cs 116 object oriented programming ii lecture 11 george koutsogiannakis copyright: 2015 / illinois...

CS 116

OBJECT ORIENTED PROGRAMMING IILECTURE 11

GEORGE KOUTSOGIANNAKIS

Copyright: 2015 / Illinois Institute of Technology/George Koutsogiannakis

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Last Week’ s Topics

• Exception Handling– Using try and catch Blocks– Catching Multiple Exceptions– User-Defined Exceptions

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New Topics

• The java.io Package• Reading Text Files• Reading Structured Text Files Using

StringTokenizer• Reading using Streams• Writing to Structured Text Files

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File Types

• Java basically supports two types of files:– text files: data is stored as characters – binary files: data is stored as raw bytes

The type of a file is determined by the classes used to write to the file.

• Types of Files can be:– Files written using Bytes Streams (1s and 0s).– Files written using Character Streams (char types).– Files written in terms of Strings (text).– Files written using primitive data types (int, double etc)– Files written using entire object streams (recording all

information about an object in a coded format).

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File Types

• To read an existing file, you must know the file's type in order to select and import the appropriate library classes for reading the file.

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Writing to Text Files

• Several situations can exist:– the file does not exist– the file exists and we want to replace the current contents– the file exists and we want to append to the current

contents

• We specify whether we want to replace the contents or append to the current contents.

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Streams

• File writing and reading is accomplished with streams.

• There are input and output streams• The term input means input to your program

or consul from a file.• The term output means from your program or

consul to a file .

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Streams

• Therefore:• To read a file input stream is needed.• To write to a file output stream is needed.

• The java.io package provides the classes for establishing input and output streams.

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Selected Input Classes in the java.io Package

Class DescriptionInputStream Abstract superclass representing a

stream of raw bytesFileInputStream Input stream to read raw bytes of data

from filesObjectInputStream Class to read/recover objects from a

file written using ObjectOutputStream

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Hierarchy for Input Classes

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Selected java.io Output Classes

Class DescriptionWriter Abstract superclass for output classes

OutputStreamWriter Class to write output data streams

OutputStream Abstract superclass representing an output stream of raw bytes

FileWriter Class for writing to character files

BufferedWriter More efficient writing to character files

PrintWriter Prints basic data types, Strings, and objects

PrintStream Supports printing various data types conveniently

FileOutputStream Output stream for writing raw bytes of data to files

ObjectOutputStream Class to write objects to a file

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Hierarchy for Output Classes

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Reading Text Files with Scanner• In CS115 we used the File object and the Scanner object to read a text file• i.e.import java.io.File;import java.io.IOException;import java.util.Scanner;Public class ReadFile {Public static void main(String[] args) {

try{

File myfile=new File(“mytextfiel.txt”);Scanner scan=new Scanner(myfile);

while(scan.hasMoreTokens()) { String str=scan.next();

System.out.println(str); }

catch)(IOException ioe) { System.out.println(ioe.toString()); } }}

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Opening and Closing an Input Stream• When we construct an input stream or output stream object, the

JVM associates the file name, standard input stream, or standard output stream with our object. This is opening the file.

• When we are finished with a file, we optionally call the close method to release the resources associated with the file.

Return value Method name and argument list

void close( )

releases resources associated with an open input stream. Throws an IOException.

Closing an output stream

• When closing a Buffered output stream in addition to the close() method for the stream the methdod flush() should also be invoked:

datastreamobject.flush();– Flushing the output stream forces any buffered

output bytes to be written out.

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Opening and Closing Standard Streams

• Some streams are standard and the are available to any java program.

• The standard input stream (System.in), the standard output stream (System.out), and the standard error stream (System.err) are open when the program begins. They are intended to stay open and should not be closed.

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Calling the close method is optional. When the program finishes executing, all the resources of any unclosed files are released.

It is good practice to call the close method, however, especially if you will be opening a number of files (or opening the same file multiple times.)

Do not close the standard input, output, or error

devices, however. They are intended to remain open.

SOFTWARE ENGINEERING TIP

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Exceptions While Reading from a File We can catch this exception:

FileNotFoundException thrown by the Scanner constructor if the filename is not found when opening the file

We do not expect these exceptions to occur, so we will catch them as subclasses of IOException, and print the stack trace.

InputMismatchException if the input does not match the expected data type. (The next method does not throw this exception, so we don’t need to catch this exception).

NoSuchElementException if we attempt to read beyond the end of the file. IllegalStateException if we attempt to read after calling the close method.

See Example 11.8 ReadTextFile.java18

Reading Structured Text Files

Some text files are organized into lines that represent a record -- a set of data values containing information about an item.

• The data values are separated by one or more delimiters; that is, a special character or characters that separate one value from the next.

• As we read the file, we need to parse each line; that is, separate the line into the individual data values called tokens.

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Example• An airline company could store data in a file where each line

represents a flight segment containing the following data: – flight number– origin airport– destination airport– number of passengers– average ticket price

• These are called headers or field names of the record.

• Such a file could contain the following data: AA123,BWI,SFO,235,239.5 AA200,BOS,JFK,150,89.3 …

In this case, the delimiter is a comma. This example shows two records (or two rows of data).

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Remember the StringTokenizer Class?

• The StringTokenizer class is designed to parse Strings into tokens.

• StringTokenizer is in the java.util package.• When we construct a StringTokenizer object,

we specify the delimiters that separate the data we want to tokenize. The default delimiters are the whitespace characters.

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Example Using StringTokenizer

The file flight.txt contains the following comma-separated flight data on each line:

flight number, origin airport, destination airport, number of passengers, average ticket price

The FlightRecord class defines instance variables for each flight data value.

The ReadFlights class reads data from flights.txt, instantiates FlightRecord objects, and adds them to an ArrayList.

See Examples 11.12 FlightRecord.java and Example 11.13 ReadFlights.java

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Reading using Streams (without the Scanner object)

• Suggestion: from now on import the entire java.io package (java.io.*;)• Text files: files created by using Strings based streams ( written using the class

PrintWriter).– Use: class PrintReader to read it.

• Character Files: files created using char types (written using FileWriter class).– Use: class FileReader to read it.

• Bytes Files -binary data. (written using class FileOutputStream)– Use: class FileInputStream to read it.

• Primitive Data Types Files: files that have recorded strictly primitive data types (written using various methods depending on the primitive data type of class DataOutputStream )– Use: class DataInputStream methods to read file.– These classes offer different write methods for primitive data types i.e. public final void writeDouble(double v) throws IOException

• Files that have Objects recorded: files written using ObjectOutputStream class methods.– Use: class ObjectInput methods to read the objects from the file.

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Example: Reading Text File try{ FileInputStream file=new FileInputStream("MyFile.txt"); BufferedReader filereader=new BufferedReader(new InputStreamReader(file));

int index=0;int count=0;while(index>=0) {count++;index=filereader.read();

} System.out.println(count);

file.close(); filereader.close(); }// end of trycatch(IOException ioe){

System.out.println(ioe.toString(); }

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Example Reading Text File • In this example a buffer stream was used.• The buffer stream wraps the File Stream.• The while loop and when we encounter -1, since this is the indicator that the

end of a file has been reached (-1 is added to the end of the file when we create the file).

• Calling the read method of InputStreamReader class returns an int .If the end of the file is reached the value of the int is -1. The method reads one character at a time.

• In this example we are not saving what we read. The count counts the number of readings made in order to possibly create an array.

• We can repeat the loop once we have created an array and this time save the readings into the array by using a different format of the read method:

int index=filereader.read(myarray, 0, count-1); this call places the readings into the array, where array is an array of char

data types (from 0th index to the last index).

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Example Reading Text File • The char array myarray, needs to be instantiated to the correct size

before it is used in a while loop to capture the characters read. Therefore we need to loop twice through the file. – The first time to capture the count of characters (as in slide 24). We use the value of

count to set the size of the char array. Close the stream objects used!!!– The second time we create a new set of stream objects and we loop again through the

file , this time using the read method that take s the char array as argument as in slide 25

• Keep in mind that an array of char types (myarray) can be converted to a String by using:String str=new String(myarray);

We can output the String by using: System.out.println(str);

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Constructors for Writing to Text Files

Class ConstructorFileWriter FileWriter( String filename,

boolean mode )

constructs a FileWriter object from a String representing the name of a file. If the file does not exist, it is created. If mode is false, the current contents of the file, if any, will be replaced. If mode is true, writing will append data to the end of the file. Throws an IOException.

BufferedWriter BufferedWriter( Writer w )

constructs a BufferedWriter object from a Writer object

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Buffering

Writing data to the disk is much slower than writing data into main memory.

When a class uses buffering, instead of writing one character at a time to the file, it accumulates the data in a temporary location in memory, called a buffer.

When the buffer is full or is flushed, the accumulated

data is written to the file in one efficient operation.

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Methods of the BufferedWriter Class

See Example 11.9 WriteTextFile.java and Example 11.10 AppendTextFile.java

Return value Method name and argument listvoid write( String s )

writes a String to the current OutputStream object. This method is inherited from the Writer class. Throws an IOException.

void newLine( )

writes a line separator. Throws an IOException.

void close( )

releases resources allocated to the BufferedWriter object. Throws an IOException.

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Writing Primitive Types to Text Files

FileOutputStream, a subclass of the OutputStream class, is designed to write a stream of bytes to a file.

The PrintWriter class is designed for converting primitive data types to characters and writing them to an output stream. – print method, writes data without a newline– println method, writes data then adds a newline

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Constructors for Writing to Structured Text Files

Class ConstructorFileOutputStream FileOutputStream( String filename,

boolean mode )

constructs a FileOutputStream object from a String representing the name of a file. If the file does not exist, it is created. If mode is false, the current contents of the file, if any, will be replaced. If mode is true, writing will append data to the end of the file. Throws a FileNotFoundException.

PrintWriter PrintWriter( OutputStream os )

constructs a PrintWriter object from an OutputStream object

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Useful PrintWriter Methods

The argument can be any primitive data type (except byte or short), a char array, or an object.

See Example 11.15 WriteGradeFile.java

Return value Method name and argument list

void print( dataType argument ) writes a String representation of the argument to an output stream

void println( dataType argument ) writes a String representation of the argument to an output stream followed by a newline.

void close( ) releases the resources associated with the PrintWriter object

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Example: Writing to a Text FileSuppose we want to write a String aString to a File. Notice that the file name in the

FileOutputStream constructor is a String type.try {FileOutputStream file = new FileOutputStream(filename, true);OutputStreamWriter filestream = new OutputStreamWriter(new BufferedOutputStream(file));filestream.write(aString);filestream.flush();file.close();}Catch(IOException ioe){ System.out.ptintln(ioe.toString());}• If more than one String is to be written then a loop can be used.• Notice that the flush method needs to be called at the end to flush out the stream into the

file. If this is not done the String is not written in the file!

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Study Guide

• Chapter 11 Sections: 11.2, 11.3, 11.4, 11.5, 11.6

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