Chapter 19: I/O

Question 1

What package can be used to interact with files and streams (I/O) in Java?

Answer 1

java.io

Question 2

What is the File class used for?

Answer 2

The File class is used to read information about existing files and directories, list the contents of a directory, and create/delete files and directories.

Question 3

What are two ways to retrieve the local separator character?

Answer 3

• System.getProperty(“file.separator”).

- java.io.File.separator.

Question 4

Given the following code, how can we check to see if the file exists?

File zooFile = new File(“data/stripes.txt”);

Answer 4

zooFile.exists();

Question 5

What constructors (and parameters) does the File class have?

Answer 5

• public File(String pathname)
• public File(File parent, String child)
• public File(String parent, String child)

Question 6

True or false? The following File instance points to a file.

var someFile = new File(“/data/zoo.txt”);

Answer 6

Maybe.

/data/zoo.txt looks like a file but could be a directory. Don’t assume it is either unless the question tells you it is.

Question 7

The java.io API defines two sets of stream classes for reading and writing streams. What are they and what are their differences?

Answer 7

• Byte streams
• Character streams

Byte streams read/write binary data and have class names that end with InputStream or OutputStream.

Character streams read/write text data and have class names that end in Reader or Writer.

Question 8

Why aren’t character streams strictly necessary to be used for io?

Answer 8

Since the byte stream classes can write all types of binary data, including strings.

Question 9

When working with text, which type of stream class should be used?

Answer 9

Character streams

Question 10

Most InputStream stream classes have a corresponding OutputStream class and most Reader classes have a corresponding Writer class.

example: FileInputStream and FileOutputStream, FileWriter and FileReader.

What classes are an exception to this rule?

Answer 10

• PrintWriter has no accompanying PrintReader class.

- PrintStream (is an OutputStream) has no accompanying form of InputStream

Question 11

What is the difference between a low-level and high-level stream?

Answer 11

A low-level stream connects directly with the source of the data while a high-level stream is built on top of another stream using wrapping.

Question 12

Why should you -usually- always wrap a file stream with a Buffered class?

Answer 12

Buffered classes read/write data in groups, rather than a single byte or character at a time. This drastically increases performance as I/O operations are very costly.

Question 13

What are the four abstract classes that are the parents of all stream classes in the java.io library?

Answer 13

• InputStream
• OutputStream
• Reader
• Writer

Question 14

Why does the following code not compile?

new BufferedInputStream(new FileReader(“x.txt”));

Answer 14

BufferedInputStream takes an InputStream, not a Reader.

Question 15

Why does the following code not compile?

new BufferedInputStream(new InputStream());

Answer 15

InputStream is abstract and cannot be instantiated

Question 16

What is the abstract stream base class for all input character streams?

Answer 16

• Reader

Question 17

What is the abstract stream base class for all output character streams?

Answer 17

Writer

Question 18

What is the abstract stream base class for all input byte streams?

Answer 18

InputStream

Question 19

What is the abstract stream base class for all output byte streams?

Answer 19

OutputStream

Question 20

What is the FileInputStream class used for? is it high/low level?

Answer 20

Reads file data as bytes

Low level

Question 21

What is the FileOutputStream class used for? is it high/low level?

Answer 21

Writes file data as bytes

Low level

Question 22

What is the FileReader class used for? is it high/low level?

Answer 22

Reads file data as characters

Low level

Question 23

What is the FileWriter class used for? is it high/low level?

Answer 23

Writes file data as characters

Low level

Question 24

What is the BufferedInputStream class used for? is it high/low level?

Answer 24

Reads byte data from an existing InputStream in a buffered manner, which improves efficiency and performance

High level

Question 25

What is the BufferedOutputStream class used for? is it high/low level?

Answer 25

Writes byte data to an existing OutputStream in a buffered manner, which improves efficiency and performance High level

Question 26

What is the BufferedReader class used for? is it high/low level?

Answer 26

Reads character data from an existing Reader in a buffered manner, which improves efficiency and performance High level

Question 27

What is the BufferedWriter class used for? is it high/low level?

Answer 27

Writes character data to an existing Writer in a buffered manner, which improves efficiency and performance High level

Question 28

What is the ObjectInputStream class used for? is it high/low level?

Answer 28

Deserializes primitive Java data types and graphs of Java objects from an existing InputStream High level

Question 29

What is the ObjectOutputStream class used for? is it high/low level?

Answer 29

Serializes primitive Java data types and graphs of Java objects to an existing OutputStream High level

Question 30

What is the PrintStream class used for? is it high/low level?

Answer 30

Writes formatted representations of Java objects to a binary stream High level

Question 31

What is the PrintWriter class used for? is it high/low level?

Answer 31

Writes formatted representations of Java objects to a character stream High level

Question 32

All I/O streams include a method to release any resources within the stream when it is no longer needed. What method is this?

Answer 32

close()

Question 33

True or False All I/O streams inplement the Closeable interface?

Answer 33

True

Question 34

When working with a wrapper stream, do we have to close the wrapped stream before closing the wrapper stream when we want to close resources?

Answer 34

No. It is only necessary to use close() on the topmost object.

Question 35

What methods exist to manipulate the order in which data is read from a stream?

Answer 35

- markSupported() - mark(int readLimit) - reset() throws IOException - skip(long n) throws IOException

Question 36

What do the mark(int readLimit) and reset() methods do? (input stream class methods)

Answer 36

the mark(int readLimit) and reset() methods return a stream to an earlier position.

Question 37

What method should be called before calling either the mark(int readLimit) or reset methods? (input stream class methods)

Answer 37

markSupported(), which returns true only if mark() is supported (not all input stream classes support mark() and reset()).

Question 38

What does the skip(long n) method do? (input stream class method)

Answer 38

The skip() method reads data from a stream and discards the contents.

Question 39

Given the following code, how can we manipulate the order in which data is read such that it prints LIOION instead of LION? ``` ... InputStream is... ... System.out.print((char) is.read()); //L System.out.print((char) is.read()); //I System.out.print((char) is.read()); //O System.out.print((char) is.read()); //N ```

Answer 39

``` ... InputStream is... ... System.out.print((char) is.read()); //L if(is.markSupported()) { is.mark(100); // call reset after at most 100 bytes System.out.print((char) is.read()); //I System.out.print((char) is.read()); //O is.reset(); } System.out.print((char) is.read()); //I System.out.print((char) is.read()); //O System.out.print((char) is.read()); //N ```

Question 40

Given the following code, how can we manipulate the order in which data is read such that it prints TRS instead of TIGERS? ``` ... InputStream is... ... System.out.print((char) is.read()); //T System.out.print((char) is.read()); //I System.out.print((char) is.read()); //G System.out.print((char) is.read()); //E System.out.print((char) is.read()); //R System.out.print((char) is.read()); //S ```

Answer 40

``` ... InputStream is... ... System.out.print((char) is.read()); //T is.skip(3); System.out.print((char) is.read()); //R System.out.print((char) is.read()); //S ```

Question 41

What does the flush() method do in Output Streams (OutputStream and Writer)?

Answer 41

The flush() method requests that all accumulated data be written immediately to disk, so that no data is lost when the program terminates unexpectedly.

Question 42

Why is it not needed to call the flush() method when you have finished writing data?

Answer 42

The close() method automatically calles the flush() method too.

Question 43

What are the rules for a Serializable class?

Answer 43

- The class must implement Serializable | - Every instance member of the class is serializable, marked transient, or has a null value at the time of serialization

Question 44

Is the following Cat class serializable? ``` public class Cat implements Serializable { private Tail tail = new Tail(); } ``` ``` public class Tail implements Serializable { private Fur fur = new Fur(); } ``` public class Fur { }

Answer 44

Cat and Tail both are marked Serializable, but Fur is not, making the Cat class not serializable.

Question 45

Is the following Cat class serializable? ``` public class Cat implements Serializable { private Tail tail = new Tail(); } ``` ``` public class Tail implements Serializable { private transient Fur fur = new Fur(); } ``` public class Fur { }

Answer 45

Cat and Tail both are marked Serializable, and Fur is transient, making the Cat class serializable.

Question 46

Is the following Cat class serializable? ``` public class Cat implements Serializable { private Tail tail = new Tail(); } ``` ``` public class Tail implements Serializable { private Fur fur = new Fur(); } ``` public class Fur implements Serializable { }

Answer 46

All classes are marked Serializable making the Car class serializable.

Question 47

Is the following Cat class serializable? ``` public class Cat implements Serializable { private Tail tail = new Tail(); } ``` ``` public class Tail implements Serializable { private Fur fur = null; } ``` public class Fur { }

Answer 47

Cat and Tail both are marked Serializable, and Fur is not. However, fur is assigned null. If it is null during serialization, the Cat class is serializable.