如何获得某个目录下所有的文件名，包括子目录

public String search(File file)
 {
  String resultfile = "";    
  if (file.isFile() )
  {   
      return  resultfile+=file.toString()+"/n";
  }
  else if (file.isDirectory() )
  {  
   resultfile = resultfile +" "+file +"/n";
   String[] directory = file.list ();
   for (int i=0;i<directory.length ;i++)
   {
    File ff = new File (file.toString()+"//"+directory[i]);  
    resultfile+=search(ff);   
   }   
  } 
  return resultfile;
 }