First thing is to fix the generic parameters used by the SwingWorker. It should be SwingWorker< "Result Type", "Publish Type" >
"Result Type" is the class of the final result we want from running this task. "Publish Type" is the class encapsulating data we're going to send to GUI components.
In this case, the result we want is convertedFile; which is a File. ("Result Type"). The data we want to send to the UI (JTextArea) is counter, which is an int. ("Publish Type). NB: Java Generics can't use primitive types directly, instead must use their equivalent wrapper class.
class FileProcessor extends SwingWorker<File, Integer>
It keeps telling me INPUT isn't recognized in the FileProcessor class.
In the constructor of FileProcessor, you need to store the object reference to "INPUT" as an instance variable to be used later by "doInBackground()". See next answer
How would I get counter in FileProcessor to constantly update to outerTextArea in class X?
This can be done using "publish/process" methods of SwingWorker. First you also need to store an object reference to the JTextArea.
The complete constructor will look something like this: