File IO (Input Output)

What do streams do?

Transfer data to and from a program

What is an input stream?

Data flowing into the program (reading from a file)

What is an output stream?

Data flowing out of the program (writing to a file)

What are the two types of streams?

Text streams (Characters). Byte streams

Features of Text streams (2)

• Handles 16-bit unicode characters

• Used for .txt/.csv and source code

What classes are used on text streams? (2)

Reader and Writer

What classes are used on byte streams? (2)

InputStream and OutputStream

Features of Byte streams (2)

• Handles raw 8-bit binary data

• Used for images/audio and compiled files

What does delimited mean?

Using a character such as space/comma (CSV) or tab to mark the end of a field

What does fixed-width mean?

When every field takes up exactly 4 characters regardless of its value

What are the text packing methods for Text streams (2)

Delimited. Fixed-width

Do byte streams use delimiters?

No

What do byte streams use to separate fields?

Size and order

Both … and … must agree on number of …

Reader. Writer. Bits

What is the “drift” problem?

If input stream reads a smaller value (32) where output stream wrote larger (64)

What does the “drift” problem cause?

Creates error and entire stream becomes garbage

Can java give an error when working with files?

No

Name 2 classes that can handle text files

OutputFile. InputFile

What is the OutputFile Class?

Provides methods to write basic java types to a text file

What is the InputFile Class?

Provides methods to read primitives and Strings back from a text file

What is a path?

Location of a file

What is an absolute path?

Full specific address e.g. C:\Users\Cinema\theater.txt

Negative of absolute files

If the project is moved the path breaks

What is a relative path?

Address relative to the Project Root. e.g. “data/logs.txt” or “theater.txt“

Benefit of relative path

Portability

What is portability

Project is portable without changes

Example of an output file object creation

OutputFIle out = new OutputFile(“bpm.txt“)

What is “out”? (Example of outputfile object creation)

Name of object

What is “new”? (Example of outputfile object creation)

Opens file

What does out.println do? (OutputFile)

Writes value to file

What does out.close do? (OutputFile)

Saves and closes file

What does .eof() return?

Boolean as to whether you are at the end of a file

What should you check before reading from an input file?

If .eof() is false then read

What does it mean when .eof() is false?

There’s still data

What does it mean when .eof() is true?

When end of file is reached

Explain the hierarchal structure of classes

Wrapped inside one another to add functionality

What does FileWriter/FileReader do?

Connects directly to file on disk

What does BufferedWriter/BufferedReader do?

Adds a memory buffer for efficiency

What does PrintWriter do?

Adds high-level methods like println() and printf()

Explain wrapping

Wrap a low-level FileWriter inside a high-level PrintWriter to get disk connectivity + easy formatting

Is Disk access slower or RAM access?

Disk access

Explain the benefit of buffering

Writing 1 byte at a time to hardrive is slow. Fill buffer in RAM then drive to disk

Can you write primitives directly?

Yes using println() PrintWriter class

Benefit of using try with resources (classes in try-catch bracket)

Automatically closes and causes chain-reaction which closes all nested classes

What does .exists() do?

Checks to see if file exists

What does .canRead() do?

Checks to see if file is readable

What does .getAbsolutePath() do?

Gets exact path of file

What does .length() do?

Metadata- finds length of file

What is metadata?

Data about a file e.g. size/name/last modified

What can be used instead of BufferedReader?

Scanner

Benefit of using Scanner (2)

Extract primitives. Allows to check data type before it is read (type-safe reading)

When to use BufferedReader instead of Scanner?

Bulk text

When to use Scanner instead of BufferedReader?

Handles mixed data

Features of text files (3)

• Readable (Unicode)

• Variable width (1 takes 1 byte. 1000 takes 4)

• Sequential

Features of binary files (3)

• Machine-readable

• Fixed width (int always 4 bytes)

• Random access

File types of binary (2)

.dat and .bin

What is sequential access?

Data read in strict order from start to finish

What is random access?

Data accessed via a pointer that jumps to any address

Formula to find byte address

(record index) x (fixed record size)

How is File IO handled in a project

By one class (high cohesion + low coupling)

What is the GIGO Principle?

“Garbage In Garbage Out.” If a corrupt file is loaded the entire code’s logic will fail