Main classes in the JFreeReport class library.

Some rules when working with the report definition objects:

- Functions cannot be added to the report once the report generation has started.
- Do not modify the report structure after the report generation is started, unless you know exactly what you are doing.
- It is safe to modify the report definition before the report is started.
- Once the report processing has started, the report from the report processor must not be reused for other purposes.

## Groups

A group is a set of database rows, where the contents of the group fields are equal for all rows.

Lets look at the following table:

Car | Color | Price
--- | --- | ---
Toyota | blue | 10.000
Toyota | blue | 20.000
Toyota | red | 15.000
Lexus | blue | 25.000
Lexus | yellow | 15.000
BMW | yellow | 40.000
BMW | blue | 35.000
BMW | green | 5.000

Groups are always defined on sorted tables. A group is defined by selecting one or more rows, which should form the group. A group instance will contain all rows from the beginning of the group until one of the group fields changes.

Lets select the row car for our group:

Selecting that row as group argument will result in 3 group instances. The first group instance will contain the first 3 rows (as the contents of these rows are equal).

The next instance will contain all rows of the "Lexus" car and the last group contains all rows from the "BMW" rows.

If a group is defined by more than one field, then the group will end, if one of the group member fields changes.

Lets define a group which consists of "Car" and "Color".

The first instance of the group will contain the first 2 rows (Toyota,blue). Then the value in the column "Color" changes, so a new group instance is started. This new group contains a single row, as the column "Car" in the next row will contain a different value.

The next two group instances are "Lexus, blue" and "Lexus, yellow", both group instances contain 1 row, as the color changes after 1 row.

The next row starts a new group, as the column "Car" contains a different value (BWM instead of Lexus). It doesn't matter, that the color is still equal, to end a group instance, it is sufficient that one column value changed.