The current state of specification for
https://download.java.net/java/early_access/jdk17/docs/api/java.desktop/javax/swing/JFormattedTextField.AbstractFormatter.html#getDocumentFilter()
https://download.java.net/java/early_access/jdk17/docs/api/java.desktop/javax/swing/JFormattedTextField.AbstractFormatter.html#getNavigationFilter()
is the following
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protected DocumentFilter getDocumentFilter()
Subclass and override if you wish to provide a DocumentFilter to restrict what can be input. install will install the returned value onto the JFormattedTextField.
Returns:
    DocumentFilter to restrict edits

protected NavigationFilter getNavigationFilter()
Subclass and override if you wish to provide a filter to restrict where the user can navigate to. install will install the returned value onto the JFormattedTextField.

Returns:
    NavigationFilter to restrict navigation
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Actually as per OpenJDK - null is returned by both "default" implementations and it means "no filtering", as other places in the code indicate