question: @ 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 ; }
target:  Retrieve the names from all files in dir