Transcript PowerPoint

Chapter 15
Text Processing and
File Input/Output
Lecture Slides to Accompany
An Introduction to Computer
Science Using Java (2nd Edition)
by
S.N. Kamin, D. Mickunas, E. Reingold
Chapter Preview
In this chapter we will:
• describe the java.io package
• introduce the Java StringBuffer class
• show how files can be read and written
• discuss how to handle file input and output
exceptions
• demonstrate how to perform console input and
output
Strings
• Java provides a number of methods for
operating on String objects
• String objects are immutable
• Immutable objects cannot be changed once
they are created
StringBuffer
• Java provides a mutable staring class called
StringBuffer that allows strings to grow
dynamically during program execution
• Several StringBuffer methods are the same as
those found in String
• To convert between String objects and
StringBuffer objects Java provides constructors
for each class
• The StringBuffer class also contains a ToString
method to allow easier output
Sequential Files
• Files are stored are stored on disks
• In this section we will assume that files
consist of multiple lines composed of
characters
• Each line ends with an end of line character
• The file itself may have an end of file
character
• Programmers often need to read or write files
stored on disks
File Input
• Java classes that support file input are found
in the java.io package
• FileReader allows us to open a file for
reading
• BufferedReader is a wrapper class that
provides methods that
– allow us to treat the file as a stream of characters
– increases the efficiency of reading
– allows line-oriented reading
Wrapper Classes
•
Class W is said to wrap class Y if:
1. Y is a concrete (not abstract) class
2. W’s constructor takes Y as an argument and
stores a local copy of Y
3. W reimplements all of Y’s methods
•
A wrapper can wrap a class and be the
subclass of another class at the same time
Buffered Input
• We can make a file available for reading one
character at a time by using
FileReader fr = FileReader(filename);
• We can read the file more efficiently by
reading a block of characters at a time, this is
called buffering the read
• To perform a buffered read we use this
BufferedReader br =
new BufferedReader(
new FileReader(filename));
Input Example – Part 1
import CSLib.*;
import java.io.*;
public class Copy {
// Copy file to an OutputBox
private FileBuffer fr;
private BufferedReader br;
public Copy(String filename) throws IOException {
// open local file given by filename
br = new BufferedReader(new FileReader(filename));
}
Input Example – Part 2
public void copy( ) throws IOException {
OutputBox out = new OutputBox( );
int I;
while (true) {
i = br.read();
if (i == -1)
// check for end of file
return;
out.print((char) i);
// must have char to print
}
}
}
File Output
• Java classes that support file output are found in the
java.io package
• FileWriter and BufferedReader provide
methods to write
–
–
–
–
a single character
an array of characters
a string
the end of line
• PrintWriter is a wrapper class provided to convert
several data type values to printable forms
Writing to a File
• To make a file available for printing
PrintWriter pr =
new PrintWriter(
new BufferedWriter(
new FileWriter(filename)));
• PrintWriter has void methods print and
println that behave the same way as they
for OutputBox objects
Mail Merge Application – part 1
import java.io.*;
public class MailMerge{
// mail merge application
BufferedReader template, maillist;
PrintWriter out;
public void openFiles(String fn) throws Ioexception {
template = new BufferedReader(
new FileReader(fn + “.template”));
maillist = new BufferedReader(
new FileReader(fn + “.list”));
out = new PrintWriter(
new BufferedWriter(
new FileReader(fn + “.out”)));
}
Mail Merge Application – part 2
private StringBuffer
readUpto(BufferedReader br, char delim)
throws Ioexception {
StringBuffer sb = new StringBuffer( );
int inputChar;
while (true) {
inputchar = br.read( );
if ((inputchar == -1) || (inputchar == delim))
return sb;
sb.append((char) inputchar);
}
}
private StringBuffer[] st = new StringBuffer[10];
Mail Merge Application – part 3
private int readTemplate( ) throws Ioexception {
int n;
// Read the first portion of the template
st[0] = readUpro(template, ‘%’);
// If there is there is an ‘%’ as the first char
// read the rest of the template
for (n = 1; n < 10; n++) {
st{n] = readUpto(template, ‘%’);
if (st[n].length() == 0)
// might be empty
return n;
}
return n;
}
Mail Merge Application – part 4
public void merge( ) throws Ioexception {
Stringbuffer sm;
in n = readTemplate(); // Read the first field
sm = readUpto(mailList, ‘#’);
while (true) {
// if no more fields
if (sm.length() == 0)return;
out.print(st[0]);
// interleave template portions and fields
for (int i = 1; i < n; i++) {
mailList.read( );
// get past new line
out.print(sm);
// print field
out.print(st[i]);
// print next template
sm = readUpto(mailList, ‘#’);
}
out.println(“--------------------------------”);
}
Mail Merge Application – part 5
public void closeFiles( ) {
out.close( );
}
}
• A typical client
import java.io.*;
public class MailMergeClient {
public static void main (String{] args)
throws exception IOException {
MailMerge mm = new MailMerge( );
mm.openFiles(args[0]);
mm.merge( );
mm.closeFiles( );
}
}