Streams and Files

Streams and Files CMPS 2143

Overview • Stream classes • File objects • File operations with streams • Examples in C++ and Java

File I/O • A stream is a general name given to a flow of data. • Disk file I/O is a special case of the general I/O system

C++ Stream class hierarchy (simplified) ios ios ios ios ostream istream ios ifstream ios ios iostream ofstream The classes used specifically for disk file I/O are declared in the file fstream ios fstream

C++ Disk file I/O with Streams • Three relevant classes: • ifstreamfor input // ifstreaminfile; • ofstreamfor output // ofstreamoutfile; • fstreamfor both input and output // fstreamiofile;

Opening and Closing a file void ifstream::open(const char *filename, ios::openmode mode= ios::in) void ofstream::open(const char *filename, ios::openmode mode= ios::out | ios::trunc) void fstream::open(const char *filename, ios::openmode mode= ios::in | ios::out) ioFile.open (”my file”); ioFile.close();

open (filename, mode);

Reading and writing Text Files • Use << and >> operators the same way you do when performing console I/O, except that instead of using cin, cout substitue a string that is linked to a file

What is Needed to Use Files • Include the fstream header file • Define a file stream object • ifstream for input from a file ifstream infile; • ofstream for output to a file ofstream outfile;

Open the File • Open the file • Use the open member function infile.open("inventory.dat"); outfile.open("report.txt"); • Filename may include drive, path info. • Output file will be created if necessary; existing file will be erased first • Input file must exist for open to work

Use the File • Use the file • Can use output file object and << to send data to a file outfile << "Inventory report"; • Can use input file object and >> to copy data from file to variables infile >> partNum; infile >> qtyInStock >> qtyOnOrder;

Close the File • Close the file • Use the close member function infile.close(); outfile.close(); • Don’t wait for operating system to close files at program end • May be limit on number of open files • May be buffered output data waiting to be sent to a file

Reading Input from ANY File and Writing Output to ANY File void openFiles (ifstream & infile, ofstream & outfile) { char infileName[40]; char outfileName[40]; //open input and output files cout<< “Enter name of input file: “ ; cin >> infileName; infile.open (infileName); cout<< “Enter name of output file: “; cin >> outfileName; outfile.open (outfileName); }

Example #include <iostream> #include <fstream> #include <string> using namespace std; int main ( ) { ofstreamoutfile; ifstreaminfile; double price, discount, newPrice; //open input and output files openFiles (infile, outfile) //get price and discount infile >> price >> discount; //calculate newPrice newPrice = price * discount; //echoprint input outfile << “Price is $ “ << price << endl; outfile << “Discount is “ << discount <<endl << endl; //display result outfile << “New price is $” << newPrice << endl; //close files infile.close( ); outfile.close( ); return 0; }

JavaFile I/O • Lots of stream classes!! • Designed to work with Exceptions/Exception Handling

Stream Zoo • C++ gives you istream, ostream, iostream, ifstream, ofstream, fstream, wistream, wifstream, istrsteam… (18) • Java goes overboard, gives you separate classes for selecting buffering, lookahead, random access, text formatting, zip format, or binary data. • 4 abstract classes at base: • InputStream, OutputStream, Reader and Writer.

Stream Zoo • InputStream and OutputStream deal with bytes • DataInputStream and DataOutputStream deal with basic Java types read in as binary data: • DataInputStreams • Read binary data from InputStream • Methods read, readByte, readChar, readDouble... • DataOutputStreams • Write binary data to OutputStream • Methods write, writeChar, writeInt...

Stream Zoo • File processing • Import java.io • Definitions of stream classesFileInputStreamandFileOutputStream • Inherit fromInputStreamandOutputStream • abstract classes • FileInputStream and FileOutputStream give you streams attached to disk files. • FileInputStream fin = new FileInputStream (“file.dat”) • Only support at byte level

Problem • FileInputStream has no methods to read numeric types • DataInputStream has no methods to get data from a file • Java has creative mechanism to combine two into filtered streams by feeding existing stream to the constructor of another stream.

Example FileInputStream fin = new FileInputStream (“file.dat”); DataInputStream din = new DataInputStream (fin); Double s = din.readDouble ( ); • The newly created filtered stream still accesses data from the file attached to the file input stream, but it now has more capable interface. • NOT BUFFERED

Example 2 • Not buffered: every call to read contacts OS to ask it to dole out another byte. IF you want buffering, data input for a file, you combine 3 types. DataInputStream din = new DataInputStream (new BufferedInputStream (new FileInputStream (“file.dat”))); • Note that DataInputStream is last because we want to use DataInputStream methods (and we want them to use buffered read method).

Files and Streams • Buffering • Improves I/O performance • I/O is slow compared to processor • Buffer stores data of many I/O operations • When full, sends data • Can be explicitly flushed (forced to send data)

Comparisons • Other languages offer niceties such as buffering and lookahead automatically in stream libraries • Java’s ability to combine provides greater flexibility

Text Streams • Set of stream filters that bridges gap between Unicode-encoded text and character encoding used by local OS. • Use classes that descend from Reader and Writer (similar methods to InputStream and OutputStream) • FileReader and FileWriterattach a reader or writer to a file.

Reader Class Hierarchy Reader StringReader CharacterArrayReader PipedReader BufferedReader FileInputStream InputStreamReader FilterReader PushbackReader FileReader

Reader - operations

Reader - example • import java.io.*; • public class CountSpaces{ • public static void main (String[] args) • throws IOException • { • Reader infile; // infilecan also be FileReader • infile = new FileReader("FileIn.txt"); • int ch, total, spaces; • spaces = 0; • for (total = 0 ; (ch = infile.read()) != -1; total++){ • if(Character.isWhitespace((char) ch)) • spaces++; • } • System.out.println(total + " chars " + spaces + " spaces "); • } • } Count total number of spaces in the file

Buffered Streams • Java supports creation of buffers to store temporarily data that read from or written to a stream. This process is known as buffered I/O operation. • Buffered stream classes – BufferedInputStream, BufferedOutputStream, BufferedReader, BufferedWriter buffer data to avoid every read or write going to the stream. • These are used in file operations since accessing the disk for every character read is not efficient.

Buffered Streams • Buffered character streams understand lines of text. • BufferedWriter has a newLine method which writes a new line character to the stream. • BufferedReader has a readLine method to read a line of text as a String. • For complete listing of methods, please see the Java manual/documentation.

BufferedReader - example import java.io.*; class ReadTextFile { public static void main(String[] args) throws FileNotFoundException, IOException { BufferedReader in; in = new BufferedReader( new FileReader(“Command.txt”)); String line; while (( line = in.readLine()) != null ) { System.out.println(line); } } } • Use a BufferedReader to read a file one line at a time and print the lines to standard output

Writer Class Hierarchy Writer BufferedWriter CharacterArrayWriter FilterWriter PrintWriter PipedWriter OutputStreamWriter StringWriter FileWriter

Text Streams • For text output, use PrintWriter. • Can print strings and numbers in text format • Must be combined with destination writer PrintWriter out = new PrintWriter (new FileWriter (“file.out”)); String name = “Harry Hacker”; double salary = 75000.00; out.print (name); out.print (‘ ‘); out.println (salary); //don’t throw exceptions

Text Streams • No analog to read data in text format. • Only possibility for processing text input is BufferedReader BufferedReader in = new BufferedReader (new FileReader (“descriptions.txt”)); • readLine method • reads a line of text. • Returns null when no more input available String line; while ((line = in.readLine () ) != null) { … }

Text Streams • To read numbers from text input, you need to read a string first, and then convert it. String s = in.readLine ( ); Double x = Double.parseDouble (s); • If more than one number on a line, you must break up the input string Use StringTokenizer utility class.

String Tokenizers and Delimited Text • Must specify delimiters to separate out tokens StringTokenizer t = new StringTokenizer (line, “|”); StringTokenizer t = new StringTokenizer (line, “ \t\n\r”); //white space StringTokenizer t = new StringTokenizer (line); //default is white space

Reading Delimited Input bufferedReader in = new BufferedReader (new FileReader (“file.dat”)); String s = in.readLine ( ); StringTokenizer t = new StringTokenizer (s); String name = t.nextToken (); double Salary = Double.parseDouble (t.nextToken()); int year = Integer.parseInt (t.nextToken());

Files and Exceptions • When creating files and performing I/O operations on them, the system may generate errors. The basic I/O related exception classes are given below: • EOFException – signals that end of the file is reached unexpectedly during input. • FileNotFoundException – file could not be opened • InterruptedIOException – I/O operations have been interrupted • IOException – signals that I/O exception of some sort has occurred – very general I/O exception.

Syntax • Each I/O statement or a group of I/O statements much have an exception handler around it/them as follows: try { …// I/O statements – open file, read, etc. } catch(IOException e) // or specific type exception { …//message output statements }

Example import java.io.*; class CountBytesNew { public static void main (String[] args) throws FileNotFoundException, IOException // optional in this case { FileInputStream in; try { in = new FileInputStream("FileIn.txt"); int total = 0; while (in.read() != -1) total++; System.out.println("Total = " + total); } catch(FileNotFoundException e1) { System.out.println("FileIn.txt does not exist!"); } catch(IOException e2) { System.out.println("Error occured while read file FileIn.txt"); } } }

Standard Input/Output • C++ - cin and cout are streams • cout is an ostream object • cin is an istream object • constructed by ios_base::Init before the body of main begins execution • Java – System.in and System.out are streams • System.in is an InputStream object • System.out is an OutputStream object • Instantiated by the JVM

Files and Directories • Sometimes you may need access to information about a file rather than its content. • For instance, if you need to know the file size or the file attributes of a file. • The same may be true for a directory. • For instance, you may want to get a list of all files in a given directory. • Both file and directory information is available via the File class in Java. infile.length();

C++ file information example std::ifstream::pos_typefilesize(const char* filename) { std::ifstream in(filename, std::ifstream::in | std::ifstream::binary); in.seekg(0, std::ifstream::end); return in.tellg(); }

Random Access to Files • You can get random access to files. • Random doesn't mean that you read or write from truly random places. • It just means that you can skip around the file and read from or write to it at the same time. • This makes it possible to write only parts of an existing file, to append to it, or delete from it.

References • http://www.cplusplus.com/doc/tutorial/files/ • http://tutorials.jenkov.com/java-io/index.html http://docs.oracle.com/javase/7/docs/api/

Streams and Files