JAVA生成随机数字和字母组合

import java.util.Random;

/**
 * 随机数
 * @author YXP
 * 2012-9-29 上午10:38:06
 */
public class RandomTest {
	
	/**
	 * 方法1：生成随机数字和字母组合 
	 * @param length
	 * @return
	 */
	public static String getCharAndNumr(int length) {
		Random random = new Random();
		StringBuffer valSb = new StringBuffer();
		String charStr = "0123456789abcdefghijklmnopqrstuvwxyz";
		int charLength = charStr.length();
		
		for (int i = 0; i < length; i++) {
			int index = random.nextInt(charLength);
			valSb.append(charStr.charAt(index));
		}
		return valSb.toString();
	}
	
	/**
	 * 方法2：生成随机数字和字母组合
	 * @param length
	 * @return
	 */
	public static String getCharAndNumr2(int length) {
		StringBuffer valSb = new StringBuffer();
		Random random = new Random();
		for (int i = 0; i < length; i++) {
			String charOrNum = random.nextInt(2) % 2 == 0 ? "char" : "num"; // 输出字母还是数字
			if ("char".equalsIgnoreCase(charOrNum)) {
				// 字符串
				int choice = random.nextInt(2) % 2 == 0 ? 65 : 97;  // 取得大写字母还是小写字母
				valSb.append((char) (choice + random.nextInt(26)));
			} else if ("num".equalsIgnoreCase(charOrNum)) {
				// 数字
				valSb.append(String.valueOf(random.nextInt(10)));
			}
		}
		return valSb.toString();
	}
	
	/**
	 * 方法3：生成随机数字和字母组合
	 * @param length
	 * @return
	 */
	public static String getCharAndNumr3(int length) {
		StringBuffer valSb = new StringBuffer();
		for (int i = 0; i < length; i++) {
			String charOrNum = Math.round(Math.random()) % 2 == 0 ? "char" : "num"; // 输出字母还是数字
			if ("char".equalsIgnoreCase(charOrNum)) {
				// 字符串
				int choice = Math.round(Math.random()) % 2 == 0 ? 65 : 97;  // 取得大写字母还是小写字母
				valSb.append((char) (choice + Math.round(Math.random()*25)));
			} else if ("num".equalsIgnoreCase(charOrNum)) {
				// 数字
				valSb.append(String.valueOf(Math.round(Math.random()*9)));
			}
		}
		return valSb.toString();
	}

	public static void main(String[] args) {
		System.out.println(getCharAndNumr(5));
		System.out.println(getCharAndNumr2(5));
		System.out.println(getCharAndNumr3(5));
		System.out.println(Math.round(0.01));
		System.out.println(Math.ceil(0.01));
		System.out.println(Math.floor(0.91));
	}

}