How do I get result from SQL query in custom liquibase precondition?

I am trying to create a custom precondition as defined in https://docs.liquibase.com/concepts/changelogs/preconditions.html and https://javadocs.liquibase.com/liquibase-core/liquibase/precondition/CustomPrecondition.html

The interface has a void check(Database database) method but I'm not sure how I'm supposed to actually run any checks.

For example, if I wanted to run a sql query select max(len(user_name)) from users against the database and get an Integer result from this, how would I write this within my check method? I can't see anything about this in the docs

check method has Database object as an argument, which has getConnection() method which gets you the connection to the database.

From here you can make the needed DB query, return a result set and verify it.

E.g.:

@Override
public void check(Database database) throws CustomPreconditionFailedException,
        CustomPreconditionErrorException {
    try (Connection connection = ((JdbcConnection) database.getConnection()).getWrappedConnection();
         Statement statement = connection.createStatement();
         ResultSet resultSet = statement.executeQuery("select max(len(user_name)) from users")) {
        if (resultSet.next()) {
            int count = resultSet.getInt(1);
            // implement your check here
        } else {
            throw new CustomPreconditionFailedException("something went wrong");
        }
    } catch (Exception e) {
        throw new CustomPreconditionErrorException(e.getMessage());
    }
}