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Class de.tudarmstadt.ito.xmldbms.Column

java.lang.Object
   |
   +----de.tudarmstadt.ito.xmldbms.Column

public class Column

extends Object

Column contains information about a column in a table. The table is not named; instead, Columns are included in Tables and ColumnMaps.

Note that column values are stored in three separate objects in XML-DBMS:

• Result sets: These are JDBC ResultSet objects either passed to DBMSToDOM or generated by DBMSToDOM when transferring data from the database to XML.
• Row object: This is an intermediate object used by both DBMSToDOM and DOMToDBMS to buffer column values.
• INSERT statement: These are JDBC PreparedStatement objects created by DOMToDBMS when transferring data from XML to the database.

The position of a column value can be different in each of these three objects. For example, it could be in column 5 in the result set, array index 3 in the Row object, and parameter 4 in the INSERT statement. Thus, three sets of column numbers are needed:

• Result set column number: This is the number of the column in the result set in which the value is stored. The number is 1-based and is stored in the Table.rsColumnNumbers array. (Note that the order in which result set column numbers are stored in this array guarantees that columns in the result set will be accessed in increasing order.)
• Row object column number: This is the index of the position in the Row.columnValues array in which the value is stored. The index is 0-based and is stored in the rowObjectIndex variable of Column. It is also the index of the Table.rsColumnNumbers array; that is, column values are stored in the Row object in the same order they occur in the result set. In XML-DBMS version 1.0, this was named Column.number.
• INSERT statement parameter number: This is the number of the parameter in the INSERT statement in which the column value is stored. The number is 1-based and corresponds to the order in which the Column object is stored in the Table.columns array; it is therefore not stored separately.

In virtually all cases, these three column numbers are the same, except for differences due to being 0- or 1-based. This is because XML-DBMS generates SELECT and INSERT statements itself and therefore places the columns in the same order in each. The only time that they differ is when the calling application passes a result set to DBMSToDOM.retrieveDocument. In this case, the number and order of columns in the result set can differ from the order of the columns as they appear in the Table.columns array. Because of this, the three different numbers are needed.

In the future, the Column class should contain information about the nullability and data type of the column as well. This will be used when creating CREATE TABLE statements.

Version:

1.01

Author:

Ronald Bourret, Technical University of Darmstadt

length

Length of the column.

name

Name of the column.

rowObjectIndex

Index of the column value in the Rows.columnValues array.

type

Data type of the column.

Column()

Column(String, int, int, int)

 public String name

Name of the column.

 public int rowObjectIndex

Index of the column value in the Rows.columnValues array. 0-based.

 public int type

Data type of the column. This must be one of the values in java.sql.Types.

 public int length

Length of the column. Used only for character and binary data.

 public Column()

 public Column(String name,
               int rowObjectIndex,
               int type,
               int length)

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