simple help about JFileChooser

here is a method i writte in java
public void aaa(){
        JFileChooser chooser=new JFileChooser();
           chooser.setFileFilter(new FileFilter() {//
setFileFilter(javax.swing.Filechooser.filefilter)
//in javax.swing.JFileChooser can't applied to (<anonymous
java.io.fileFilter>)
            public boolean accept(File f) {
                String fileName = f.getName().toLowerCase();
                return fileName.endsWith(".txt") || f.isDirectory();
            }
            public String getDescription() {
                return "Text Files";
            }
        });
        chooser.showOpenDialog(this);
    }

when used in some class it run well.but when it appear in some
class,it goes wrong .the IDE tell me that
setFileFilter(javax.swing.Filechooser.filefilter)
//in javax.swing.JFileChooser can't applied to (<anonymous
java.io.fileFilter>).
who can tell me what's that means. and why that happen