uk.ac.starlink.topcat

Class SyntheticColumn

java.lang.Object

uk.ac.starlink.table.ColumnData

uk.ac.starlink.topcat.SyntheticColumn

public class SyntheticColumn
extends uk.ac.starlink.table.ColumnData

A column which produces read-only values based on an algebraic expression. This expression may include other columns in the table referenced by column name, and can use any of the java language expression constructs as well as the static methods in the java.lang.Math class. This list could be extended quite easily if new arithmetic tricks were required.

The engine used for expression evaluation is the GNU Java Expressions Library (JEL).

Author:

Mark Taylor (Starlink)

See Also:

JEL

Constructor Summary

Constructors

Constructor and Description
SyntheticColumn(TopcatModel tcModel, uk.ac.starlink.table.ColumnInfo cinfo, String expression, Class<?> resultType) Constructs a new synthetic column from an algebraic expression applied to a table.

Method Summary

Modifier and Type	Method and Description
String	getExpression() Returns the JEL expression that provides this column's value.
Object	readValue(long lrow)
void	setExpression(String expression, Class<?> resultType) Sets the algebraic expression which this column uses to calculate its results.

Methods inherited from class uk.ac.starlink.table.ColumnData

getColumnInfo, isWritable, setColumnInfo, storeValue

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

SyntheticColumn

public SyntheticColumn(TopcatModel tcModel,
                       uk.ac.starlink.table.ColumnInfo cinfo,
                       String expression,
                       Class<?> resultType)
                throws gnu.jel.CompilationException

Constructs a new synthetic column from an algebraic expression applied to a table.

Parameters:

tcModel - context for JEL expression evaluation

cinfo - metadata for the new column; note this object may be modified as required by the supplied expression

expression - algebraic expression for the value of this column

resultType - a Class for the result, presumably one of the primitive wrapper types or String.class. If null a suitable class is chosen automatically.

Throws:

gnu.jel.CompilationException

Method Detail

setExpression

public void setExpression(String expression,
                          Class<?> resultType)
                   throws gnu.jel.CompilationException

Sets the algebraic expression which this column uses to calculate its results. Column names are used to refer to values of cells in other columns of this table in the same row as that for which the result is being generated.

Parameters:

expression - the string giving the algebraic exprssion for this column's value in terms of other columns

resultType - a Class for the result, presumably one of the primitive wrapper types or String.class. If null a suitable class is chosen automatically.

Throws:

gnu.jel.CompilationException

getExpression

public String getExpression()

Returns the JEL expression that provides this column's value.

Returns:

expression

readValue

public Object readValue(long lrow)
                 throws IOException

Specified by:

readValue in class uk.ac.starlink.table.ColumnData

Throws:

IOException