Streams and File I/O (That is, Input/Output)

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Streams and File I/O (That is,
Input/Output)

• OR
• How you read data from files and write data to
  files

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Streams

• Abstractly, a stream is a flow of data
• Data could be characters, numbers, bytes
  consisting of binary digits, bytes consisting of
  binary encoding of objects, etc.
• If the data flows out of your program (and,
  say, to a file or the monitor) then the stream is
  an output stream
• If the data flows in to your program, then the
  stream is an input stream

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Streams

• In Java, file I/O (and also simple
  keyboard/monitor I/O) is handled by streams
• In Java, a stream is an object that either
  delivers data to its destination (such as a file
  or monitor) or takes data from a source (such as
  a file or keyboard) and delivers it to your
  program
• System.out is an example of an output stream

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Streams
File, Monitor, Network, Etc.
output stream
Program
File, Keyboard, Network, Etc.
input stream
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Remember

• An input stream moves data into your program (not
  into a file)
• An output stream moves data out of your program
  (not out of the file)

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Binary vs Text Files

• All data in any file is stored as a sequence of
  bits. But, sometimes we think of the file as
  consisting of a sequence of characters (for
  example, your Java source code files), and some
  we think of as simply containing a sequence of
  binary digits (such as a file containing the
  machine code for a program)
• The files of characters are called text files
• The files of bits are called binary files
• Java has objects to handle I/O to both kinds of
  files. Well only work with text files

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Text File I/O

• Best to just start with an example well add
  some file I/O to StudentRecord

Must be here!
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Text File I/O (cont.)

• We use the println() method in the class
  PrintWriter (not System.out.println(), but acts
  the same)

Returns a reference to an OutputStream object
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Whats with these?!
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Another Look
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Whats With This trycatch Thing?!

• This is an example of exception handling in Java
  (which we may cover more completely at a later
  date)
• For now, know that this says execute the
  statements in the try block. If something goes
  wrong, then stop and execute the statements in
  the catch block
• Lots can go wrong with file I/O (e.g. the file
  may not exist, or you may not have permission to
  access it)

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Subtle and Very Important
If written this way, the variable outputStream
is local to the try block!
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Still Another Look
Here its visible throughout the whole method
Whats with this?!
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So, How Do We Use These Things?!

• Just like you use System.out.println(), though
  there are also methods specially designed to
  print doubles, chars, etc.

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Writing To a File (cont.)
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Writing To a File (cont.)
Why dont I need to use getName() here?
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Still More Writing To a File
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close() the Stream!

• Calling the close() method
• Flushes the stream
• Operating system releases all resources needed to
  connect the stream to the file, and performs
  other housekeeping
• If stream isnt closed, Java closes it when the
  program ends, but youre taking a chance
• If program ends abnormally, then Java may not be
  able to automatically close the stream and you
  could lose data
• You need to close a stream before reading from
  the same file

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A True Story
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Why Ever flush() a Stream?

• There are situations where you want the stream to
  remain open (youre still using it), but you need
  to be sure data goes to output device
• Writing to a network interface
• During long operations on a file, flush stream in
  case there is some kind of abnormality
• OS crash, etc.

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Miscellaneous

• File names The file name you give Java is simply
  a String. It doesnt know about suffixes and the
  like. Thats the OSs thing.
• Opening a text file for appending

Indicates open for appending
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Streams and File I/O Part II

• Reading from Files

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BufferedReader Class

• The BufferedReader class is the input stream
  equivalent of the PrintWriter class
• The constructor requires a similar setup
• The class methods are analogous
• Remember to import java.io.
• Remember this is for reading text files

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The FileReader class is a subclass of the Reader
class, so using it in the constructor is legal
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Whats with this override thing?
Note the exceptions that can be thrown
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Reading From Files

• As with writing to files, you need to know
  where you are in the file when reading
• Unlike with writing, you need to know when you
  have run out of stuff to read in the file
• When reading, you generally need to have a place
  (i.e. a variable) to put the data you have read
• Beware there are many subtleties here

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Example

• Some new file-reading code added to the code that
  created the file profsGrades.txt

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Example (cont.)
Adding a simple cast to a char will fix this
problem, but there are others (see next slide)
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Note the Changes

• The entirety of the input code is in the try
  block (including closing the stream), not just
  the memory allocation for the stream
• There are two different classes of exceptions
  that are caught here FileNotFoundException and
  IOException
• This is how you should write your I/O code!

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Running the Example Code
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Augmenting Our Code

• What the read documentation didnt tell you is
  that the read() method returns the integer value
  ?1 if there is no more file to read
• Well use this to modify our code to read (and
  print to standard output) the entire file

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NOTE! (see next slide)
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Notes From Last Slide

• myChar is declared an int, since well need to
  check an integer value to see if were at the end
  of the file
• We use an infinite loop to keep iterating until
  weve read the entire file
• We call the read() method within the conditional
  for the if statement. Regardless of whether the
  condition is true, the call to read() occurs
  (this is a standard hack for this kind of code)

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More Notes

• We use the print() method as opposed to the
  println() method when a newline character is
  encountered, this will cause a new line in the
  output
• We perform the cast to char inside the
  System.out.print () statement

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There Are Easier Ways

• Use another version of the read() method to read
  characters into a char array
• Use the readline() method, which reads a whole
  line at a time into a String

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More read() Documentation
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Often the Best Way
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A problem
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The Problem

• The readLine() method returns the value null if
  it reaches the end of file. When this happens,
  the variable currentLine is assigned the value
  null, which causes a NullPointerException when we
  try to call its equals() method in the if
  statement. See the fix on either of the next two
  slides (the first is the recommended fix)

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A fix
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Another fix
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Reading Numeric Data Types

• Note that using any of the BufferedReader methods
  results in reading either a String or a char. To
  read a double or int or other numerical data
  type, you need to
• Use methods like stringToInt()
• Use methods like Double.parseDouble(String input)
• The method Integer.parseInt(String input) is the
  exact equivalent of stringToInt()

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Finally, Reading Input From the Keyboard

• The keyboard is an input device like any other,
  and we can read it with an input stream
• We use the read() method in System.in
• This works like our other read() method, but
  reads from the keyboard
• We use the above read() method to create a static
  readLine() method that we can call to read from
  the keyboard

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This is needed in order to handle both Unix and
Windows style end of line conventions
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A Tester For KeyboardIn

• Dont forget that we need to use a try block with
  this