7.1.3 Column Metadata

There is a one-to-one correspondence between a StarTable's ColumnInfo objects (accessed using getColumnInfo) and the FIELD elements contained in the corresponding TABLE. The attributes of each fields are interpreted (for reading) or determined (for writing) in a number of different ways:

• datatype and arraysize values depend on the class and shape of objects held in the column.
• name, unit, ucd and Utype values can be accessed using the corresponding methods on the ColumnInfo object (get/set Name(), UnitString(), UCD() and Utype() respectively).
• ID width, precision and type are held as String-type auxiliary metadata items in the ColumnInfo object, keyed by constants defined by the VOStarTable class (ID_INFO, WIDTH_INFO, PRECISION_INFO and TYPE_INFO respectively).
• LINK elements are represented by URL-type auxiliary metadata items in the ColumnInfo object, keyed by their title or, if it doesn't have one, ID attribute.
• Magic bad values for integer columns are represented by the Tables.NULL_VALUE_INFO auxiliary metadata item.
• Columns representing unsignedByte values are marked with a Boolean.TRUE value of their Tables.UBYTE_FLAG_INFO auxiliary metadata item, as well as having a java short integer data type (the java byte type is no good because it's signed).

So if you have read a VOTable and want to determine the name, ucd and ID attributes of the first column and its magic blank value, you can do it like this:

    StarTable table = readVOTable();
    ColumnInfo col0 = table.getColumnInfo(0);
    String name0 = col0.getName();
    String ucd0 = col0.getUCD();
    String id0 = (String) col0.getAuxDatumValue(VOStarTable.ID_INFO,
                                                String.class);
    Number blank0 = (Number) col0.getAuxDatumValue(Tables.NULL_VALUE_INFO,
                                                   Number.class);

And if you are preparing a table to be written as a VOTable and want to set the name, ucd and ID attributes of a certain column, fix it to use a particular magic null integer value, and have it contain an element <LINK title='docs' href='...'>" you can set its ColumnInfo up like this:

    ColumnInfo configureColumn(String name, String ucd, String id, Number blank,
                               URL docURL) {
        ColumnInfo info = new ColumnInfo(name);
        info.setUCD(ucd);
        info.setAuxDatum(new DescribedValue(VOStarTable.ID_INFO, id));
        info.setAuxDatum(new DescribedValue(Tables.NULL_VALUE_INFO, blank));
        info.setAuxDatum(new DescribedValue(new URLValueInfo("docs",null), docURL));
        return info;
    }