JAVA IO

Question 1

Some key points on the File System

Answer 1

1. A file is a record within the file system that stores data.
2. Files are organized into directories.
3. Directory is a record in the file system that contains files as well as dircetories

Question 2

What is a Path

Answer 2

String representation of a file/directory within the file system

Question 3

What is the package of a File Class?

Answer 3

java.io.File

Question 4

What is the File class used for?

Answer 4

1. Read information about existing files and directories
2. list the content of a directory
3. create/delete files and directories

Question 5

What does the instance of the File class represents?

Answer 5

Path name of a particular file/directory

Question 6

What can a File class cannot do?
* very important

Answer 6

Cannot read/write data within the File system.

Question 7

Can a File class used to represent directories?

Answer 7

Yes

Question 8

What is a relative path and what is absolute path?

Answer 8

1. Relative path is the path from the current working directory to file or directory
2. Absolute Path is the path of file/directory from root directory

Question 9

Which slashdoes Unix and windows use

Answer 9

Unix: /
Windows: \

Question 10

What are the two options to retrieve the local seperator character?

Answer 10

1. System property
   System.getProperty(“file.separator”);
2. Static variable
   java.io.File.separator

Question 11

Method to find whether the file exists or not?

Answer 11

file.exists()

Question 12

What are the two constructors generally used for creating a file Instance

Answer 12

1. One is taking String as argument representing relative/absolute path
2. two arguments, one which is existing file path and other relative child path.

File file = new File(“C:\Dilli.txt”);

File file2 = new File(file, “hello\Dilli.txt”);

Question 13

What is getName() does?

Answer 13

getName returns the name of the directory / file denoted by this path

Question 14

What is getAbsolutePath()?

Answer 14

returns the absolute pathname String of the file/directory

Question 15

lastModified()

Answer 15

returns the no of milliseconds since the epoch when the file was last modified

Question 16

delete()

Answer 16

deletes the file/directory

If the pathname denotes a directory, then the directory must be empty in order to be deleted

Question 17

mkdirs()

Answer 17

creates the directory named by this paths including any non-existent parent directories

Question 18

listFiles()

Answer 18

returns the File[] Array returning the files on the directory

Question 19

How can the contents of the file accessed/written?

Answer 19

Streams, which is a list of data elements presented sequentially

Question 20

How to conceptually remember a Stream?

Answer 20

Think Stream as a long, never ending Stream of water with data presented one “wave” at a time

Question 21

What are the three input streams java provides?

Answer 21

System.in, System.err, System.out

Question 22

Difference b/w Streams and Reader/Writer classes?

Answer 22

1. Stream classes are used for inputting and outputting all types of binary or byte data
2. Reader/Writer classes are used for inputting and outputting only character and String data

Question 23

Are Readers/Writers are streams?

Answer 23

Yes, even though their names doesnt contain Streams in it, they are streams

Question 24

Why we use character Streams?

Answer 24

1. There are advantages of using reader/writer clsses as they are specifically focussed on managing character and string data
2. WE DONT WANT TO WORRY ABOUT THE UNDERLYING BYTE ENCODING OF THE FILE

Question 25

How can we differentiate streams as levels?

Answer 25

Low-Level Stream and High-Level Streams.

Question 26

What is Low-level Streams?

Answer 26

Low level Streams connects directly with the source of the data such as files, arrays or Strings. They process the raw data or resources and are accessed in a direct or unfiltered manner.

Question 27

What are high level Sreams?

Answer 27

High level Streams is built on anoter Stream using wrapping. (An instance is passed as constructor to another instance)

Question 28

What are the four abstract classes that are parent for all streams?

Answer 28

1. InputStream 2. OutputStream 3. Reader 4. Writer

Question 29

Which is the parent class of PrintStream?

Answer 29

OutputStream

Question 30

``` new BufferedInputStream(new FileReader("C:\\hello.txt")); new BufferedWriter(new FileOutputStream("C:\\hello.txt")); new ObjectInputStream(new FileOutputStream("C:\\hello.txt")); new BufferedInputStream(new InputStream("hello.txt")); ``` will these comile?

Answer 30

No. First two mix Reader/Writer classes with InputStream/OutputStream classes. Third does not compile bcause it mixes with Input Stream with Output Steam 4th doesnt compile because, InputStream is a abstract class, no constructor is allowed

Question 31

What happens if we do not close() a file operation?

Answer 31

1. Failing to close may leave the OS lock the file, so that no other processes could read/write until program is terminated.

Question 32

Why do we need to use flush() methods?

Answer 32

When the data is written to the OutputStream, the underlying operations system doesnt necessarily guarantee that the data will be written to File system. Sometimes, the data will be cached and written after sometime. INI the meantime, if application termintates then the data is lost

Question 33

What are the two methods InputStream and Reader classes have?

Answer 33

mark(int) and reset to move the stream back to earlier position

Question 34

Which method should we call before calling the MARK / reset method? Why?

Answer 34

``` We should call markSupported() method. Because, not all Stream classes support these operations and calling mark(int) or reset on a class that does not support these operations will throw runtime Exceptions. ``` So, markSupported() returns true only if mark is supported.

Question 35

method to skip over some data?

Answer 35

skip(long)

Question 36

What are FileInputStream and FileOutPutStream?

Answer 36

They are used to read bytes from files / write bytes to the files

Question 37

What data can the constructor can take as argument for FileInputStream/FileOutputStream?

Answer 37

They take file objects or String, representing the path to the file

Question 38

What does the read method in FileInputStream method returns?

Answer 38

It returns int. So that when the EOF is reached, it returns '-1', from which we can predict it is the end of file.

Question 39

What does the overloaded read() method do?

Answer 39

1. It takes a pointer to a byte array where the data is written. iT returns the integer representing how many data can be read into byte array

Question 40

which method does FileOutputStream takes to write to a file and its overloaded version?

Answer 40

1. write(int) method - writes successive bytes to file | 2. Overloaded write methodmethod, that allow byte array to be passed and can be used by biffered class

Question 41

What are PrintStream and printWriter classes?

Answer 41

1. High level stream classes | 2. Writes formatted representation of java objects to a text-based output stream

Question 42

Which instances does the PrintStream and PrintWriter classes operates?

Answer 42

1. PrintStream classes operates on OutputStream instances and writes data as bytes 2. PrintWriter operates on Writer instances and writes data as characters

Question 43

What is special about PrintStream and PrintWriter?

Answer 43

1. They can open and write to files directly. PrintWriter class can even take an OutPutStream as input, allowing us to wrap the PrintWriter class

Question 44

Which object does the System.out and System.err classes belongs to?

Answer 44

PrintStream classes

Question 45

Does PrintBased methods throw any exceptions? Cool way to remember this?

Answer 45

1. They do not throw any exceptions 2. Whenever, we used System.out.println() we have not checked for any exceptions, which indicates that they dont throw any exceptions

Question 46

What is the method used to detect any error in Print based methods?

Answer 46

checkError(), used to detect te presence of a problem after attempting to write data to stream

Question 47

print() method is overloaded with what?

Answer 47

with all java primitives and String and Objects. It also internally calls String.valueOf() method of arguments passed

Question 48

Key point to remember for PrintStream on methods used in PrintWriter

Answer 48

We can use all te methods used in PrintWriter with printStream

Question 49

Whar are all the arguments that teh format method takes?

Answer 49

1. It takes a String, an Optional locale and set of arguments

Question 50

What are the two methods that could be used interchangebly in java formatting Strings?

Answer 50

printf() and format() methods

Question 51

Which is the recommended methods for interacting with the user and displaying information in the test-based environment?

Answer 51

java.io.console

Question 52

Which pattern does the Console class follows?

Answer 52

Console follows SingletonPattern. It is created automatically by JVM and accessd by calling System.console()

Question 53

What are the methods provided by the Console for the reading and writing operations?

Answer 53

reader() and writer() to access an istance of Reader and PrintWriter

Question 54

How does the format() method differs from the Stream format() method?

Answer 54

It does not take an argument of locale variable

Question 55

How does the basic readLine mthod works?

Answer 55

1. It works by retreiving the single line of text from the user and the user presses the Ener button to terminate it

Question 56

What about the overloaded readLine() method?

Answer 56

signature is: readLine(String format, Object... args), which displays the formatted prompt to the user

Question 57

What is different in readPassword() method when compared to readLine() method?

Answer 57

1. Echoing is disabled in readPassword() | 2. readPassword() returns an arrayof characters instead of String

Question 58

How can we wipe an arrays data?

Answer 58

Arrays.fill(array_variable, 'x');

Question 59

What happens if we call reset() after we have pases the mark position?

Answer 59

An exception may be thrown at run time. may- because some implementations may use buffer to allow extra data to be read