输出N位包含数字和字母的字符串

public static String generateStr(int length) {
		StringBuilder sb = new StringBuilder();
		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
				sb.append((char) (97 + random.nextInt(26)));//大写字母ASCII码97-122，小写字母ASCII码65-90
			else if ("num".equalsIgnoreCase(charOrNum)) // 数字
				sb.append(String.valueOf(random.nextInt(10)));//0-10				
		}
		String returnStr = sb.toString();
		if(isAllDigit(returnStr) || isAllLetter(returnStr))//全为字母或数字时重新生成		
			        generateStr(length);							
		return sb.toString();		
	}
	public static boolean isAllDigit(String str) {  
	    return str.matches("[0-9]{1,}");  
	} 
	public static boolean isAllLetter(String str) {  
		return str.matches("[a-zA-Z]+");  
	} 
	public static void main(String[] args) {		
		for (int i = 0; i < 1000000; i++) {
			System.err.println(generateStr(6));
		}			
	}