Java compact1, compact2, compact3 java.nio.file Interface DirectoryStream<T>（JDK8）

While DirectoryStream extends Iterable, it is not a general-purpose Iterable as it supports only a single Iterator; invoking the iterator method to obtain a second or subsequent iterator throws IllegalStateException.

An important property of the directory stream's Iterator is that its hasNext method is guaranteed to read-ahead by at least one element. If hasNext method returns true, and is followed by a call to the next method, it is guaranteed that the next method will not throw an exception due to an I/O error, or because the stream has been closed. The Iterator does not support the remove operation.

A DirectoryStream is opened upon creation and is closed by invoking the close method. Closing a directory stream releases any resources associated with the stream. Failure to close the stream may result in a resource leak. The try-with-resources statement provides a useful construct to ensure that the stream is closed:

   Path dir = ...
   try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir)) {
       for (Path entry: stream) {
           ...
       }
   }
 

Once a directory stream is closed, then further access to the directory, using the Iterator, behaves as if the end of stream has been reached. Due to read-ahead, the Iterator may return one or more elements after the directory stream has been closed. Once these buffered elements have been read, then subsequent calls to the hasNext method returns false, and subsequent calls to the next method will throw NoSuchElementException.

A directory stream is not required to be asynchronously closeable. If a thread is blocked on the directory stream's iterator reading from the directory, and another thread invokes the close method, then the second thread may block until the read operation is complete.

If an I/O error is encountered when accessing the directory then it causes the Iterator's hasNext or next methods to throw DirectoryIteratorException with the IOException as the cause. As stated above, the hasNext method is guaranteed to read-ahead by at least one element. This means that if hasNext method returns true, and is followed by a call to the next method, then it is guaranteed that the next method will not fail with a DirectoryIteratorException.

The elements returned by the iterator are in no specific order. Some file systems maintain special links to the directory itself and the directory's parent directory. Entries representing these links are not returned by the iterator.

The iterator is weakly consistent. It is thread safe but does not freeze the directory while iterating, so it may (or may not) reflect updates to the directory that occur after the DirectoryStream is created.

Usage Examples: Suppose we want a list of the source files in a directory. This example uses both the for-each and try-with-resources constructs.

   List<Path> listSourceFiles(Path dir) throws IOException {
       List<Path> result = new ArrayList<>();
       try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir, "*.{c,h,cpp,hpp,java}")) {
           for (Path entry: stream) {
               result.add(entry);
           }
       } catch (DirectoryIteratorException ex) {
           // I/O error encounted during the iteration, the cause is an IOException
           throw ex.getCause();
       }
       return result;
   }