6. Java 7: File NIO2

Question 1

How would you write the following code to point to a file path using Java 7 new features?
File file = new File("readme.txt");

Answer 1

Path path = Paths.get(“readme.txt”);

Question 2

Given the following code, how can a File be converted to a Path object?
File file = new File("readme.txt");

Answer 2

file.toPath();

Question 3

Given the following code what will be printed?
Path path = Paths.get(“//home/user/jordi”);
System.out.println(path.toString());

Answer 3

/home/user/jordi

Paths.get or FileSystem.getDefault().getPath will preform minor syntatic cleanup on paths.

Question 4

Given the following file path how would one loop over the directories and print them?

/home/user/jordi/readme.txt

Answer 4

Path path = Paths.get(“/home/user/jordi/readme.txt”);
for(int i = 0; i < path.getNameCount; i++) {
System.out.println(path.getName(i));
}

Question 5

Given the following path how would you print the filename?

Path path = Paths.get(“//home/user/jordi/readme.txt”);

Answer 5

path.getFileName();

Question 6

What is the result of subPath?
Path path = Paths.get(“//home/user/jordi/readme.txt”);
Path subPath = path.subPath(0,2);

Answer 6

/home/user

Question 7

What is the result of root?
Path path = Paths.get(“./user/jordi”);
Path root = path.getRoot();

Answer 7

null

path.getRoot(); is only usefull when using absolute paths. For example if the path (on windows) would have been C:\users it would have returned C:\

Question 8

Given the following how would one replace jordi with bar?

Path path = Paths.get(“/user/jordi”);

Answer 8

path.resolveSibling(“bar”);

Question 9

Given the following code, what is printed?
Path path = Paths.get(“/user/jordi/../../foo”);
path.normalize();
System.out.println(path.getNameCount());
System.out.println(path.toString());

Answer 9

5
/foo

Normalize will remove redundant paths from the complete path. However the getNameCount(); will still return the orignal number of Name parts.

Question 10

What is the result of the following:
Path p1 = Paths.get("home");
Path p2 = Paths.get("home/users/jordi");
Path relative_1_2 = p1.relativize(p2);
Path relative_2_1 = p2.relativize(p1);

Answer 10

users/jordi

../..

Question 11

How would one move into a sub-directory using a path?

Answer 11

path.resolve(“subdir”);

Question 12

What is the Files class used for?

Answer 12

Class with static methods that uses Path objects to work with files and folders.

Question 13

What are the available options when using Files to do a copy operation?

Answer 13

LinkOption.NOFOLLOW_LINK
Indicates that if the file is symbolic link it should copy the link and not the file linked to.
StandardCopyOption.COPY_ATTRIBUTES
Copies the originals file attributes to the new file, the attributes are platform independent but the last-modified-time is supported accross platform.
StandardCopyOption.REPLACE_EXISTING
Performs the copy operation even if the target file already exists.If the target is a non-empty directory the copy fails with FileAlreadyExistsException.

Question 14

How to copy a file from /home/readme.txt to /home/readme2.txt using the FIles class? The last modified time should also be copied to the new file and existing files should be overwritten.

Answer 14

Path p1 = Paths.get(“/home/readme.txt”);
Path p2 = Paths.get(“/home/readme2.txt”);
Files.copy(p1, p2, StandardCopyOption.COPY_ATTRIBUTES,
StandardCopyOption.REPLACE_EXISTING
);

Question 15

What are the available options when using Files to do a move operation?

Answer 15

StandardCopyOption.REPLACE_EXISTING
Performs the copy operation even if the target file already exists.If the target is a non-empty directory the copy fails with FileAlreadyExistsException.
StandardCopyOption.ATOMIC_MOVE
It will move the file into a directory any process watching the directory will only access the file when it’s completly moved (not partially). Throws a exception when a ATOMIC_MOVE is not supported by the filesystem.

Question 16

How to move a file from /home/readme.txt to /readme2.txt using the Files class? It should replace any already existing files.

Answer 16

Path p1 = Paths.get("/home/readme.txt");
Path p2 = Paths.get("/readme2.txt");
Files.move(p1, p2,
StandardCopyOption.REPLACE_EXISTING
);

Question 17

What is the result of the following (given p1 and p2 are valid Path objects):
Files.move(p1, p2,
StandardCopyOption.COPY_ATTRIBUTES
);

Answer 17

throws UnsupportedOperaionException

COPY_ATTRIBUTES is not a valid option for move.

Question 18

How to use the Files class to read all bytes from a file names /home/readme.txt?

Answer 18

Path p1 = Paths.get(“/home/readme.txt”);

byte[] bytes = Files.readAllBytes(p1);

Question 19

Files class provides two methods for creating a BufferedReader and a BufferedWriter, name these two method signatures.

Answer 19

Files.createNewBufferedReader(Path path, Charset cs, OpenOption… options);
Files.createNewBufferedWriter(Path path, Charset cs, OpenOption… options);

Question 20

How to create an InputStream using the Files class for file /home/readme.txt.

Answer 20

Path p1 = Paths.get("/home/readme.txt");
Files.newInputStream(p1);

Question 21

The Files class methods sometimes take none or more OpenOptions. What are the StandardOpenOptions?

Answer 21

APPEND - When opened for WRITE, append new bytes to the end of the file.
CREATE - Create a new file if it doesn’t exist. Ignored if CREATE_NEW is also present
CREATE_NEW - Create a new file, fails if the file already exists.
DELETE_ON_CLOSE - When the JVM closes the file it will do a best effort to try and delete the file (not guaranteed).
DSYNC - Requires that every change of the data is synchronously written to the underlying storage device.
SYNC - Requires that every change of the date or metadat is synchronously written to the underlying storage device.
READ - Open for read access.
SPARSE - when used with CREATE_NEW then it will mark the file as a Sparse file (only possiible if the File system supports it).
TRUNCATE_EXISTING - if opened with WRITE access the file will be emptied on open.
WRITE - Open for write access.

Question 22

How would one open a directory called /home and loop ofer it’s contents that end with .java?

Answer 22

Path p1 = Paths.get("/home");
Files.newDirectoryStream(p1, *.java);

Question 23

Name the two methods for creating temp files using the Files class?

Answer 23

createTempFile(String prefix, String suffix, FileAttribute>… atrs);
- Create a temp file using the default directory. The prefix and suffix can be used to control the filename that is generated. For example if prefix is myapp_ and suffix is .myapp then the generated filename could be myapp_182391082930.myapp.
createTempFile(Path dir, String prefix, String suffix, FileAttribute>… atrs);
-Creates a temp file in the specified dir.

Question 24

Describe the interface to recursively access a directory tree?

Answer 24

public interface FileVisitor {
FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs) throws IOException;
FileVisitResult visitFile(T file, Basic, BasicFileAttributes attrs) throws IOException;
FileVisitResult visitFileFailed(T file, IOException e) throws IOException;
FileVisitResult postVisitDirectory(T dir, IOException e) throws IOException;
}

Question 25

Describe the FileVisitResult interface?

Answer 25

CONTINUE
- Continue. When returned from preVisitDirectory then the entries of the dir should also be visited.
TERMINATE
- Terminate the process.
SKIP_SUBTREE
- Continue without visiting the entries in this directoory. Only meaninful when returned from the preVisitDirectory method.
SKIP_SIBLINGS
- Continue without visiting the siblings of this file or directory.

Question 26

Use a SimpleFileVisitor to loop through the directories and files of the following directory tree and print the path and the size:

/home
/home/readme.txt
/home/jordi/
/home/jordi/readme2.txt

Answer 26

FileVisitor visitor = new SimpleFileVisitor() {
public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
   System.out.println(dir.toString() + "-" attrs.size());
   return FileVisitResult.CONTINUE;
}

public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
System.out.println(file.toString() + “-“ + attrs.size());
return FileVisitResult.CONTINUE;
}
}

Paths.walkFileTree(Paths.get(“/home”), visitor);

Question 27

How to create a PathMatcher using the default filesystem and based on glob pattern matching?

Answer 27

FileSystem fs = FileSystems.getDefault();

fs.getPathMatcher(“glob:” + pattern);

Question 28

Given a Path object how to use a PathMatcher to print if it matches against the glob pattern for either a java or class file?

Answer 28

FileSystem fs = FileSystems.getDefault();
PathMatcher matcher = fs.getPathMatcher(“glob:*.{java,class}”);
System.out.println(matcher.matches(Path.get(“/home”));

Question 29

Descirbe how to create a file watcher that watches the directory “/home/jordi” for create and modify events

Answer 29

WatchService watchService = FileSystems.getDefault().newWatchService();
Path path = Paths.get("/home/jordi");
path.register(watchService, StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_MODIFY);

Question 30

What are the events that can be listened to using a WatchService?

Answer 30

StandardWatchEventKinds.ENTRY_CREATE
StandardWatchEventKinds.ENTRY_MODIFY
StandardWatchEventKinds.ENTRY_DELETE
StandardWatchEventKinds.OVERFLOW

Question 31

What are the three WatchService methods to retrieve a WatchKey with the events that occured?

Answer 31

WatchService.poll(); returns and removes the next WatchKey that has had some its events occur or NULL if no events occurred.
WatchService.polll(long timeout, TimeUnit unit); If an event occurs during the specified time period this method exits and returning and removing the relevant WatchKey.
WatchService.take() similair to preceding methods but will wait until a watchkey is available.

Question 32

What are the states a WatchKey can ben in?

Answer 32

Ready - When first created.
Signalled - indicates that one or more events are queued.
Invalid - The key is no longer active. This happens when
* cancel() is called on the WatchService.
* The directory becomes inaccessible
* The watch service is closed.

Question 33

How to get the events from a WatchKey?

Answer 33

pollEvents();

returns a list of occured events.

Question 34

What is the result:
WatchKey key = watchService.take(); //waits until a key is available
System.out.println(key.isValid());
for (WatchEvent> watchEvent : key.pollEvents()) {
Kind> kind = watchEvent.kind();
}
System.out.println(key.isValid());

Answer 34

true true

A WatchKey does not become invalid until it is cancelled or the WatchService is closed.

Question 35

Which of the following statements are valid usages of StandardOpenOption options that determine how the file is opened?

1. new OpenOption[]{StandardOpenOption.WRITE, StandardOpenOption.DSYNC}
2. new OpenOption[]{StandardOpenOption.READ, StandardOpenOption.APPEND}
3. new OpenOption[]{StandardOpenOption.APPEND, StandardOpenOption.TRUNCATE_EXISTING}
4. new OpenOption[]{StandardOpenOption.APPEND, StandardOpenOption.SYNC}
5. new OpenOption[]{StandardOpenOption.READ, StandardOpenOption.SYNC}

Answer 35

1,4,5

Question 36

What is the result of the following?
Path p1 = Paths.get(“\personal\readme.txt”);
Path p2 = Paths.get(“\index.html”);
Path p3 = p1.relativize(p2);
System.out.println(p3);

Answer 36

....\index.html