Password Generate

package password;

import java.util.Random;

/**
 * @author 丛培雪 E-mail:[e-mail]
 * @version 创建时间：2008-6-9 下午08:34:37 类说明
 */
public class PasswordGenerator {
	public static void main(String[] args) throws Exception {
		PasswordGenerator passwordGenerator = new PasswordGenerator();
		for (int i = 20; i <= 100; i++) {
			System.out.println(passwordGenerator.generate(PRINTABLE_CHARACTERS,
					i));
		}
	}

	protected Random m_generator = new Random();

	public static final String DIGITS = "0123456789";
	public static final String LOCASE_CHARACTERS = "abcdefghijklmnopqrstuvwxyz";
	public static final String UPCASE_CHARACTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	public static final String PRINTABLE_CHARACTERS = DIGITS
			+ LOCASE_CHARACTERS + UPCASE_CHARACTERS;

	public String generate(String chars, int passLength) throws Exception {
		if (passLength > chars.length()) {
			throw new Exception("Password generation is imposible");
		}
		char[] availableChars = chars.toCharArray();
		int availableCharsLeft = availableChars.length;
		StringBuffer temp = new StringBuffer(passLength);
		for (int i = 0; i < passLength; i++) {
			int pos = (int) (availableCharsLeft * m_generator.nextDouble());
			temp.append(availableChars[pos]);
			// 出现过的下一次就不会出现了
			availableChars[pos] = availableChars[availableCharsLeft - 1];
			
			// 替换其他单词的单词将不会出现2遍以上，防止增加后面单词的出现几率
			--availableCharsLeft;
		}
		return String.valueOf(temp);
	}
}