java递归所有文件

import java.io.File;
public class Filep {
    public static File f = new File("E:\\javatest");
    public static File ff;
    public static String findfile(File f){
        if(f.list()==null) return null;
        String[] str = f.list();
        for(String ss: str)
        {    
            System.out.println(f.getAbsolutePath()+"\\"+ss);
            ff = new File(f.getAbsolutePath()+"\\"+ss);
            findfile(ff);
        }        
        return null;
    }
  public static void main(String argv[])
  {
    findfile(f);
  } 
}