幕后发生了什么？

Question

I came across the following code and I need some clarification about this code.

http://www.vogella.com/articles/AndroidSQLite/article.html#contentprovider_overview

private void checkColumns(String[] projection) {
        String[] available = { TodoTable.COLUMN_CATEGORY,
                TodoTable.COLUMN_SUMMARY, TodoTable.COLUMN_DESCRIPTION,
                TodoTable.COLUMN_ID };
        if (projection != null) {
            HashSet<String> requestedColumns = new HashSet<String>(
                    Arrays.asList(projection));
            HashSet<String> availableColumns = new HashSet<String>(
                    Arrays.asList(available));
            // Check if all columns which are requested are available
            if (!availableColumns.containsAll(requestedColumns)) {
                throw new IllegalArgumentException(
                        "Unknown columns in projection");
            }
        }
    }

My question is: As hashset is used to compare the values what is going on behind the scenes? Is hashset storing the data of tables in it ? If yes then may I use this value in another program and also print out in consoles ?

I am not sure what is happening , please explain it.

thanks.

Answer 1

The purpose of the code you pasted in is only to check that the user did not give columns in the projection that don't exist in a table. There's no query made against the specified table, no data extracted. This is only a validity check.