Chapter 7 Input Output Stream

Object serialization To store an object, an array of objects, or some other complex structure: • Storing all the pieces of an object separately and reconstruct the object when that da

Input/Output StreamChapter 7

Outline

Input/Output streams

destination

and write information to an output stream

Standard I/O

 There are three standard I/O streams:

• standard output – defined by System.out

• standard input – defined by System.in

• standard error – defined by System.err

statements

 System.in typically represents keyboard input, which

we've used many times with Scanner objects

particular window on the monitor screen

Exception in I/O

 FileNotFoundException is thrown if the file cannot be

located (when the FileReader constructor is executed)

IOException

• A file might not exist

• Even if the file exists, a program may not be able to find it

• The file might not contain the kind of data we expect

Outline

Writing text files

Writing text files

• write method: allows to write a content to a file

• newLine method: allows to break a line

FileWriter fw = new FileWriter (" Lop.txt ");

BufferedWriter bw = new BufferedWriter (fw);

bw.write(" TCTH30A - Trung cap tin hoc 30A ");

bw.write(" TCTH30B - Trung cap tin hoc 30B ");

bw.write(" CDTH7K - Cao dang tin hoc 7K ");

bw.close();

}

}

FileWriter fw = new FileWriter (" Lop.txt “);

BufferedWriter bw = new BufferedWriter (fw);

bw.write(" TCTH30A - Trung cap tin hoc 30A\r\n ");

bw.write(" TCTH30B - Trung cap tin hoc 30B\r\n ");

bw.write(" CDTH7K - Cao dang tin hoc 7K\r\n ");

bw.close();

Reading text files

Reading text files

which allows us to read an entire line of characters in one operation

• the readLine method returns a string

• the readLine method returns null when no more input is available

FileReader fr = new FileReader (filename);

BufferedReader br = new BufferedReader (fr);

TCTH30A - Trung cap tin hoc 30A

TCTH30B - Trung cap tin hoc 30B

CDTH7K - Cao dang tin hoc 7K

Lop.txt

catch ( FileNotFoundException exception) {

System.out.println (" The file " +

filename + " was not found ");

Writing - Reading text files

FileOutputStream fis = new FileOutputStream (“LopHoc.txt”);

OutputStreamWriter isr = new OutputStreamWriter (fis, "UTF-8");

br.write(" Trung cấp tin học 30K ");

FileInputStream fis = new FileInputStream (filename);

InputStreamReader isr = new InputStreamReader (fis, " UTF-8 ");

BufferedReader br = new BufferedReader (isr);

catch ( FileNotFoundException exception){

System.out.println (" The file " + filename + " was not found ");

}

catch ( IOException exception){

Outline

Object serialization

 To store an object, an array of objects, or some other

complex structure:

• Storing all the pieces of an object separately and reconstruct the

object when that data is read back in  more complex the

information, the more difficult

• Object serialization: represents an object as a sequence

of bytes that can be stored in a file or transferred to another

computer

Serialization - Deserialization

 Serialization refers to the process of saving the

state of an object by sending it to an output stream

object back into memory

public final void writeObject (Object writeObject x ) throws IOException

public final Object readObject() throws readObject IOException, ClassNotFoundException

public final Object readObject() throws readObject Exception

public final void writeObject (Object writeObject x ) throws Exception

 We have the CountryInfo objects that we want to save to

a file

objects have been serialized and written out to a file, after that let’s deserialize them

import java.io.Serializable;

public class CountryInfo implements Serializable

{

private String name, abbreviation, capitol;

private long area, population;

String result = "Name: " + name + "\n";

result += "Abbreviation: " + abbreviation + "\n";

result += "Capitol: " + capitol + "\n";

result += "Area: " + area + " square kilometers\n";

result += "Population: " + population + "\n";

result += "^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^";

return result;

This class implements the Serializable interface

Example programs

 Write – Read a CountryInfo object

 Write – Read an array of CountryInfo objects by separately

 Write – Read an array object

 Write – Read an ArrayList

country = new CountryInfo (“ Viet Nam ", “ VIE ", “ Ha Noi ", 9629091L, 278058900L);

FileOutputStream file = new FileOutputStream (" countries.dat ");

// Serialize the object to a file

outStream writeObject (country);

CountryInfo country = new CountryInfo() ;

FileInputStream file = new FileInputStream (" countries.dat ");

ObjectInputStream inStream = new ObjectInputStream (file);

// Deserialize the objects country = ( CountryInfo ) inStream readObject ();

inStream.close();

// Print the objects System.out.println (country); Read an object CountryInfo

Output

Example programs

 Write – Read a CountryInfo object

 Write – Read an array of CountryInfo objects by separately

 Write – Read an array object

 Write – Read an ArrayList

CountryInfo[] countries = new CountryInfo[5];

countries[0] = new CountryInfo ("United States of America", "USA", "Washington, D.C.",

FileOutputStream file = newFileOutputStream ("countries.dat");

ObjectOutputStream outStream = new ObjectOutputStream (file);

// Serialize the objects to a filefor (int i = 0; i < countries.length; i++) outStream.writeObject (countries[i]);

outStream.close();

}

Write an object separately

// Deserialize the objects

countries[i] = (CountryInfo) inStream.readObject();

inStream.close();

// Print the objects

Read an object separately

// Deserialize the objects

countries[i] = (CountryInfo) inStream.readObject();

inStream.close();

// Print the objects

System.out.println (c);

}

Read an object separately

Output

Example programs

 Write – Read a CountryInfo object

 Write – Read an array of CountryInfo objects by separately

 Write – Read an array object

 Write – Read an ArrayList

CountryInfo[] countries = new CountryInfo[5];

countries[0] = new CountryInfo ("United Statesof America", "USA", "Washington, D.C.",

FileOutputStream file = newFileOutputStream ("countries.dat");

ObjectOutputStream outStream = new ObjectOutputStream (file);

// Serialize the objects to a file

outStream.writeObject (countries);

Write an array of object

FileInputStream file = new FileInputStream (" countries.dat ");

ObjectInputStream inStream = new ObjectInputStream (file);

// Deserialize the objects

inStream.close();

// Print the objects

for ( CountryInfo c : countries) System.out.println (c);

}

}

countries = ( CountryInfo []) inStream.readObject();

Read an array of object

Output

Example programs

 Write – Read a CountryInfo object

 Write – Read an array of CountryInfo objects by separately

 Write – Read an array object

 Write – Read an ArrayList

Write – Read an ArrayList

 The ArrayList class also implements the

Serializable interface, so we can store an entire list

of objects in one operation

 If we had stored the CountryInfo objects in an ArrayList,

we should write the entire set of objects out in one

operation

 Likewise, we should then read the entire ArrayList of

CountryInfo objects from the file in one operation

 Keep in mind that the objects stored in the ArrayList must also implement the Serializable interface for this to work

import java.io.*;

import java.util.ArrayList;

public class WriteCountryInfo3 {

public static void main (String[] args) throws IOException

arrCountries.add (new CountryInfo ("Poland", "POL", "Warsaw", 312685L, 38633900L));

FileOutputStream file = newFileOutputStream ("countries.dat");

ObjectOutputStream outStream = new ObjectOutputStream (file);

// Serialize the object arraylist to a file

ArrayList<CountryInfo> arrCountries = new ArrayList<CountryInfo>();

FileInputStream file = new FileInputStream ("countries.dat");

ObjectInputStream inStream = new ObjectInputStream (file);

// Deserialize the objects

arrCountries = (ArrayList) inStream.readObject();

inStream.close();

// Print the objects

System.out.println ("DOC MOT TAP CAC DOI TUONG VAO ARRAYLIST");

System.out.println ("+++++++++++++++++++++++++++++++++++++++++++++++++");

for (Object c : arrCountries)

Read an ArrayList object