HOW-TO Create variables and define specific behavior

If there is a need to decouple how a variable is calculated, and how the result must be shown, then a combination of CustomExpresion and DJValueFormatter may be used.

Suppose we have a column that must show an amount of time (a duration of something, like a phone call) in a format like 14′ 23″, and the field for that column is an Integer that represents the duration in seconds. Besides that, we want to have some groups, and display totals in its footer. They must also be shown as mm’ ss”

First, when creating the column, two different custom expression are passed, one defines how data is shown in the column (setCustomExpression(…)), the other how data must be passed to report engine, so it makes calculations (setCustomExpressionForCalculation(…))

...
AbstractColumn columnaCustomExpression = ColumnBuilder.getNew()
.setCustomExpression(getCustomExpression())
.setCustomExpressionForCalculation(getCustomExpression2())
.setTitle("Duration").setWidth(new Integer(90))
.setStyle(amountStyle).setHeaderStyle(headerStyle).build();
...

Also, add a group with a variable in its footer to show to total amount of time (the sum). Pay attention to the getValueFormatter() call

...

drb.addGlobalFooterVariable(columnaCustomExpression, DJCalculation.SUM,amountStyle, getValueFormatter());

...

And now the definition of the CustomExpression and DJValueFormatter calls.

private DJValueFormatter getValueFormatter() {
    return new DJValueFormatter(){
        public Object evaluate(Object value, Map fields, Map variables, Map parameters) {
                return "Total time: " + getAsMinutes((Long) value);
        }

        public String getClassName() {
                return String.class.getName();
        }
    };
}

private CustomExpression getCustomExpression() {
    return new CustomExpression() {
        public Object evaluate(Map fields, Map variables, Map parameters) {
            Long amount = (Long) fields.get("quantity");
            return getAsMinutes((Long) amount);
        }

        public String getClassName() {
            return String.class.getName();
        }
    };
}

private CustomExpression getCustomExpression2() {
    return new CustomExpression() {
        public Object evaluate(Map fields, Map variables, Map parameters) {
            return fields.get("quantity");
        }
        
        public String getClassName() {
                return Long.class.getName();
        }
    };
}

public static String getAsMinutes(Long value) {
    Long amount = (Long) value;
    int sec = amount.intValue() % 60;
    int mins = amount.intValue() / 60;
    return mins + "' " + sec + """;
}

Refer to VariableValueFormatterReportTest for a working example.