Fall 2002CS 150: Intro. to Computing1 Streams and File I/O (That is, Input/Output) OR How you read data from files and write data to files

Fall 2002 CS 150: Intro. to Computing 1

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

OR

How you read data from files and write data to files


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


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


Streams

Program

File,Monitor,Network,

Etc.

File,Keyboard,Network,

Etc.

output stream

input stream


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)


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. We’ll only work with text files


Text File I/O

• Best to just start with an example: we’ll add some file I/O to StudentRecord

Mustbe here!


Text File I/O (cont.)

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

Returns a referenceto an OutputStreamobject




What’s with these?!


Another Look…


What’s With This try…catch 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)


Subtle and Very Important…If written thisway, the variableoutputStream is local to the try block!


Still Another Look…Here it’svisible throughoutthe whole method

What’s with this?!


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.


Writing To a File (cont.)This is a variable name!I could have called it anylegal variable name.


Writing To a File (cont.)

Why don’t I needto use getName() here?


Still More Writing To a File



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 isn’t closed, Java closes it when the program ends, but you’re 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


A True Story…


Why Ever flush() a Stream?

• There are situations where you want the stream to remain open (you’re 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.


Miscellaneous

• File names: The file name you give Java is simply a String. It doesn’t know about suffixes and the like. That’s the OS’s thing.

• Opening a text file for appending:Indicates open for appending


Streams and File I/O Part II

Reading from Files


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



The FileReader class is a subclass of the Reader class,so using it in the constructoris “legal”



What’s withthis overridething?

Note the exceptionsthat can be thrown


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


Anything look strange here?

It should!



Example

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


Example (cont.)

Adding a simple cast to a char will fix this problem,but there are others (see next slide)



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!


Running the Example Code


Augmenting Our Code

• What the read documentation didn’t tell you is that the read() method returns the integer value 1 if there is no more file to read

• We’ll use this to modify our code to read (and print to standard output) the entire file


NOTE! (see next slide)


Notes From Last Slide• myChar is declared an int, since we’ll need

to check an integer value to see if we’re at the end of the file

• We use an infinite loop to keep iterating until we’ve 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)


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


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



More read() Documentation


Often the Best Way…


A problem


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)


A fix


Another fix


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()


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



This is needed in order to handle bothUnix and Windows style end of line conventions


A Tester For KeyboardIn

• Don’t forget that we need to use a try block with this

Documents

Fall 2002CS 150: Intro. to Computing1 Streams and File I/O (That is, Input/Output) OR How you read data from files and write data to files