Strings and IO

Question 1

How can ranges be extracted from a string?

Answer 1

using string.subString(start, end).

Note that the second parameter is the index of the character AFTER the last character.

Question 2

What is the result of “abcd”.subString(1, 3)?

Answer 2

“bc” - the second index is of the character AFTER

Question 3

How are strings compared?

Answer 3

Using string.compareTo(other).

This returns 0 if they are the same, negative value if the parameter precedes the string alphabetically, and a positive value if the parameter comes after the string in the alphabet.

Question 4

How are substrings found in a string?

Answer 4

With string.indexOf(substring)

This returns the index of the first occurrence. A second parameter can be used to start the search from a particular character index

Question 5

How can regular expressions be matched to a whole string?

Answer 5

use string.matches(pattern) to check the format of a whole string OR test if it contains a particular substring

Question 6

What is the basic regex syntax?

Answer 6

^ is the beginning of a string
[a-z] is a lowercase character from a to z
{3,4} isi the previous character pattern occurring 0 to 9 times
\d is the digit 0 to 9
$ is end of string

Question 7

How are backspaces used in Java strings?

Answer 7

They are escaped with a another backslash i.e. “\” = \

Question 8

How are groups represented in regex?

Answer 8

With brackets i.e. ([A-z][0-9]){3}

Question 9

What does a full stop represent in regex?

Answer 9

Any character.

Hence use . to match a literal period

Question 10

How is any digit matched?

Answer 10

with .

Question 11

How are 0 or 1 repetitions matched?

Answer 11

With ?

Question 12

How are 1 or more repetitions matched?

Answer 12

With +

Question 13

What does a ? represent?

Answer 13

The previous character / pattern may occur 0 or 1 time

Question 14

What does a + represent?

Answer 14

The previous character / pattern may occur one of more times

Question 15

What does a * represent?

Answer 15

The previous character / pattern may occur any number of times

Question 16

How are any number of repetitions matched?

Answer 16

With *

Question 17

What does [^a] match?

Answer 17

Anything but a

Question 18

How is whitespace / non-whitespace matched?

Answer 18

\s matches any whitespace character; \S matches anything that isn’t whitespace

Question 19

How is any letter matched?

Answer 19

[a-zA-Z]

Question 20

How is anything buy x, y, or z matched?

Answer 20

[^xyz]

Question 21

How can strings be replaced based on regex?

Answer 21

With string.replaceAll(pattern, substring)

Alternatively using .replaceFirst()

Question 22

How is regex implemented behind the scenes?

Answer 22

The pattern is compiled into a Pattern object.

Together with the string that is to be matched, this pattern is used to generate a Matcher object that does the actual matching

Question 23

How many bytes is a character?

Answer 23

Two

Question 24

How are file paths represented and what behaviours do they have?

Answer 24

They are represented with Path. To create one from a string use the Paths.get(string) static method.

Path objects implement .getFileName(), .isAbsolute() and .toAbsolutePath()

Question 25

How are files represent and what behaviour does this allow?

Answer 25

The Files class provides static methods to access information about objects. Note that path objects are used for the inputs. For example :

Files.exists(path)
Files.isDirectory(path)
Files.size(path)
Files.getLastModifiedTime(path)
Files.newDirectoryStream(path)

Question 26

What does a DirectoryStream path do?

Answer 26

Provide an iterable data structure of objects with the Path interface

Question 27

How are files written to?

Answer 27

Create a Formatter with the file path (string) as its parameter.

use .format(…) to write and then .close() to finish with it

Question 28

How are files read?

Answer 28

Create a scanner Scaner(path). Then use the scanner as usual.

Question 29

What is the process of converting an object to a series of bytes called?

Answer 29

Serialisation.

This is commonly used to save objects to files

Question 30

How can an object be made serialisable?

Answer 30

By implementing the Serializable interface. Extra work must be done only if the class contains instance variables that are no themselves serialisable.

Specifically, ObjectsStreamField[] is a static instance variable that identifies which fields should be serialised. Then the methods writeObject and readObject are implemented.

Question 31

How is a serialisable object written to a file?

Answer 31

By creating an ObjectOutputStream using Files.newOutputStream(path). Note that .bin is often used as the file extension.

Each object can then be written using output_stream.writeObject(). Finally, .close() is used to close it

Question 32

How is a serialisable object read from a file?

Answer 32

An input stream is created using Files.newInputStream(path).

The object is then read using (ClassType) input.readObject(). Note that the result must be case to the appropriate type.

Finally the stream is closed with .close()

Question 33

How is information streamed to and from disk?

Answer 33

Using a DataOutputStream and DataInputStream.

These have readByte(), readInt(), readDouble() and corresponding write methods.

Behind the scenes this is stored as a binary file but one that doesn’t store any Java objects