public Item withStringSet(String attrName, String ...val) {
        checkInvalidAttribute(attrName, val);
        Set<String> strSet = new LinkedHashSet<String>(Arrays.asList(val));
        if (strSet.size() != val.length)
            throw new IllegalArgumentException(DUPLICATE_VALUES_FOUND_IN_INPUT);
        attributes.put(attrName, strSet);
        return this;
    }
Sets the value of the specified attribute in the current item to the
given value.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/document/Item.java