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2017-06-09T05:10:42Z

Created page with "'''Title of script:''' Custom Exceptions '''Author:''' Joms Antony '''Keywords:''' Custom Exceptions, User defined exceptions, throw keyword, throws keyword, video tutorial,..."

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'''Title of script:''' Custom Exceptions

'''Author:''' Joms Antony

'''Keywords:''' Custom Exceptions, User defined exceptions, throw keyword, throws keyword, video tutorial, Java

{| style="border-spacing:0;"
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| <center>'''Visual Cue'''</center>
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| <center>'''Narration'''</center>

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 1'''

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Welcome to the spoken tutorial on''' custom exceptions.'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 2'''

'''Learning Objectives'''

* '''Custom exceptions'''
* Usage of '''throw''' and '''throws''' keywords

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| In this tutorial we will learn about:

* '''Custom exceptions '''and
* Usage of '''throw''' and '''throws''' keywords

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 3'''

'''Software Requirements'''

* '''Ubuntu Linux '''16.04
* '''JDK 1 .7'''
* '''Eclipse''' 4.3.1

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| To record this tutorial, I am using

* '''Ubuntu Linux 16.04 OS'''
* '''JDK 1 .7 '''and
* '''Eclipse 4.3.1 '''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 4'''

'''Prerequisites'''

* Basic knowledge of '''Exceptions Handling'''
* For relevant tutorials, please visit http://www.spoken-tutorial.org

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| To follow this tutorial, you must have basic knowledge of '''Exceptions Handling '''in''' Java.'''

If not, for relevant''' Java '''tutorials, please visit the link shown.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 5'''

'''Custom Exception'''

* '''Custom exception''' is a user defined exception class.
* It is usually created as checked exceptions.
* It is used to customize the exception according to user need.

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| First let us learn about '''custom exceptions'''.

* '''Custom exception''' is a '''user defined exception class'''.

* It is usually created as '''checked exceptions.'''

* It is used to customize the exception according to user need.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| In Eclipse IDE, create a project called '''CustomExceptionDemo '''
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Now we will open''' eclipse''' and create a new project called''' CustomExceptionDemo.'''

Inside this project we will create the necessary classes to demonstrate '''custom exceptions.'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Right click on '''src''' folder and click '''New'''-> '''Class '''and type the class name as '''InvalidMarkException''' and hit '''Enter'''
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| We will create a new '''class InvalidMarkException.'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''public''' '''class''' InvalidMarkException

Type

'''extends''' '''Exception'''
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| To make this as a type of '''exception''' class, it should be a subclass of Java '''exception''' class.

To do so, type''' extends''' '''Exception'''.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Click on '''Source''' -> '''Generate constructors from Superclass'''

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Click on '''Source''' menu and then select '''Generate constructors from Superclass'''.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Click on '''Deselect All'''

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Now click on '''Deselect All''' button in the right hand side.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Select '''Exception(String) '''

Click '''OK '''button.

Highlight the inserted code
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Then select the constructor with a single string argument and click on '''OK''' button at the bottom.

This '''string''' argument can be used to customize the message shown when this '''exception''' occurs.

<nowiki><<PAUSE>></nowiki>

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Right click on '''src''' folder and click '''New'''-> '''Class -> StudentMarks'''
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Let us add another class named '''StudentMarks.'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Copy the following code

'''int''' marks;

'''public''' StudentMarks('''int''' marks) {

'''this'''.marks = marks;

}

Highlight “'''int''' marks;”

Highlight “'''public''' StudentMarks('''int''' marks) { '''this'''.marks = marks;}”
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Then type the following code.

This class contains only one variable named '''marks.'''

This constructor initializes the value of '''marks.'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Copy the following code

'''public''' '''void''' validate()

{

'''if'''<nowiki> (marks<0 || marks>100)</nowiki>

'''throw''' '''new''' InvalidMarkException(marks+" is not a valid entry");

'''else'''

System.''out''.println("Entry OK");

}

Highlight “'''if'''<nowiki> (marks<0 || marks>100)</nowiki>”

Highlight “'''throw''' '''new''' InvalidMarkException(marks+" not a valid entry");”

Highlight “'''throw'''” and '''“marks not a valid entry”'''

Highlight "'''Entry OK'''"
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Let us now add a method to validate the marks.

The normal range of marks is from 0 to 100.

If '''marks less than 0 or greater than 100''' is processed, '''InvalidMarkException''' will be thrown.

For this, we need to use the '''throw''' keyword explicitly to throw a custom exception.

If the mark is a valid one, the message '''“Entry OK” '''will be displayed.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Highlight

'''throw''' '''new''' InvalidMarkException(marks+" is not a valid entry");
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| We can see that there is an error '''InvalidMarkException.'''

Let us check and resolve it.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Click on the error and double click “Add throws declaration”

Highlight the “'''throws''' InvalidMarkException”

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| So click on the error and double click '''“Add throws declaration'''”.

We can see that the error disappears once the '''“throws InvalidMarkException”''' is added to the method signature.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Highlight'''

'''public''' '''void''' validate() '''throws''' InvalidMarkException
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Here we can see that the '''throws''' keyword is used along with methods.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Highlight'''

InvalidMarkException
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| It indicates that the method will raise the specified exception.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''throw''' '''new''' InvalidMarkException(marks+" not a valid entry");
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| We have to provide the exception handling code when such a method is called.

<nowiki><<PAUSE>></nowiki>

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Next to '''System.out.println("Entry OK");''' statement, type

'''FileReader fr=null;'''
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Next, let us perform a file access operation which will raise a '''FileNotFoundException'''.

So type the following code to create an instance of a '''FileReader '''class'''.'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Point the error

Click on the error and double click '''import 'FileReader' (java.io).'''

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Eclipse''' will show some errors as we have not imported the corresponding '''Java packages.'''

To rectify the same, click on the error and then double click '''import 'FileReader' (java.io).'''

We will learn about '''package''' and its usage in detail in a later tutorial.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| fr='''new '''FileReader(“/home/spoken/Marks");

Highlight "/home/spoken/Marks

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| To allow '''fr '''to access a file called''' Marks''' which is located in the home folder, type the following code.

The path shown here is to be replaced with that of your system's home folder.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Point the error

d

Click on the error and double click '''Add throws declaration'''

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| An error shows that this line of code can raise a '''FileNotFoundException.'''

We can resolve it by adding this exception in the '''throws clause'''.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Highlight FileNotFoundException

Highlight '''throws''' InvalidMarkException, FileNotFoundException
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| We can see that '''FileNotFoundException''' is also added to the '''throws''' clause.

We can handle multiple exceptions using '''throws''' as shown here.

<nowiki><<PAUSE>></nowiki>

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Copy the following code

'''public''' '''static''' '''void'''<nowiki> main(String[] args) {</nowiki>

StudentMarks m1='''new''' StudentMarks(40);

m1.validate();

System.''out''.println("rest of the code");

}
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| We will now create the '''main''' method inside the '''StudentMarks '''class and verify the results.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Highlight StudentMarks m1='''new''' StudentMarks(40);

Highlight '''m1.validate();'''
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Here we create an object '''m1''' initialized with 40 as the value for '''marks.'''

In the next line we invoke the method '''validate''' using the object '''m1.'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Point the error
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| We can see that there is an error when the '''validate''' method is invoked.

It says that this method will raise the

'''InvalidMarkException''' and '''FileNotFoundException'''

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Point to '''throws '''in the drop down options

Double click '''Surround with try/catch'''

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| To resolve the error, we can add '''throws''' clause to '''main''' method as we did earlier

But it is recommended to use '''try''' and '''catch''' '''block.'''

So, double click '''Surround with try/catch'''.

Now the necessary '''try-catch blocks''' are added and the exception has been handled.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Click '''run''' icon
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Now let us run this program.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Point to Entry OK, rest of the code
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| It displays “Entry OK” and “rest of the code”.

This happens because the value of '''marks''' 40 is a valid entry.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Modify “Marks m1='''new''' Marks(-10);”
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Let us now change the value to -10 which is an invalid entry.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Click '''run''' icon
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| We will run the program again.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Highlight the output

InvalidMarkException: -10 is not a valid entry

Highlight the message “rest of the code”
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Now we can see that the '''InvalidMarkException''' is thrown as -10 is an invalid entry.

Since we have handled the exception,

we can see the message “rest of the code”

Instead if we use “throws” clause,

this message “rest of the code” will not be printed.

Also the program will be terminated.

So it is better to use a '''try catch block''' when a method is called inside the '''main''' method.

With this we come to the end of this tutorial.

<nowiki><<PAUSE>></nowiki>

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 8'''

'''Summary'''

* What is a Custom Exception
* Usage of throw and throws keywords
* How to create and use custom exceptions

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Let us summarize.

In this tutorial we have learned about

* What is a Custom Exception
* Usage of throw and throws keywords
* How to create and use custom exceptions

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 9'''

'''Assignment'''

* Create a custom exception class

'''InvalidAgeException'''

* Create another class '''Age''' and create a constructor to initialize the value of age
* Create validate() method to<nowiki> throw the above exception if age < 18 </nowiki>

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| As an assignment

Create a custom exception class called '''InvalidAgeException'''

Create another class '''Age''' and create a constructor to initialize the value of age

Also create a method '''validate''' to throw an exception if the age is less than 18

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Slide 9A'''

'''Assignment'''

* Create objects inside the main method and invoke the validate() method

* Provide exception handling using

try-catch blocks

* Verify the custom exception class

| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| Create objects inside the '''main'''

method and invoke the '''validate()''' method

Provide exception handling using

'''try-catch''' blocks wherever required.

Verify the custom exception class.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''About Project'''

(retain the slide as in TEX file)
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| The video at the following link summarizes the''' Spoken Tutorial Project.'''

Please download and watch it.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''About Workshops'''

(retain the slide as in TEX file)
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| The''' Spoken Tutorial Project Team'''

• Conducts workshops using spoken tutorials and

• Gives certificates on passing the online tests

For more details, please write to us.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''About NMEICT'''

(retain the slide as in TEX file)
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Spoken Tutorial Project''' is funded by the NMEICT, MHRD, Government of India.

More information on this mission is available at the link shown.

|-
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| '''Contributor slide'''

(retain the slide as in TEX file)
| style="background-color:#ffffff;border:1pt solid #000001;padding-top:0cm;padding-bottom:0cm;padding-left:0.173cm;padding-right:0.191cm;"| This script has been contributed by:

'''Department of Information Technology, Amal Jyothi College of Engineering'''

This is Priya from IIT Bombay, signing off. Thanks for joining.

|}

Java/C3/Custom-Exceptions/English - Revision history

Nancyvarkey at 10:28, 16 June 2017

Priyacst: Created page with "'''Title of script:''' Custom Exceptions '''Author:''' Joms Antony '''Keywords:''' Custom Exceptions, User defined exceptions, throw keyword, throws keyword, video tutorial,..."