question: public static void concat ( Iterable < File > toMerge , File dst ) throws IOException { try ( FileChannel dstChannel = FileChannel . open ( dst . toPath ( ) , StandardOpenOption . CREATE , StandardOpenOption . TRUNCATE_EXISTING , StandardOpenOption . WRITE ) ) { for ( File file : toMerge ) { try ( RandomAccessFile raf = new RandomAccessFile ( file , "r" ) ) { FileChannel channel = raf . getChannel ( ) ; dstChannel . write ( channel . map ( FileChannel . MapMode . READ_ONLY , 0 , raf . length ( ) ) ) ; } } } }
target:  Concats the first files and writes them to the last file. 