Groups allow you to organize the records of a report with a better structure. A group is defined through an expression, so it can contain fields and variables, JasperRepors evaluates this expression and puts in the same group all the elements that have the same final result of the expression. When the result change then a new group is started. To create a new group go into the Outline view, right click on the root element (the report you have created) and select "Create Group", as shown in the image below:
Now the window to create a new group will appear:
From here, enter the group name and select your method for creating the new group:
- Create group from a report object: this is a simplified way to create a group without creating an expression in the expression editor. With this option, all the fields and variables defined in the selected report object are listed, and you can choose which to use in your group. Selecting an element from this list is equivalent to create a group where the expression include only that element.
- often the "create group from a report object" option will be sufficent to do what you want, but sometimes a more complex selection is needed. When you provide an expression for the group, JasperReport will evaluate it against each record. When the expression changes in value, a new group is created. Suppose you have the file that contains the amount of money you spent each day, and a report that get the data from this file, into the field Money_Spent. Then define a group with an expression Iike $F{Money_Spent}>150. This expression returns a boolean value, doing this the consecutive records with a value greater than 150 will be grouped toghether. If you have a variable containing that expression already, you can use the simpler method of creating a group from a report object, as described above.
In the previous example, we used the word "consecutive" for the grouped values. Suppose to spent $120 for two consecutive days, $200 the third day and $100 the fourth day. With these values three groups will be created: one with the first two elements, one with the third, and one with the fourth. This happens because the JasperReport:
- Reads the first record and evaluates the group expression: $120 is lesser than $150, so the result of the expression is "false". But since this is the first evaluation a new group is created.
- Reads the second value and evaluates the group expression: $120 is still lesser than $150 and the result is still false. Since it is the same value of the previous evaluation this value goes in the first group.
- Reads the third value and evaluates the group expression: $200 is greater than $150 and the result is true. It is different from the previous evaluation, so a new group for this value is created.
- Reads the fourth and last value and evaluates the group expression: $100 is not greater than $150 and the result is false. Even this time it is different from the previous evaluation so another group is created.
This should clarify how the evaluation of the expression affects the creation of a group. Returning to the creation of the group, after we have selected the way how the group is evaluated we can click "Next" or "Finish". If we click next the following window will appear:
There you can select if you want the header and the footer of the group in the report. The group will be evaluated even without this two bands, but without any of them there aren't visually changes between a group and another (but the group can still be used for other calculations, see for example the evaluation time, reset type and the other fields in the variable section that uses the group to do some calculations). This because the bands Header and Footer will be always created, and them are visible in the outline view, but if one of the checkbox is unchecked then the relative band will not be placed into the report (in the outline view a band not visible in the report has its name in lightgray color). If you click finish in the previous screen instead of Next both the bands of the group will be created.