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Form editor's trying to use declared superclass of the form makes problems to many users. As it did not work this way sooner, people had many forms extending classes that are not beans (cannot be instantiated). However the form still works (using a substitute type), error message is shown each time the form is opened, and many people have problems understand what it means (mainly when everything worked well in previous release). The type substitution should be probably done silently (although the error information should not be lost), maybe the substitute type should be also definable by the user.
It turns out this issue has impact also on performance and projects. For performance aspect, see issue 27183 (not obtaining form superclass would speed up opening for the first time). As for projects, if forms could be opened from filesystems (i.e. not from any project), the information about superclass would not be available - so the error would be displayed everytime. So raising priority...
Giving up the performance aspect, but otherwise fixed. If the declared form superclass cannot be resolved/used, the FormInfo type is used silently, just printing warning message to console and log. For the case described in issue 29700 (form code moved to innerclass), non-visual declared superclass is not used if there is FormInfo of visual form. Trying to improve performance by not asking for the superclass (so not requiring source parsing) has been found complicated and not reliable (we would have to save the superclass in the .form file, so upgrade the format version again, check it always on save, but still could be not in sync with source code if modified externally). /cvs/form/src/org/netbeans/modules/form/GandalfPersistenceManager.java revision 1.112
*** Issue 19185 has been marked as a duplicate of this issue. ***