java生成4位不重复字符(包含大写字母、小写字母、数字)

需要生成4位不重复的字符作为唯一参数，要求只能使用大写字母、小写字母和数字的组合。大写字母26个、小写字母26个、10个数字共62个字符。不重复排列个数：62*62*62*62=14776336个。

/**
 * 
 * @author wzp
 * 2016-3-18
 */
public class DmSequenceUtil {

	static String bwords[] = { "A", "B", "C", "D", "E", "F", "G", "H", "I",
			"J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V",
			"W", "X", "Y", "Z", "a", "b", "c", "d", "e", "f", "g", "h", "i",
			"j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v",
			"w", "x", "y", "z", "0", "1", "2", "3", "4", "5", "6", "7", "8",
			"9" };
	static Random random = new Random();
	static int index1 = 0;
	static int index2 = 0;
	static int index3 = 0;
	static int index4 = 0;
	static boolean isRead = true;

	/**
	 * 支持14776336个不重复4位字符串 2016-3-18
	 * 
	 * @return
	 * @throws Exception
	 */
	public static synchronized String getNoRepeatId() throws Exception {
		// 判断是否需要读取文件记录
		if (isRead) {
			String sequence = read();
			if (sequence != null && sequence.length() == 4) {
				index1 = getIndex(String.valueOf(sequence.charAt(0))) + 10;
				index2 = getIndex(String.valueOf(sequence.charAt(1))) + 10;
				index3 = getIndex(String.valueOf(sequence.charAt(2))) + 10;
				index4 = getIndex(String.valueOf(sequence.charAt(3))) + 10;
			}
			isRead = false;
		}

		getRandom();
		String id = "" + bwords[index1 == 0 ? 0 : index1 - 1]
				+ bwords[index2 == 0 ? 0 : index2 - 1]
				+ bwords[index3 == 0 ? 0 : index3 - 1] + bwords[index4 - 1];
		write(id);
		return id;

	}

	private static int getIndex(String word) {
		for (int i = 0; i < bwords.length; i++) {
			if (bwords[i].equals(word)) {
				return i;
			}
		}
		return 0;
	}

	private static void getRandom() throws Exception {
		if (index4 < 62) {
			index4++;
		} else {
			index4 = 1;
			if (index3 < 62) {
				index3++;
			} else {
				index3 = 1;
				if (index2 < 62) {
					index2++;
				} else {
					index2 = 1;
					if (index1 < 62) {
						index1++;
					} else {
						throw new Exception("结束");
					}
				}
			}
		}

	}

	private static void write(String sequence) throws IOException {
		File file = new File("sequence.dat");
		FileWriter fw = new FileWriter(file, false);
		fw.write(sequence);
		fw.flush();
		fw.close();
	}

	private static String read() throws IOException {
		String sequence = "";
		File file = new File("sequence.dat");
		try {
			FileReader fr = new FileReader(file);
			BufferedReader br = new BufferedReader(fr);
			sequence = br.readLine();
			br.close();
			fr.close();
		} catch (Exception e) {
			System.err.println("读取文件出错");
		}
		return sequence;
	}

}

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