IO

Question 1

File class

Answer 1

used to read information about existing les and directories, list the contents of a directory, and create/delete les and directories.

Question 2

Two ways to retrieve the file separator character for the current file system

Answer 2

System.getProperty(“file.separator”)

java.io.File.separator

Question 3

What does the following code do?

File file = new File("/home/smith/data/zoo.txt");
System.out.println(file.exists());

Answer 3

Checks if the file exists within the file system.

Question 4

How to create a new File object

Answer 4

File file = new File(String path);

Question 5

Difference between the “Stream” named classes and the “Reader/Writer” classes

Answer 5

The stream classes are used for inputting and outputting all types of binary or byte data, whereas the reader/writer classes can only work with characters and strings

Question 6

What does it mean when a class has the word “Buffered” in it?

Answer 6

The class reads and writes data in groups rather than a single byte or character at a time.

Unless you are doing something very specialized in your application, you should always wrap a file stream with a Buffered class in practice.

You create these instances by wrapping the corresponding stream type, which means, for example, that if you had a BufferedWriter, you would create it with: 
 new BufferedWriter(new FileWriter(filename))

Question 7

Where should you open your file streams?

Answer 7

In the opening of a try-with-resources statement so that it will close at the end. Otherwise you must call .close() on it when you are done

Question 8

flush()

Answer 8

When called on a stream, requests that all accumulated data be written immediately to disk.

Useful for if the application terminates unexpectedly

Question 9

What should you do before calling the mark(int) method on a stream?

Answer 9

Call markSupported() on the stream to make sure it supports mark(int). Otherwise an exception will be thrown if it doesn’t support it.

Question 10

mark(int num)

Answer 10

Sets a marker at the current point in the stream and allows you to read up to num amount of bytes with the ability to call reset() if you want to return to the original marked point

Question 11

skip(long num)

Answer 11

Skips over the next num bytes, or if there are less than num bytes left then skips to the end.

Returns the number of bytes skipped.

Question 12

How do you know when a FileInputStream object has reached the end of the file?

Answer 12

It returns -1

Question 13

What exception is thrown if the file you try to read from doesn’t exist?

Answer 13

FileNotFoundException

Question 14

copy(File file1, File file2)

Answer 14

Copies the contents of file1 to file2, overwriting any content that file2 may already have

Question 15

write(byte[],int,int)

Answer 15

Used with BufferedOutputStream, stores the bytes in the byte array until filled with the third arguments value of bytes. The second argument is an offset to start writing which is usually set to 0

Question 16

serialization

Answer 16

The process of converting an in-memory object to a stored data format

Question 17

Serializable interface

Answer 17

The interface that classes must implement if you are going to serialize objects of that type

Any class can implement the Serializable interface since there are no required methods to implement.

Question 18

NotSerializableException

Answer 18

Thrown when attempting to serialize an object when its class or an object contained in its class is not marked as Serializable

Question 19

Which members of a class will be ignored when serializing?

Answer 19

Those marked static or transient

Question 20

How to create an ObjectInputStream

Answer 20

ObjectInputStream in = new ObjectInputStream (new BufferedInputStream( new FileInputStream(filename)));

Question 21

Method to deserialize an object given

ObjectInputStream in = new ObjectInputStream (new BufferedInputStream( new FileInputStream(filename)));

Answer 21

Object object = in.readObject();

Note: you need to do an explicit cast if you want it to be something other than Object

Question 22

Method to serialize an object given

ObjectOutputStream out = new ObjectOutputStream(
new BufferedOutputStream(new FileOutputStream(filename)))

Answer 22

out.writeObject(objectName);

Question 23

How do we know when an ObjectInputStream has reached the end of the input file?

Answer 23

An EOFException is thrown.

Question 24

What do ObjectInputStream and ObjectOutputStream do with null values?

Answer 24

Accept them!

Question 25

What is ignored during the deserialization (writing) process?

Answer 25

The constructor and initialization blocks

Question 26

PrintStream

Answer 26

Operates on OutputStream objects and writes data as bytes

Question 27

PrintWriter

Answer 27

Operates on Writer objects and writes data as bytes

Question 28

What type of object are System.out and System.err?

Answer 28

PrintStream

Question 29

PrintWriter methods

print()

println()

format()

printf()

Answer 29

prints the argument by calling String.valueOf on it

Like print() but with a new line at the end

Takes a string and prints it based on the formatting arguments you give it

Same thing as format(), just there to make it easier on C programmers

Question 30

How to obtain the console object

Answer 30

Console console = System.console()

Note: will return null if the environment doesn’t support text interactions.

Question 31

Console methods reader() and writer()

Answer 31

Provide access to an instance of Reader and PrintWriter, respectively

Question 32

Console method format()

Answer 32

Lets you just pass it a string to print so that you don’t need to use the reader() or writer() method then print()

Question 33

Console method readLine(prompt)

Answer 33

Displays the String prompt to the user and accepts input terminated by the Enter key

Question 34

Console method readPassword()

Answer 34

functions the same as the readLine method but does not show the text being typed.

Also returns an array of chars instead of a String to avoid having it be stored in the String pool