) {
    int triangle[][] = new int[row][];
    for (int i = 0; i < triangle.length; ++i) {
        triangle[i] = new int[i+1];
        for (int j = 0; j < triangle.length-i-1; ++j) {
            System.out.print(" ");
        }
        for (int j = 0; j <= i; ++j) {
            if (i == 0 || j == 0 || j == i) {
                triangle[i][j] = 1;
            } else {
                triangle[i][j] = triangle[i-1][j-1] + triangle[i-1][j];
            }
            System.out.print(triangle[i][j] + " ");
        }
        System.out.println();
    }
}

输出结果

         1 
        1 1 
       1 2 1 
      1 3 3 1 
     1 4 6 4 1 
    1 5 10 10 5 1 
   1 6 15 20 15 6 1 
  1 7 21 35 35 21 7 1 
 1 8 28 56 70 56 28 8 1 
1 9 36 84 126 126 84 36 9 1

输入数字，输出中文大写

如输入101，输出壹佰零壹
输入100010，输出拾万零壹拾
自己闲来无聊就写了一个小程序，希望大家多测试

/**
     * 生成中文描述的数字串
     * @param originStr
     * @return
     */
    private static String generateChinese(String originStr) {
        String pointStr = "";
        // 小数处理
        if (originStr.contains(".")) {
            pointStr = originStr.substring(originStr.indexOf('.') + 1); // 得到小数点后的数
            originStr = originStr.substring(0, originStr.indexOf('.'));
        }
        originStr = filterStartWithZero(originStr); // 去除前面的0
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < originStr.length(); ++i) {
            if (i % 4 == 0) {
                if (i / 4 == 1) {
                    sb.append("万");
                } else if (i / 4 == 2) {
                    sb.append("亿");
                }
            } else {
                if (i % 4 == 1) {
                    sb.append("十");
                } else if (i % 4 == 2) {
                    sb.append("百");
                } else if (i % 4 == 3) {
                    sb.append("千");
                }
            }
            sb.append(getChinese(originStr.charAt(originStr.length()-i-1)));
        }
        String result = reverse(sb.toString());
        result = filterAfterGenerate(result);   // 小数点之前的数处理完毕

        /** 处理小数点后的数 **/
        StringBuilder pointSB = new StringBuilder();
        if (!"".equals(pointStr)) {
            for (int i = 0; i < pointStr.length(); ++i) {
                pointSB.append(getChinese(pointStr.charAt(i)));
            }
            result = result + "点" + pointSB.toString();
        }
        return result;
    }
    
/**
     * 最后产生结果后进行过滤，把不相关的数据替换掉
     * @param originStr
     * @return
     */
    private static String filterAfterGenerate(String originStr) {
        String[] filterData = {"零亿", "零万", "零千", "零百", "零十", "零零"};
        String[] replaceData = {"亿", "万", "零", "零", "零", "零"};
        int flag = 0;
        /** 替换filterData中的数据为replaceData中的数据 **/
        while (true) {
            for (int i = 0; i < filterData.length; ++i) {
                if (!originStr.contains(filterData[i])) {
                    flag++;
                } else {
                    flag = 0;
                    originStr = originStr.replace(filterData[i], replaceData[i]);
                }
            }
            if (flag == filterData.length) {
                break;
            } else {
                flag = 0;
            }
        }
        /** 去除末尾的零，比如 20: 二十零 **/
        while (originStr.endsWith("零")) {
            originStr = originStr.substring(0, originStr.length()-1);
        }
        return originStr.equals("") ? "零" : originStr;  // 如果是空，就返回零
    }
    
/**
     * 根据传入的数字得到中文
     * @param num
     * @return
     */
    private static char getChinese(char num) {
        char[] digit = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'};
        char[] chinese = {'零', '壹', '贰', '叁', '肆', '伍', '陆', '柒', '捌', '玖'};
        for (int i = 0; i < digit.length; ++i) {
            if (digit[i] == num) {
                return chinese[i];
            }
        }
        return '零';
    }
    
/**
     * 反转字符串
     * @param str
     * @return
     */
    private static String reverse(String str) {
        StringBuilder sb = new StringBuilder();
        for (int i = str.length() - 1; i >= 0; --i) {
            sb.append(str.charAt(i));
        }
        return sb.toString();
    }

/**
     * 去除输入字符串开头的零，如：0023输出为23
     * @param originStr
     * @return
     */
    private static String filterStartWithZero(String originStr) {
        int i = 0;
        for (; i < originStr.length(); ++i) {
            if (originStr.charAt(i) != '0') {
                break;
            }
        }
        return originStr.substring(i);
    }

    public static void main(String[] args) {
        System.out.println(generateChinese("100010220.2")); // 壹亿零壹万零贰百贰十点贰
        System.out.println(generateChinese("12003.2341"));  // 壹万贰千零叁点贰叁肆壹
        System.out.println(generateChinese("0000.00")); // 零点零零
        System.out.println(generateChinese("00001.230"));   // 壹点贰叁零
        System.out.println(generateChinese("12003943490.12"));  // 壹百贰十亿零叁百玖十肆万叁千肆百玖十点壹贰
    }

输出九九乘法表

private static void nineNineMulitTable() {
    for (int i = 1; i <= 9; ++i) {
        for (int j = 1; j <= i; ++j) {
            System.out.print(i + "*" + j + "=" + (i * j) + "\t");
        }
        System.out.println();
    }
}

Java面试题总汇