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

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

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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

Program

File,Monitor,Network,

Etc.

File,Keyboard,Network,

Etc.

output stream

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. We’ll only work with text files

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Text File I/O

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

Mustbe 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 referenceto an OutputStreamobject

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What’s with these?!

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Another Look…

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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)

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

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Still Another Look…Here it’svisible throughoutthe whole method

What’s 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.)This is a variable name!I could have called it anylegal variable name.

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Writing To a File (cont.)

Why don’t I needto 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 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

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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 (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.

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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

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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 constructoris “legal”

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What’s withthis overridething?

Note the exceptionsthat 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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Anything look strange here?

It should!

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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 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

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NOTE! (see next slide)

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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)

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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 bothUnix and Windows style end of line conventions

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A Tester For KeyboardIn

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