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public static String generateRandomString(Integer len) {
final String chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz';
String randStr = '';
while (randStr.length() < len) {
Integer idx = Math.mod(Math.abs(Crypto.getRandomInteger()), chars.length());
randStr += chars.substring(idx, idx+1);
}
return randStr;
}