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2.2 Column Metadata

Each column in a StarTable is assumed to contain the same sort of thing. More specifically, for each table column there is a ColumnInfo object associated with each column which holds metadata describing the values contained in that column (the value associated with that column for each row in the table). A ColumnInfo contains information about the name, units, UCD, class etc of a column, as well as a mechanism for storing additional ('auxiliary') user-defined metadata. It also provides methods for rendering the values in the column under various circumstances.

The class associated with a column, obtained from the getContentClass method, is of particular importance. Every object in the column described by that metadata should be an instance of the Class that getContentClass returns (or of one of its subtypes), or null. There is nothing in the tables infrastructure which can enforce this, but a table which doesn't follow this rule is considered broken, and application code is within its rights to behave unpredictably in this case. Such a broken table might result from a bug in the I/O handler used to obtain the table in the first place, or a badly formed table that it has read, or a bug in one of the wrapper classes upstream from the table instance being used. Because of the extensible nature of the infrastructure, such bugs are not necessarily STIL's fault.

Any (non-primitive) class can be used but most table I/O handlers can only cope with certain types of value - typically the primitive wrapper classes (numeric ones like Integer, Double and Boolean) and Strings, so these are the most important ones to deal with. The contents of a table cell must always (as far as the access methods are concerned) be an Object or null, so primitive values cannot be used directly. The general rule for primitive-like (numeric or boolean) values is that a scalar should be represented by the appropriate wrapper class (Integer, Float, Boolean etc) and an array by an array of primitives (int[], float[], boolean[] etc). Non-primitive-like objects (of which String is the most important example) should be represented by their own class (for scalars) or an array of their own class (for arrays). Note that it is not recommended to use multidimensional arrays (i.e. arrays of arrays like int[][]); a 1-dimensional Java array should be used, and information about the dimensionality should be stored in the ColumnInfo's shape attribute. Thus to store a 3x2 array of integers, a 6-element array of type int[] would be used, and the ColumnInfo's getShape method would return a two-element array (3,2).


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STIL - Starlink Tables Infrastructure Library
Starlink User Note252
STIL web page: http://www.starlink.ac.uk/stil/
Author email: m.b.taylor@bristol.ac.uk