Chapter 15 Text Processing and File Input/Output

1. Chapter 15Text Processing andFile Input/Output Lecture Slides to Accompany An Introduction to Computer Science Using Java (2nd Edition) by S.N. Kamin, D. Mickunas, E. Reingold

2. 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

3. 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

4. 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

5. 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

6. 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

8. Wrapper Classes • Class W is said to wrap class Y if: • Y is a concrete (not abstract) class • W’s constructor takes Y as an argument and stores a local copy of Y • W reimplements all of Y’s methods • A wrapper can wrap a class and be the subclass of another class at the same time

9. 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));

10. 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)); }

11. 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 } } }

12. 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

14. 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

16. 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”))); }

17. 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];

18. 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; }

19. 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(“--------------------------------”); }

20. 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( ); } }