5. I/O & NIO

Question 1

What is the File class?

Answer 1

the file class isn’t used to actually read or write data; its used to work at a higher level,
making new empty files, searching for files, deleting files, making directories en working with
paths.

Question 2

What is the FileReader class?

Answer 2

this class is used to read character files. Its read() methods allowing you to read
single characters, the whole stream of characters or a fixed number of characters.
FileReaders are wrapped by higher-level objects (BufferedReaders)

Question 3

What is the BufferedReader class?

Answer 3

this class is used to make lower-level Reader classes (FileReader) more
efficiënt and easier to use. BufferedReaders read relatively large chunks of data from a file at
once and keep this data in a buffer.

Question 4

What is the FileWriter class?

Answer 4

this class is used to write to character files. Its write() allow you to write
characters or strings to a file. These are wrapped by higher-level writer objects
(BufferedWriters or PrintWriters)

Question 5

What is the BufferedWriter class?

Answer 5

this class is used to make lower-level Writer classes (FileWriter) more
efficiënt and easier to use. It writes relatively large chunks of data to a file at once.

Question 6

What is the PrintWriter class?

Answer 6

because of newly created methods and contructors you might find that you can
use PrintWriter in places where you previously needed a wirter to be wrapped. New
methods make printwriter quite flexible and powerful

Question 7

What is the FileInputStream class?

Answer 7

this class is used to read bytes from files and can be used for binary as well
as tekst. We use FileInputStream with higher-level objects (ObjectInputStream)

Question 8

What is the FileOutputStream class?

Answer 8

this class used to write bytes to files. We use FileOutputStream with higher-
level objects (ObjectOutputStream)

Question 9

What is the ObjectInputStream class?

Answer 9

this class is used to read an input stream and deserialize objects. Its
used with FileOutputStream so that you can read objects rather than characters or bytes.
This proces is called deserialisation

Question 10

What is the ObjectOutputStream class?

Answer 10

this class is used to write objects to an ouput stream and is used with
classes like FileOutputStream to write to a file (serialization)

Question 11

What is the Console class?

Answer 11

this class provides methods to read input from the console and write formatted
output to the console.

Question 12

What are Objects of type File representing?

Answer 12

Objects of type File are used to represent the actual files(but not the data in the files) or directories that exist on a computers physical disk.

Question 13

Does new File(); creates a file?

Answer 13

No

Question 14

What does boolean createNewFile(); do?

Answer 14

Returns true if the named file does not exist and was successfully created; returns cals if the named file already exist

Question 15

What does boolean exists(); do?

Answer 15

The exists() look if the named file already exists

Question 16

What does the code FileWriter fw = new FileWriter(file); do?

Answer 16

does two things:

• It created a FileWriter reference variable (fw), and a FileWriter object, then assigned to fw
• It created an actual empty file out on the disk

Question 17

What happens when you write data out to a stream?

Answer 17

When you write data out to a stream, some amount of buffering will occur, and you never know for
sure exactly when the last data will actually be sent.

Question 18

What does the flush() method do?

Answer 18

invoking the flush() method guarantees that the last of the data you thought you had already written actually gets out to the file.

Question 19

What does the close() method do?

Answer 19

Whenever you’re done using a file you should invoke the close() method. This method has following
results:
- If the stream is open, it closes the stream releasing the resources
- If the stream is already closed, it will have no effect.
- If any read or other similar operation is performed on the stream, after closing, it raises
IOException

Question 20

What are 2 ways to create a file?

Answer 20

• Invoke the createNewFile() method on a File object.
• Create a Writer or a Stream. Specifically, create a FileWriter, Printwriter or a
  FileOutputStream. Whenever you create an instance of one of these classes, you
  automatically create a file, unless one already exists.

Question 21

How to create a directory?

Answer 21

Creating a directory is similar to creating a file. As with creating a file, creating a directory is a 2-step
process; first we create a Directory (File) object, then we create an actual directory using the mkdir()
method.

Question 22

What do you need to keep in mind with directories?

Answer 22

Be careful when you’re creating new directories! As we have seen, constructing a Writer of a Stream
will often create a file for you automatically if one doesn’t exist, but that is not true for a directory. If
you omit to call mkdir() method no directory will created

Question 23

What does readLine() returns?

Answer 23

When there is no more data to read, readLine() returns a null – this our signal to stop reading the file.
When reading a file no flushing (flush()) is requires.

Question 24

What are the rules for the delete and renaming methods in File?

Answer 24

The File class lets you do things like renaming and deleting files. Most common ins and outs of
deleting files and directories (via delete()) and renaming files and directories (via renameTo()). Here
are some rules for it:
- delete(): you cannot delete a directory if it is not empty
- renameTo(): You must give the existing File object a valid new File object with the new name
that you want (if newName had been null, we would have gotten an NullPointerException())
- renameTo(): its okay to rename a directory, even it isn’t empty

Question 25

What does the File.list() method do?

Answer 25

This method returns a string array of files and
directories. We then use the enhanced fo loop to iterate through and print. If you leave the path
section empy -> NullPointerException()

Question 26

When do you have a Console Object?

Answer 26

If you are running Java from the command line, you will have access to a console object, to which you
can get a reference by invoking System.console() (this method returns – if found – a valid console
reference, if you don’t run it from the command line or it can’t find a console -> null)

Question 27

what does the readLine() method do?

Answer 27

returns a string containing whatever the user keyed in.

Question 28

what does the readPassword() method do?

Answer 28

returns a character array. (not a String cause once you have got the password, you can verify it and then absolutely remove it from memory. If a string was
returned, it could exist in a pool somewhere in memory, and a hacker could find it

Question 29

What is the Path interface?

Answer 29

this interface is one of the key classes of file-based I/O under NIO.2 and replaces File as the representation of a file or a directory when working in NIO.2. It is a lot more powerful than a File

Question 30

What is the Paths class?

Answer 30

this class contains static methods that create Path objects

Question 31

What is the Files class?

Answer 31

this class contains static methods that work with Path objects. You will find basic operations in here like copying or deleting files.

Question 32

Does the file automatically be created when creating a new Path?

Answer 32

When you create a Path to a new file, that file does not exist until you actually create the file using
Files.createFile(Path target).

Question 33

What are 2 different paths?

Answer 33

• Absolute path: since they begin with the root (c: = windows, / = linux).
• Relative path: when you do not begin with the root, which means java looks from the current
  directory.

Question 34

What is the file://c:/ws. syntax?

Answer 34

If you are on windows you might deal with a URL that looks like file://c:/ws. The file:// is a protocol
(such as http). This syntax allows you to browse to a folder in internet explorer. But you can only use
these syntax this way: convert the String to a URI, before trying to create a Path:

Path p = Paths.get(URI.create(file:///C:/ws));

Question 35

What are the copy() and move() methods?

Answer 35

If you try to overwrite a file that already exists, java doesn’t want us to lose the file, so it “asks” if we
are sure by throwing an exception. Copy() and move() actually take an optional third parameter –
CopyOptions (vb. StandardCopyOption.REPLACE_EXISTING – you tell your jvm, I know what i’m
doing).

Question 36

What happens if you try to delete a file that don’t exist?

Answer 36

If you try to delete a file that don’t exist, you get a NoSuchFileException. There is an alternative,
Files.deleteIfExists(path) returns true and deletes the file only if it exists. If not, it just quietly returns
false.

Question 37

What are the important methods in the File class to know?

Answer 37

Method Description
- Path copy(Path source, Path target, CopyOption…
option)

Copy the file from source to target and return
target

• Path move(Path source, Path target, CopyOption…
  options)

Move the file from source target and return target

• void delete(Path path)

Delete the file and throw an exception if it does
not exist

• boolean deleteIfExists(Path path)

Delete the file if it exists and return whether file
was deleted

• Boolean exists(Path path, LinkOption… options)

Return true if file exists

• Boolean notExists(Path path, LinkOption…
  options)

Return true if file doesn’t exists

Question 38

What are the important methods in the Path class to know?

Answer 38

• String getFileName()
  Return the filename or the last element of the sequence of name elements
• Path getName(int index)
  Return the path element corresponding to the
  specified index. The 0th element is the one closest
  to the root (C:\ or /) (Throws
  illegalArgumentException if its out of bound).
• int getNameCount()
  Returns the number of elements in this path, excluding the root
• Path getParent()
  Returns the parent path, or null if this path does not have a parent
• Path getRoot()
  Return the root of this path, or null if this path does not have a root
• Path subPath(int beginIndex, int endIndex)
  Returns a subsequence of this path (not including
  the root element) as specified by the beginning
  (included) and ending (not included) indexes.
  (Throws illegalArgumentException if its out of
  bound).
• String toString()
  Returns the string representation of this path

Question 39

What are the NIO.2 attributes?

Answer 39

• BasicFileAttributes: You can rely on these attributes being available to you unless you are
  writing java code for some new funky operating system. Attributes common to many file
  systems
• PosixFileAttributes: This interface is implemented by both UNIX- and Linux-based operating
  systems. (ezelsbruggetje: Posix eindigd met x net als UNIX en Linux)
• DosFileAttributes: This interface is part of all Windows operating systems.

Question 40

Which methods do the BasicFileAttributes interface provide?

Answer 40

• DateTime creationTime()
• DateTime lastAccessTime()
• DateTime lastModifiedTime()
• Boolean isDirectory()

The key takeaway here are that the FileAttributes classes are read only and the FileAttributeView
classes allow updates.

Question 41

What is the directoryStream interface?

Answer 41

The directoryStream interface lets you iterate through a directory. You can filter data when iterating
through a directory with DirectoryStream as follows:
DirectoryStream stream = Files.newDirectoryStream(dir, “[vw]*”);
[vw] means either of the character v or w. The * is a wildcard that means zero or more of any
character. There is one limitation with DirectoryStream. It can only look at one directory
(ezelsbruggetje: werkt net als dir in de cmd, en directoryStream directory is enkelvoud).

Question 42

What does the FileVisitor class do?

Answer 42

class that does, in fact, look at subdirectories. Java provides a SimpleFileVisitor class. You extend it and override the method visitFile(). Then you can call
Files.walkFileTree(Path path, FileVisitor visitor), which knows how to recursively look through a
directory structure and call methods on a visitor subclass. See blz 289 for a code example.

There are 2 parameters to visitFile()

• Path file: the Path object representing the current file.
• BasicFileAttributes: it lets you find out if the current file is a directory, and many other similar
  pieces of data

Finally visitFile() returns FileVisitResult.CONTINUE this tells walkFileTree() that it should keep looking
through the directory structure for more files.

Question 43

What are the 4 different FileVisitResult constant?

Answer 43

• CONTINUE: Continue
• SKIP_SUBTREE: Continue without visiting the entries in this directory
• TERMINATE: Terminate, hij stopt gwn met walktreeën net als system.exit doet ook geen post
  dingen meer
• SKIP_SLIBBINGS: combination of SKIP_SUBTREE and “don’t look in any folders at the same
  level”

Question 44

Gouden regel FileVisitor

Answer 44

File visitor can only skip subtrees that it has not encountered yet.

Question 45

What does serialisation do?

Answer 45

When you serialize an object, java serialization takes care of saving that object’s entire “object
graph”. That means everything the saved object needs to be restored (also the object that the
reference refers to). But these reference classes must also implement the Serializable interface.

Question 46

What does the transient keyword do?

Answer 46

You can also mark the instance reference variable with transient, then serialization will simply skip
the reference class during serialization. That means when the class is serialized, the reference
variable value will be null.

Question 47

Can a subclass be serializable? when the super class is not?

Answer 47

What happens if a superclass is not marked Serializable, but the subclass is? This works, but there are implications. The instance variables from the subclass will be serialized an deserialized correctly, but
the inherited variables from the non-serializable superclass will come back with their default values.
This is because the non-serializable class constructor will run.