Eval arg0 = args[0]; if(arg0 instanceof ErrorEval) { return arg0; } if(!(arg0 instanceof AreaEval)) { return ErrorEval.VALUE_INVALID; } double temp = 0; AreaEval area = (AreaEval)arg0; ValueEval[] values = area.getValues(); for (int i = 0; i < values.length; i++) { ValueEval ve = values[i]; if(ve instanceof ErrorEval) { return ve; } if(!(ve instanceof NumericValueEval)) { return ErrorEval.VALUE_INVALID; } temp += ((NumericValueEval)ve).getNumberValue(); } // ... } In this example, if any error is encountered while processing the arguments, an error is returned immediately. This code is difficult to refactor due to all the points where errors are returned.
Using
EvaluationException allows the error returning code to be consolidated to one place.
public Eval evaluate(Eval[] args, int srcRow, short srcCol) { try { // ... AreaEval area = getAreaArg(args[0]); double temp = sumValues(area.getValues()); // ... } catch (EvaluationException e) { return e.getErrorEval(); } } private static AreaEval getAreaArg(Eval arg0) throws EvaluationException { if (arg0 instanceof ErrorEval) { throw new EvaluationException((ErrorEval) arg0); } if (arg0 instanceof AreaEval) { return (AreaEval) arg0; } throw EvaluationException.invalidValue(); } private double sumValues(ValueEval[] values) throws EvaluationException { double temp = 0; for (int i = 0; i < values.length; i++) { ValueEval ve = values[i]; if (ve instanceof ErrorEval) { throw new EvaluationException((ErrorEval) ve); } if (!(ve instanceof NumericValueEval)) { throw EvaluationException.invalidValue(); } temp += ((NumericValueEval) ve).getNumberValue(); } return temp; } It is not mandatory to use EvaluationException, doing so might give the following advantages:
- Methods can more easily be extracted, allowing for re-use.
- Type management (typecasting etc) is simpler because error conditions have been separated from intermediate calculation values.
- Fewer local variables are required. Local variables can have stronger types.
- It is easier to mimic common Excel error handling behaviour (exit upon encountering first error), because exceptions conveniently propagate up the call stack regardless of execution points or the number of levels of nested calls.
Note - Only standard evaluation errors are represented by
EvaluationException ( i.e. conditions expected to be encountered when evaluating arbitrary Excel formulas). Conditions that could never occur in an Excel spreadsheet should result in runtime exceptions. Care should be taken to not translate any POI internal error into an Excel evaluation error code.
@author Josh Micich