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:
  • Constructor Summary

    Constructors
    Constructor
    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
    Description
    Returns the JEL expression that provides this column's value.
    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 Details

    • 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 Details

    • 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