互不重复的16位随机数生成代码.

public class RandomString {

	/**
	 * 每位允许的字符
	 */
	private static final String POSSIBLE_CHARS="0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
	
	/**
	 * 生产一个指定长度的随机字符串
	 * @param length 字符串长度
	 * @return
	 */
	private String generateRandomString(int length) {
		StringBuilder sb = new StringBuilder(length);
		SecureRandom random = new SecureRandom();
		for (int i = 0; i < length; i++) {
			sb.append(POSSIBLE_CHARS.charAt(random.nextInt(POSSIBLE_CHARS.length())));
		}
		return sb.toString();
	}
	
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Set<String> check = new HashSet<String>();
		RandomString obj = new RandomString();
		
		//生成2000000个随机字符串，检查是否出现重复
		for (int i = 0; i < 2000000; i++) {
			String s = obj.generateRandomString(16);
			if (check.contains(s)) {
				throw new IllegalStateException("Repeated string found : " + s);
			} else {
				if (i % 1000 == 0) System.out.println("generated " + i / 1000 + "000 strings.");
				check.add(s);
			}
		}
	}
}