java.nio.file.FileVisitor

public interface FileVisitor<T> {

    /**
     * Invoked for a directory before entries in the directory are visited.
     *
     * <p> If this method returns {@link FileVisitResult#CONTINUE CONTINUE},
     * then entries in the directory are visited. If this method returns {@link
     * FileVisitResult#SKIP_SUBTREE SKIP_SUBTREE} or {@link
     * FileVisitResult#SKIP_SIBLINGS SKIP_SIBLINGS} then entries in the
     * directory (and any descendants) will not be visited.
     *
     * @param   dir
     *          a reference to the directory
     * @param   attrs
     *          the directory's basic attributes
     *
     * @return  the visit result
     *
     * @throws  IOException
     *          if an I/O error occurs
     */
    FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs)
        throws IOException;

    /**
     * Invoked for a file in a directory.
     *
     * @param   file
     *          a reference to the file
     * @param   attrs
     *          the file's basic attributes
     *
     * @return  the visit result
     *
     * @throws  IOException
     *          if an I/O error occurs
     */
    FileVisitResult visitFile(T file, BasicFileAttributes attrs)
        throws IOException;

    /**
     * Invoked for a file that could not be visited. This method is invoked
     * if the file's attributes could not be read, the file is a directory
     * that could not be opened, and other reasons.
     *
     * @param   file
     *          a reference to the file
     * @param   exc
     *          the I/O exception that prevented the file from being visited
     *
     * @return  the visit result
     *
     * @throws  IOException
     *          if an I/O error occurs
     */
    FileVisitResult visitFileFailed(T file, IOException exc)
        throws IOException;

    /**
     * Invoked for a directory after entries in the directory, and all of their
     * descendants, have been visited. This method is also invoked when iteration
     * of the directory completes prematurely (by a {@link #visitFile visitFile}
     * method returning {@link FileVisitResult#SKIP_SIBLINGS SKIP_SIBLINGS},
     * or an I/O error when iterating over the directory).
     *
     * @param   dir
     *          a reference to the directory
     * @param   exc
     *          {@code null} if the iteration of the directory completes without
     *          an error; otherwise the I/O exception that caused the iteration
     *          of the directory to complete prematurely
     *
     * @return  the visit result
     *
     * @throws  IOException
     *          if an I/O error occurs
     */
    FileVisitResult postVisitDirectory(T dir, IOException exc)
        throws IOException;
}

java.nio.file.SimpleFileVisitor

public class SimpleFileVisitor<T> implements FileVisitor<T> {
    /**
     * Initializes a new instance of this class.
     */
    protected SimpleFileVisitor() {
    }

    /**
     * Invoked for a directory before entries in the directory are visited.
     *
     * <p> Unless overridden, this method returns {@link FileVisitResult#CONTINUE
     * CONTINUE}.
     */
    @Override
    public FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs)
        throws IOException
    {
        Objects.requireNonNull(dir);
        Objects.requireNonNull(attrs);
        return FileVisitResult.CONTINUE;
    }

    /**
     * Invoked for a file in a directory.
     *
     * <p> Unless overridden, this method returns {@link FileVisitResult#CONTINUE
     * CONTINUE}.
     */
    @Override
    public FileVisitResult visitFile(T file, BasicFileAttributes attrs)
        throws IOException
    {
        Objects.requireNonNull(file);
        Objects.requireNonNull(attrs);
        return FileVisitResult.CONTINUE;
    }

    /**
     * Invoked for a file that could not be visited.
     *
     * <p> Unless overridden, this method re-throws the I/O exception that prevented
     * the file from being visited.
     */
    @Override
    public FileVisitResult visitFileFailed(T file, IOException exc)
        throws IOException
    {
        Objects.requireNonNull(file);
        throw exc;
    }

    /**
     * Invoked for a directory after entries in the directory, and all of their
     * descendants, have been visited.
     *
     * <p> Unless overridden, this method returns {@link FileVisitResult#CONTINUE
     * CONTINUE} if the directory iteration completes without an I/O exception;
     * otherwise this method re-throws the I/O exception that caused the iteration
     * of the directory to terminate prematurely.
     */
    @Override
    public FileVisitResult postVisitDirectory(T dir, IOException exc)
        throws IOException
    {
        Objects.requireNonNull(dir);
        if (exc != null)
            throw exc;
        return FileVisitResult.CONTINUE;
    }
}