JAVA: Modification of Address Book Code

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Web Design & Development CS506

Lesson 9

Modification of Address Book Code

Adding Persistence Functionality

Hopefully, your address book you built previously is giving you the required results except one i.e.

persistence. You might have noticed that after adding some person records in the address book; if you

exit form the program next time on re-executing address book all the previous records are no more

available.

To overcome the above problem, we will modify our program so that on exiting/starting of address

book, all the previously added records are available each time. To achieve this, we have to provide the

persistence functionality. Currently, we will accomplish this task by saving person records in some text

file.

Supporting simple persistence by any application requires handling of two scenarios. These are

On start up of application data (person records ) must be read from file

On end/finish up of application data (person records) must be saved in file

To support persistence, we have to handle the above mentioned scenarios

Scenario 1 Start Up

Establish a data channel with a file by using streams

Start reading data (person records) from file line by line

Construct PersonInfo objects from each line you have read

Add those PersonInfo objects in arraylist persons.

Close the stream with the file

Perform these steps while application is loading up

We will read records from a text file named persons.txt. The person records will be present in the file in

the following format.

Ali,defence,9201211

Usman,gulberg,5173940

Salman,LUMS,5272670

persons.txt

As you have seen, each person record is on a separate line. Person's name, address &

phone number is separated using comma (,).

We will modify our AddressBook.java by adding a new method loadPersons into it. This method

will provide the implementation of all the steps. The method is shown below:

public void loadPersons ( ){

String tokens[] = null;

String name, add, ph;

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try {

FileReader fr = new FileReader("persons.txt");

BufferedReader br = new BufferedReader(fr);

String line = br.readLine();

while ( line != null ) {

tokens = line.split(",");

name = tokens[0];

add = tokens[1];

ph = tokens[2];

PersonInfo p = new PersonInfo(name, add, ph);

persons.add(p);

line = br.readLine();

}

br.close();

fr.close();

}catch(IOException ioEx){

System.out.println(ioEx);

}

First, we have to connect with the text file in order to read line by line person records from it. This

task is accomplished with the following lines of code

FileReader fr = new FileReader("persons.txt"); BufferedReader br = new

BufferedReader(fr);

FileReader is a character based (node) stream that helps us in reading data in the form of

characters. As we are using streams, so we have to import the java.io package in the

AddressBook class.

We passed the file name persons.txt to the constructor of the FileReader.

Next we add BufferedReader (filter stream) on top of the FileReader because BufferedReader

facilitates reading data line by line. (As you can recall from the lecture that filter streams are

attached on top of node streams). That's why the constructor of BufferedReader is receiving the

fr the FileReader object.

The next line of code will read line from file by using readLine( ) method of

BufferedReader and save it in a string variable called line.

String line = br.readLine( );

After that while loop starts. The condition of while loop is used to check whether the file is reached

to end (returns null) or not. This loop is used to read whole file till the end. When end comes (null),

this loop will finish.

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while (line != null)

Inside loop, the first step we performed is tokenizing the string. For this purpose, we have used

split method of String class. This method returns substrings (tokens) according to the regular

expression or delimiter passed to it.

tokens = line.split(",");

The return type of this method is array of strings that's why we have declared tokens as a String

array in the beginning of this method as

String tokens[];

For example, the line contains the following string

Ali,defence,9201211

Now by calling split(",") method on this string, this method will return back three substrings ali

defence and 9201211 because the delimiter we have passed to it is comma. The delimiter itself

is not included in the substrings or tokens.

The next three lines of code are simple assignments statements. The tokens[0] contains the name

of the person because the name is always in the beginning of the line, tokens[1] contains address

of the person and tokens[2] contains the phone number of the person.

name = tokens[0];

add = tokens[1];

ph = tokens[2];

The name, add and ph are of type String and are declared in the beginning of this method.

After that we have constructed the object of PersonInfo class by using parameterized constructor

and passed all these strings to it.

PersonInfo p = new PersonInfo(name, add, ph);

Afterward the PersonInfo object's p is added to the arraylist i.e. persons. persons.add(p);

The last step we have done inside loop is that we have again read a line form the file by using the

readLine() method.

By summarizing the task of while loop we can conclude that it reads the line from a file,

tokenizethat line into three substrings followed by constructing the PersonInfo object by

using these tokens. And adding these objects to the arraylist. This process continues till the file

reaches its end.

The last step for reading information from the file is ordinary one closing the streams,

because files are external resources, so it's better to close them as soon as possible.

Also observe that we used try/catch block because using streams can result in raising exceptions

that falls under the checked exceptions category that needs mandatory handling.

The last important step you have to perform is to call this method while loading up. The most

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appropriate place to call this method is from inside the constructor of AddressBook.java. So

the constructor will now look like similar to the one given below:

..................

public AddressBook () {

Persons = new ArrayList();

loadPersons();

}

..................

AddressBook.java

Scenario 2 End/Finish Up

Establish a datachanel(stream) with a file by using streams

Take out PersonInfo objects from ArrayList (persons)

Build a string for each PersonInfo object by inserting commas (,) between name

& address and address & phone number.

Write the constructed string to the file

Close the connection with file

Perform these steps while exiting from address book.

Add another method savePersons into AddressBook.java. This method will provide

the

implementation of all the above mentioned steps. The method is shown below:

Public void savePersons ( ){

try {

PersonInfo p;

String line;

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

PrintWriter pw = new PrintWriter(fw);

for(int i=0; i<persons.size(); i++)

{

p = (PersonInfo)persons.get(i);

line = p.name +","+ p.address +","+ p.phoneNum;

// writes line to file (persons.txt)

pw.println(line);

}

pw.flush();

pw.close();

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fw.close();

}catch(IOException ioEx){

System.out.println(ioEx);

}

As you can see, that we have opened the same file (persons.txt) again by using a set of streams.

After that we have started for loop to iterate over arraylist as we did in

searchPerson and deletePerson methods.

Inside for loop body, we have taken out PersonInfo object and after type casting it we have assigned its

reference to a PersonInfo type local variable p. This is achieved by the help of following line of code

p = (PersonInfo)persons.get(i);

Next we build a string and insert commas between the PersonInfo attributes and assign the newly

constructed string to string's local variable line as shown in the following line of code.

line = p.name +","+ p.address +","+ p.phoneNum;

Note: Since, we haven't declare PersonInfo attributes private, therefore we are able to directly access

them inside AddressBook.java.

The next step is to write the line representing one PersonInfo object's information, to the file. This

is done by using println method of PrintWriter as shown below

pw.println(line);

After writing line to the file, the println method will move the cursor/control to the next line.

That's why each line is going to be written on separate line.

The last step for saving information to the file is ordinary one closing the streams but before

that notice the code line that you have not seen/performed while loading persons records from file.

That is

pw.flush( );

The above line immediately flushes data by writing any buffered output/data to file. This step is

necessary to perform or otherwise you will most probably lose some data for the reason that

PrintWriter is a Buffered Stream and they have their own internal memory/storage capacity for

efficiency reasons. Buffered Streams do not send the data until their memory is full.

Also we have written this code inside try-catch block.

The last important step you have to perform is to call this method before exiting from the address

book. The most appropriate place to call this method is under case 4

(exit scenario) in Test.java. So the case 4 will now look like similar to the one given below:

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case 4:

ab.savePersons();

System.exit(0);

Test.java

Compile & Execute

Now again after compiling all the classes, run the Test class. Initially we are assuming that out

persons.txt file is empty, so our arraylist persons will be empty on the first start up of address book.

Now add some records into it, perform search or delete operations. Exit from the address book

by choosing option 4. Check out the persons.txt file. Don't get surprised by seeing that it

contains all the person records in the format exactly we have seen above.

Next time you will run the address book; all the records will be available to you. Perform the search or

delete operation to verify that.

Finally You have done it !!!

References

Example code, their explanations and corresponding figures for this handout are taken from the book

JAVA A Lab Course by Umair Javed. This material is available just for the use of VU students of the

course Web Design and Development and not for any other commercial purpose without the consent of

author.

Abstract Classes and Interfaces

Problem and Requirements

Before moving on to abstract classes, first examine the following class hierarchy shown below:

Circle

Square

Suppose that in order to exploit polymorphism, we specify that 2-D objects must be able to

compute their area.

All 2-D classes must respond to area() message.

How do we ensure that?

Define area method in class Shape

Force the subclasses of Shape to respond area() message

Java's provides us two solutions to handle such problem

Abstract Classes

Interfaces

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Abstract Classes

Abstract classes are used to define only part of an implementation. Because, information is not

complete therefore an abstract class cannot be instantiate. However, like regular classes, they can

also contain instance variables and methods that are full implemented. The class that inherits from

abstract class is responsible to provide details.

Any class with an abstract method (a method has no implementation similar to pure virtual

function in C++) must be declared abstract, yet you can declare a class abstract that has no abstract

method.

If subclass overrides all abstract methods of the super class, than it becomes a concrete (a class whose

object can be instantiate) class otherwise we have to declare it as abstract or we can not compile it.

The most important aspect of abstract class is that reference of an abstract class can point to the object of

concrete classes.

Code Example of Abstract Classes

The Shape class contains an abstract method calculateArea() with no definition.

public abstract class Shape{

public abstract void calculateArea();

}

Class Circle extends from abstract Shape class, therefore to become concrete class it must provides

the definition of calculateArea() method.

public class Circle extends Shape {

private int x, y;

private int radius;

public Circle() {

x = 5;

y = 5;

radius = 10;

}

// providing definition of abstract method

public void calculateArea () {

double area = 3.14 * (radius * radius);

System.out.println("Area: " + area);

}

}//end of class

The Test class contains main method. Inside main, a reference s of abstract Shape class is created. This

reference can point to Circle (subclass of abstract class Shape) class object as it is a concrete class.

With the help of reference, method calculateArea() can be invoked of Circle class. This is all shown

in the form of code below

public class Test {

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public static void main(String args[]){

//can only create references of A.C.

Shape s = null;

//Shape s1 = new Shape(); //cannot instantiate

//abstractclass reference can point to concrete subclass

s = new Circle();

s.calculateArea();

}

}//end of class

The compilation and execution of the above program is shown below:

Interfaces

As we seen one possible java's solution to problem discussed in start of the tutorial. The second possible

java's solution is Interfaces.

Interfaces are special java type which contains only a set of method prototypes, but doest not provide

the implementation for these prototypes. All the methods inside an interface are abstract by default thus

an interface is tantamount to a pure abstract class a class with zero implementation. Interface can

also contains static final constants

Defining an Interface

Keyword interface is used instead of class as shown below:

public interface Speaker{

public void speak();

}

Implementing (using) Interface

Classes implement interfaces. Implementing an interface is like signing a contract. A class that

implements an interface will have to provide the definition of all the methods that are present inside

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an interface. If the class does not provide definitions of all methods, the class would not compile.

We have to declare it as an abstract class in order to get it compiled.

Relationship between a class and interface is equivalent to "responds to" while "is a"

relationship exists in inheritance.

Code Example of Defining & Implementing an Interface

The interface Printable contains print() method.

public interface Printable{

public void print();

}

Class Student is implementing the interface Printable. Note the use of keyword implements after the

class name. Student class has to provide the definition of print method or we are unable to compile.

The code snippet of student class is given below:

public class Student implements Printable {

private String name;

private String address;

public String toString () {

return "name:"+name +" address:"+address;

}

//providing definition of interface's print method

public void print() {

System.out.println("Name:" +name+" address"+address);

}

}//end of class

Interface Characteristics

Similar to abstract class, interfaces imposes a design structure on any class that uses the interface.

Contrary to inheritance, a class can implement more than one interfaces. To do this separate the

interface names with comma. This is java's way of multiple inheritance.

class Circle implements Drawable , Printable { .......... }

Objects of interfaces also cannot be instantiated.

Speaker s = new Speaker(); // not compile

However, a reference of interface can be created to point any of its implementation class. This is

interface based polymorphism.

Code Example: Interface based polymorphism

Interface Speaker is implemented by three classes Politician, Coach and Lecturer. Code snippets of all

these three classes are show below:

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public class Politician implements Speaker{

public void speak(){

System.out.println("Politics Talks");

}

public class Coach implements Speaker{

public void speak(){

System.out.println("Sports Talks");

}

public class Lecturer implements Speaker{

public void speak(){

System.out.println("Web Design and Development Talks");

}

As usual, Test class contains main method. Inside main, a reference sp is created of Speaker class.

Later, this reference is used to point to the objects of Politician, Coach and Lecturer class. On calling

speak method with the help of sp, will invoke the method of a class to which sp is pointing.

public class Test{

public static void main (String args[ ]) {

Speaker sp = null;

System.out.println("sp pointing to Politician");

sp = new Politician();

sp.speak();

System.out.println("sp pointing to Coach");

sp = new Coach();

sp.speak();

System.out.println("sp pointing to Lecturer");

sp = new Lecturer();

sp.speak();

}

The compilation and execution of the above program is shown below:

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References

Example code, their explanations and corresponding figures for this handout are taken from the book

JAVA A Lab Course by Umair Javed. This material is available just for the use of VU students of the

course Web Design and Development and not for any other commercial

purpose without the

consent of author.

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