Download File I/O

File I/O
Chapter 10
Chapter Contents
Chapter Objectives
10.1 Introductory Example: Weather Data Analysis
10.2 Java/s I/O System: Readers, Writers, Streams
10.3 Exceptions
10.4 More About I/O Streams
10.5 Example: Scanning for a Virus
10.6 Example: Student Data Retrieval
10.7 Graphical/Internet Java: Student Data Retrieval GUI
Part of the Picture: Data Base Systems
Chapter Objectives




Study Java's I/O with text and binary files
Examine role of exceptions in I/O
Observe use of files in GUI programs in
context of an information retrieval problem
Learn about the role of files in database
management systems
Need for File Access


Files saved for word processors, spreadsheet
programs, etc.
Saved on secondary memory
–

hard disks, floppy disks, CD-ROMs, etc.
Requirements information stored in secondary
memory
–
–
can be retrieved in the future
kept separate from other documents, programs, etc.
Intro Example: Weather Data
Analysis

Problem: large amounts of weather-related
data is recorded and processed
–
–
–


year's worth of atmospheric pressure readings
taken every 15 minutes
stored in a text file pressure.dat
Weatherman needs minimum, maximum, and
average of the data
Data must be read from file, statistics
calculated, results written to a text output file
Solution Overview



Note source code, Figure 10.1
Names of input and output files are given to the
program via the args parameter
– input file is args[0]
– output file is args[1]
Command to open the input file
BufferedReader in = new BufferedReader (
new FileReader (args[0]));

Reading text from input file
valueString = in.readLine();
Input file is a static
object with
readLine() method
Solution Overview

Opening text output file
PrintWriter out = new PrintWriter (
new BufferedWriter(
new Filewriter (args[1])));

Writing text values to a file
out.println ( . . . );

Closing the files
in.close();
out.close();
Handling Exceptions

Many things can go wrong when dealing with
files
–
–
–

requested input file does not exist
input file has invalid data
output file unavailable
When such an error occurs, the method in
which the problem happened "throws an
exception"
–
–
Java can "catch" the exception if it happens in a "try
block"
Note the try { … } block in the source code
Handling Exceptions

A try block is followed by one or more "catch
blocks"
–

Note the source code:
catch (Exception anException)
{ Controller.fatal ( … ) ; }
This catch block will
–
–
–
determine the kind of exception
report the problem
terminate the program
10.2 Java's I/O System:
Readers, Writers, and Streams



All input and output in Java is accomplished by
classes called streams
Input streams provide ways to move bytes of
data from an input device to a program
Output streams move the data in the opposite
direction
Output
Stream
Input
Stream
Executing Program
Predefined Streams

System.in
–

System.out
–

InputStream object, usually for the keyboard
a buffered PrintStream object, usually the
screen or an active window
System.err
–
an unbuffered PrintStream object usually
associated with the screen or console window
Wrapping Classes

InputStream class provides methods for
reading bytes only
–

to read at a higher level we must "wrap"
System.in with another class
Example: BufferedReader class
BufferedReader myReader =
new (BufferedReader (
new InputStreamReader( System.in));

Now we can send myReader either
–
–
the .read() message for a single char value or …
the .readLine() message for an entire line of text
Using Readers and Writers



Reader and Writer classes provide support
for char (16-bit Unicode) I/O
InputStream and OutputStream provide
support for byte I/O
The Java language is still growing and
"maturing"
–
–
extensions of Reader and Writer classes have yet to
be fully accepted
author of this text suggests: Use Reader or Writer
class whenever possible, use Stream class only
when there is no Reader/Writer class that fits
Using a Reader

Wrap FileReader in BufferedReader
class
BufferedReader inFile =
new BufferedReader (
new FilReader (input_filename) );


Now the BufferedReader object (inFile) can
be sent a .readLine() message
A BufferedReader is so named because it
buffers the input
–
this improves program performance
Using a Reader

How to know when all the data in a file has
been read?
–
the readLine() method returns the value null
valueString = inFile.readLine():
while (valueString != null)
{
… // process valueString
valueString = inFile.readLine();

}
Also possible to check for other things such as
an empty line with no text
if (valueString.equals("")) continue;
Using a Reader
Sequence of steps for using text input
1. Build a BufferedReader by wrapping one
around a FileReader
2. Use an input loop to read lines of text from
the BufferedReader
 Convert those lines to numeric values if necessary
3. Close the BufferedReader
Using a Writer

When writing values to a text file, author
suggests use a Writer class
–
increase FileWriter capabilities by wrapping it in
BufferedWriter and PrintWriter classes
PrintWriter outFile = new PrintWriter(
new BufferedWriter (
new FileWriter (file_name) ) );
10.3 Exceptions

Easy for things to go wrong in a program
–
–

faulty logic
bad input from user, etc.
Java classifies all these as "exceptional
occurrences"
–
provides Exception class
–
tells something abnormal occurred
Try-Catch Blocks

To signal an abnormal occurrence
–
–
method can throw an exception
throw occurs inside a try block
try block followed by one or more catch blocks
Exception thrown here
Form:
try { … statement list … }
Program)searches
catch (Exception_Type1 variable_name
1
catch
blocks
looking
{ … }
for type match for
catch (Exception_type2 variable_name
2) thrown
exception
When match found, that block of
{ … }
statements executed
finally
{ … }

Try-Catch Blocks

After catch block executed
–
–

If no matching exception type is found
–

execution continues in finally { } block or …
if no finally { }, continues to next statements
following catch { }
program terminates
If no exception thrown
–
–
catch blocks skipped
execution continues in finally { } or in
statements following catch block
Exception Hierarchy
Most general class
Exception
DataFormatException
EOFException
IOException
FileNotFoundException
...
...
Exceptions become
more specialized
RuntimeException
InterruptedIOException
Try/Catch Strategy

Wrap all calls to methods that throw exceptions
in single try block
–
–

followed by single catch block .. or …
followed by sequence of catch blocks, one for each
type of exception
Wrap each call to a method that throws an
exception in its own try block
–
a catch block exists for each particular exception
thrown
Throwing Exceptions

Method that throws an exception must explicitly
state this in its heading
public void read(BufferedReader aReader)
throws EOFException

Now calling method must invoke this method
using try-catch blocks to handle the potential
exception
10.4 More About I/O Streams

Data can be stored in a file as text or in binary
format
–
–

same format used to store values in memory
takes less space to store an integer in binary format
than in text format
Java Stream classes provide byte level I/O
– FileOutputStream
– FileInputStream
– Also BufferedOutputStream and
BufferedInputStream
Writing Primitive types


Java provides DataOutputStream and
DataInputStream classes
Note methods provided, Table 10.1
–

note number of bytes used to store the data
Consider Data Stream Demonstration, Figure
10.3 in text – note the features
Why does…
contents of
–
–
–
–
numbers.dat
wrapping of Stream I/O classes
use of try-catch blocks appear to be gibberish?
specific methods used to write, then read
sample run
Writing Sequence of Values

Consider a group of values of different types
that are written to a file
–

they must be read in the same order that they were
written
Possible to create a class
–
–
–
that group of values is the private data of the class
specific methods are provided for an object of the
class to write/read those values to/from a file
the read and write methods receive the file handles
as parameters
10.5 Example: Scanning for a Virus

What is a virus?
–
–
–
–

software that hides itself within other executable
programs
that executable program is the "host" for the virus
the virus program tries to "infect" other programs
the virus can cause problems that range from
annoying to malicious
A virus that prints an annoying message stores
that string value within itself
–
we will write a program that searches through a file
to find that text, or signature
Virus-Detection Algorithm
1. Repeat the following:
a)
b)
c)
d)
read a line from a file
if no line read due to EOF, terminate
otherwise scan the line for specified string of text
if that text found, stop reading from file
2. Display message whether or not target string
was found – signifying virus found/not found
 Note source code, Figure 10.4
Simulated Virus Scanning

Note use of command line arguments
–
–

first argument is the name of the file to be read
second argument is the target string or virus
signature
Observe other features studied in this chapter
–
–
use of try/catch
wrapping of classes for file read
10.6 Example:
Retrieving Student Information
Problem: Information Retrieval
Records of information about students consist of


–
studentID, a 9 digit integer
–
–
first name, last name as strings
studentYear as a string
–
credits and GPA as reals
We seek a program which will
–
read a sequence of students from students.txt
prompt for studentID, read from keyboard
–
search for match, display if found
–
Objects
Objects
Type
Kind
Name
Individual Student
Student
varying
Number of
students
int
varying
numberOfStudents
Sequence of
students
StudentSequence
varying
students
Name of input file
String
constant
args[0]
A student ID
number
int
varying
studentID
Position of the
student
int
varying
position
Student Class

Operations needed include
–
–
–
–
–
–

initialize itself with default values
initialize itself with explicitly supplied values
read its attributes from a BufferedReader & store
them in itself
convert itself to a String (for output)
access each of its attributes
mutate any of its attributes
Note methods provided for these capabilities in
source code of Figure 10.5
Program Algorithm
1. Build students as a StudentSequence
a) initialized from an input file
b) name of file received in args[0]
2. Repeatedly do the following:
a) prompt for and read studentID
b) search students for target studentID, return its
position
c) if search successful
display student at position
otherwise
display error message
Class StudentSequence




Note source code, Figure 10.6
Reads numberOfStudents as first value in
the file
Constructor declares myStudents, an array of
Student
Use numberOfStudents as limiting value in
for loop
–

stores incoming values in myStudents array
Provides linear search method, find()
Student Information Retrieval


Note source code, Figure 10.7
Program declares students, object of type
StudentSequence
–

constructor loads values from file
Forever loop used to
–
–
–
prompt for, receive as input student id
call .find method to determine position of that
student in the array
print results (found or not)
10.7 Graphical/Internet Java:
Student Data Retrieval GUI



Implement student data retrieval problem
internet GUI context
Much of work already done can be reused
Behavior
GUIStudentRetriever
Bottom portion not
visible until id
entered and
processed
Alternatively, a "not
found" message
would appear here
Enter Student ID :
Bill
Freshman
111223333
Board
Credits
16.0
GPA 3.15
Implementation

Methods for GUIStudentRetreiver
–
–
–
–
–

constructor
action handler
enter initial state
enter found an ID state
enter not found state
Attributes
–
–
–
labels
entry fields
panes for panels
Event-Handler Method
public void actionPerformed(ActionEvent event)
{ String idString = myIdField.getText();
if (idString.equals(""))
enterInitialState();
else
{ int id = Integer.parseInt(idString);
int position = myStudents.find(id);
if (position >= 0)
enterGoodIDState(myStudents.getStudent(position));
else
enterErrorState("No student found with that ID");
}
}
Note source code, Figure 10.8
Part of the Picture:
Database Systems

Businesses must make decisions

Based on data
–
–

accuracy and detail help make better decisions
large data sets stored on computers
Data sets maintained by Database
Management Systems (DBMS)
DBMS Facilities


High level views of the data
Access routines
–

Support for large databases
–

data is sensitive or proprietary
Data sharing
–

as large as 1000 gigabytes (one terabyte)
Security
–

query language
protect data when more than one person wishes to
access/ modify same piece of data
Data integrity
Relational Model

Database viewed as a set of tables, called
relations
–
–

SQL, "Structured Query Language"
–

one row for each entry
column for each attribute or field of the database
provides commands for manipulate data
Example
SELECT * FROM Employee WHERE Rate = 10.5
Retrieve
All records
Name of
Relation
Condition