Default value for null fields in a Jasper Report

Solution #1

Your solution:

1. Use a regular Double field (doubleField) for the column value.
2. Add a static String text field at the same location.
3. Change the Double field to Blank When Null.
4. Set the PrintWhenExpression value for the String text field to: $F{doubleField} == null.

Solution #2

The problem is, as you pointed out, that a Double and a String are two different data types. You can assign a String variable to the value of the Double using an appropriate expression. Then use the String variable as the field. The expression might resemble:

($F{doubleField} == null) ?
  "N/A" : new java.text.DecimalFormat("#.##").format($F{doubleField})

(Note: My preference is to use == instead of !=. Think positive.)

Solution #3

Change the SQL statement to pre-format the Double as a text string, and use the "N/A" in the string (by using a CASE or DECODE statement in the query).

Avoid this solution, though, as it is not maintainable.

Recommendation

Do not hard-code the "N/A" string throughout the report(s); put the "N/A" text in a constant, or a parameter with a default value of "N/A".