question: public static String verifyFiles ( File ... files ) { StringBuilder sb = new StringBuilder ( ) ; for ( File f : files ) { if ( f == null || ! f . exists ( ) ) { sb . append ( "File " + f + " does not exist.\n" ) ; } if ( f == null || ! f . canRead ( ) ) { sb . append ( "File " + f + " can not be read.\n" ) ; } } if ( sb . length ( ) == 0 ) return null ; else return sb . toString ( ) ; }
target:  Checks if a file can be exists, and if it can be read.