java产生随机数的几种方式

import java.util.Random;

/**
* 随机数、随即字符串工具 

*/
public class RandomUtils {
        public static final String allChar = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        public static final String letterChar = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        public static final String numberChar = "0123456789";

        /**
         * 返回一个定长的随机字符串(只包含大小写字母、数字)
         *
         * @param length 随机字符串长度
         * @return 随机字符串
         */
        public static String generateString(int length) {
                StringBuffer sb = new StringBuffer();
                Random random = new Random();
                for (int i = 0; i < length; i++) {
                        sb.append(allChar.charAt(random.nextInt(allChar.length())));
                }
                return sb.toString();
        }

        /**
         * 返回一个定长的随机纯字母字符串(只包含大小写字母)
         *
         * @param length 随机字符串长度
         * @return 随机字符串
         */
        public static String generateMixString(int length) {
                StringBuffer sb = new StringBuffer();
                Random random = new Random();
                for (int i = 0; i < length; i++) {
                        sb.append(allChar.charAt(random.nextInt(letterChar.length())));
                }
                return sb.toString();
        }

        /**
         * 返回一个定长的随机纯大写字母字符串(只包含大小写字母)
         *
         * @param length 随机字符串长度
         * @return 随机字符串
         */
        public static String generateLowerString(int length) {
                return generateMixString(length).toLowerCase();
        }

        /**
         * 返回一个定长的随机纯小写字母字符串(只包含大小写字母)
         *
         * @param length 随机字符串长度
         * @return 随机字符串
         */
        public static String generateUpperString(int length) {
                return generateMixString(length).toUpperCase();
        }

        /**
         * 生成一个定长的纯0字符串
         *
         * @param length 字符串长度
         * @return 纯0字符串
         */
        public static String generateZeroString(int length) {
                StringBuffer sb = new StringBuffer();
                for (int i = 0; i < length; i++) {
                        sb.append('0');
                }
                return sb.toString();
        }

        /**
         * 根据数字生成一个定长的字符串，长度不够前面补0
         *
         * @param num             数字
         * @param fixdlenth 字符串长度
         * @return 定长的字符串
         */
        public static String toFixdLengthString(long num, int fixdlenth) {
                StringBuffer sb = new StringBuffer();
                String strNum = String.valueOf(num);
                if (fixdlenth - strNum.length() >= 0) {
                        sb.append(generateZeroString(fixdlenth - strNum.length()));
                } else {
                        throw new RuntimeException("将数字" + num + "转化为长度为" + fixdlenth + "的字符串发生异常！");
                }
                sb.append(strNum);
                return sb.toString();
        }

        /**
         * 根据数字生成一个定长的字符串，长度不够前面补0
         *
         * @param num             数字
         * @param fixdlenth 字符串长度
         * @return 定长的字符串
         */
        public static String toFixdLengthString(int num, int fixdlenth) {
                StringBuffer sb = new StringBuffer();
                String strNum = String.valueOf(num);
                if (fixdlenth - strNum.length() >= 0) {
                        sb.append(generateZeroString(fixdlenth - strNum.length()));
                } else {
                        throw new RuntimeException("将数字" + num + "转化为长度为" + fixdlenth + "的字符串发生异常！");
                }
                sb.append(strNum);
                return sb.toString();
        }

        public static void main(String[] args) {
                System.out.println(generateString(15));
                System.out.println(generateMixString(15));
                System.out.println(generateLowerString(15));
                System.out.println(generateUpperString(15));
                System.out.println(generateZeroString(15));
                System.out.println(toFixdLengthString(123, 15));
                System.out.println(toFixdLengthString(123L, 15));
        }
}