Fall 2002CS 150: Intro. to Computing1 Streams and File I/O (That is, Input/Output) OR How you read...
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Transcript of 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
Fall 2002 CS 150: Intro. to Computing 2
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
Fall 2002 CS 150: Intro. to Computing 3
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
Fall 2002 CS 150: Intro. to Computing 4
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)
Fall 2002 CS 150: Intro. to Computing 6
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!
Fall 2002 CS 150: Intro. to Computing 8
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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Fall 2002 CS 150: Intro. to Computing 10
Fall 2002 CS 150: Intro. to Computing 11
What’s with these?!
Fall 2002 CS 150: Intro. to Computing 12
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.
Fall 2002 CS 150: Intro. to Computing 17
Writing To a File (cont.)This is a variable name!I could have called it anylegal variable name.
Fall 2002 CS 150: Intro. to Computing 18
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
Fall 2002 CS 150: Intro. to Computing 22
A True Story…
Fall 2002 CS 150: Intro. to Computing 23
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.
Fall 2002 CS 150: Intro. to Computing 24
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
Fall 2002 CS 150: Intro. to Computing 25
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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Fall 2002 CS 150: Intro. to Computing 30
What’s withthis overridething?
Note the exceptionsthat can be thrown
Fall 2002 CS 150: Intro. to Computing 31
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
Fall 2002 CS 150: Intro. to Computing 32
Anything look strange here?
It should!
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Fall 2002 CS 150: Intro. to Computing 34
Example
• Some new file-reading code added to the code that created the file profsGrades.txt
Fall 2002 CS 150: Intro. to Computing 35
Example (cont.)
Adding a simple cast to a char will fix this problem,but there are others (see next slide)
Fall 2002 CS 150: Intro. to Computing 36
Fall 2002 CS 150: Intro. to Computing 37
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!
Fall 2002 CS 150: Intro. to Computing 38
Running the Example Code
Fall 2002 CS 150: Intro. to Computing 39
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
Fall 2002 CS 150: Intro. to Computing 40
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)
Fall 2002 CS 150: Intro. to Computing 42
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
Fall 2002 CS 150: Intro. to Computing 43
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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Fall 2002 CS 150: Intro. to Computing 45
More read() Documentation
Fall 2002 CS 150: Intro. to Computing 46
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()
Fall 2002 CS 150: Intro. to Computing 52
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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Fall 2002 CS 150: Intro. to Computing 54
This is needed in order to handle bothUnix and Windows style end of line conventions
Fall 2002 CS 150: Intro. to Computing 55
A Tester For KeyboardIn
• Don’t forget that we need to use a try block with this