确保我得到一个条目使用随机性

Question

我正在尝试使用下面的方法生成随机列，在生成列之后，我在我的SELECT sql中使用这些列。

final String columnsList = getColumns(table.getColumns());
final String selectSql = "SELECT ID, CREATION_DATE, LAST_MODIFIED_DATE, " + columnsList + "  from " + table.getTableName() + " where id = ?";
/**
 * A simple method to get the column names in the order in which it was
 * inserted
 * 
 * @param columns
 * @return
 */
private String getColumns(final List<String> columns) {
    List<String> copy = new ArrayList<String>(columns);
    Collections.shuffle(copy);
    int rNumber = random.nextInt(columns.size());
    List<String> subList = copy.subList(0, rNumber);
    Collections.sort(subList, new Comparator<String>() {
        @Override
        public int compare(String o1, String o2) {
            return columns.indexOf(o1) < columns.indexOf(o2) ? -1 : 1;
        }
    });
    return StringUtils.join(subList, ",");
}

问题陈述:-

有时我看到columnsList值是空的，如果它是空的，那么selectSql将无法工作，因为在from keyword之前会有额外的,。如何确保getColumns每次至少返回一个条目?

我相信nextInt会在0 (inclusively) and n (exclusively)之间产生随机数。所以有时它可能会选择0，任何在1(inclusively) and n (exclusively)

Answer 1

一种方法是:

return subList.isEmpty() ? "1" : StringUtils.join(subList, ",");

但我认为更好的是修改selectSql为

final String selectSql = "SELECT ID, CREATION_DATE, LAST_MODIFIED_DATE " + columnsList + "  from " + table.getTableName() + " where id = ?";

然后

return subList.isEmpty() ? "" : ","+StringUtils.join(subList, ",");

如果你想从随机中得到至少一个值，你也可以这样做:

int rNumber = 1+random.nextInt(columns.size()-1);