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private String get4DigitRandom() {
        final int length = 4;
        int number = (int) (Math.random() * Math.pow(10, length));
        StringBuilder sb = new StringBuilder();
        for (int i = length; i > 0; i--)
            sb.append("" + (int) (number / Math.pow(10, i - 1)) % 10);
        return sb.toString();
    }
                                    
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