生成验证码 数学+字母

public static String getCharAndNumr(int length)  
    {  
    StringBuffer buff = new StringBuffer();
int index = 0;
for (int i = 0; i < length; i++) {
    //char c = 'a';
    int random = (int) (Math.random() * 1000);
    if (i > 3) {
index = random % 3;
    } else {
index = i;
    }

    switch (index) {
    case 0:
buff.append((char) (97 + random % 26));
break;
    case 1:
buff.append((char) (65 + random % 26));
break;
    case 2:
buff.append((char) (48 + random % 10));
break;
    }
}     

String before = buff.toString();
System.out.println("before=" + before);

// 手机验证码过滤1Il0o等几个难以区分的字符
String after = before.replace("1", "3")//
.replace("I", "J")//
.replace("l", "m")//
.replace("0", "2")//
.replace("o", "p");
System.out.println(" after=" + after);


        return buff.toString();  
    }