JFileChooser File Filtering

Question

Why is the FileFilter class for the JFileChooser an abstract class?
How easy would it have been for Sun to do the following (see code below)?
It would certainly save on code bloat and make file handling programs just that little more compact, which is especially important when deploying applets or JWS applications.
Yes I know this code does not handle groups of file extentions. But thats not hard by using another constructor with an ArrayList or somthing.
* Sets up a file filter to be asociated with the JFileChooser.
public class FileFiltering extends FileFilter
  String description;
  String extention;
  //============================================================================
   * The constructor for the FileFiltering object.
   * @param extention The file extention to be allowed.
   * @param description A description of the filter.
  //============================================================================
  public FileFiltering(String extention, String description)
    this.extention = extention;
    this.description= description;
  }//===========================================================================
//=============================================================================
   * A required method of the parent class.
   * @param f The file to be filtered.
   * @return If the file is to be displayed return true. else return false.
//=============================================================================
  public boolean accept(File f)
      String name = f.getName();
      int i  =name.lastIndexOf(".");
      String extention =  name.substring(i);
      //if the file extention maches that of the class
      if (extention.equalsIgnoreCase(this.extention))
          return true;
  }//===========================================================================
  //============================================================================
   * Returns a description so that the fileChooser can display it in its
   * file type comboBox.
   * @return
  //============================================================================
  public String getDescription()
         return description;
  }//===========================================================================
}//end of class-===================================================================

Answer 1

Code for a FileFilter. accept() method that rejects only ".class" files where name contains a $ character
public boolean accept(File f) {
     if(f==null)
          return false;
     if(f.isDirectory())
           return true;
     String suffix=f.getName().toLowerCase();
     if(suffix.endsWith(".class") && indexOf('$')!=-1)
          return false; //Its a classfile and the name contains the $ char
     return true;