@Override public List<String> allKeys(){
  List<String> nameFiles=new ArrayList<>();
  File[] files=cacheDirectory.listFiles();
  if (files == null)   return nameFiles;
  for (  File file : files) {
    if (file.isFile()) {
      nameFiles.add(file.getName());
    }
  }
  return nameFiles;
}

@ Override public List < String > allKeys ( ) { List < String > nameFiles = new ArrayList < > ( ) ; File [ ] files = cacheDirectory . listFiles ( ) ; if ( files == null ) return nameFiles ; for ( File file : files ) { if ( file . isFile ( ) ) { nameFiles . add ( file . getName ( ) ) ; } } return nameFiles ; }
retrieve the names from all files in dir
