Jyotisolanki at 06:55, 14 September 2017

2017-09-14T06:55:44Z

Jyotisolanki: Created page with "{| border=1 | Time | Narration |- | 00:01 | Welcome to the spoken tutorial on''' custom exceptions.''' |- | 00:05 | In this tutorial we..."

2017-09-14T06:55:06Z

Created page with "{| border=1 | <center>Time</center> | <center>Narration</center> |- | 00:01 | Welcome to the spoken tutorial on''' custom exceptions.''' |- | 00:05 | In this tutorial we..."

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{| border=1
| <center>Time</center>
| <center>Narration</center>

|-
| 00:01
| Welcome to the spoken tutorial on''' custom exceptions.'''

|-
| 00:05
| In this tutorial we will learn about: '''Custom exceptions '''and Usage of '''throw''' and '''throws keywords'''

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| 00:14
|To record this tutorial, I am using :'''Ubuntu Linux 16.04 OS''' '''JDK 1 .7 '''and '''Eclipse 4.3.1 '''

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| 00:26
| 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.

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| 00:38
| First let us learn about '''custom exceptions'''.

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| 00:42
| '''Custom exception''' is a '''user defined exception class'''. It is usually created as '''checked exceptions.'''

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| 00:51
|It is used to customize the '''exception''' according to user need.

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| 00:57
| Now we will open''' eclipse''' and create a new '''project''' called''' CustomExceptionDemo.'''

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| 01:04
|Inside this '''project''' we will create the necessary '''classes''' to demonstrate '''custom exceptions.'''

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| 01:11
| We will create a new '''class InvalidMarkException.'''

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| 01:15
| To make this as a type of '''exception class''', it should be a '''subclass''' of '''Java exception class'''.

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| 01:22
|To do so, type''' extends Exception'''.

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| 01:27
| Click on '''Source''' menu and then select '''Generate constructors from Superclass'''.

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| 01:34
| Now click on '''Deselect All''' button in the right hand side.

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| 01:38
| Then select the '''constructor''' with a '''single string argument''' and click on '''OK''' button at the bottom.

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| 01:45
|This '''string argument''' can be used to customize the message shown when this '''exception''' occurs.<nowiki><<PAUSE>></nowiki>

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| 01:52
| Let us add another '''class''' named '''StudentMarks.'''

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| 01:57
| Then type the following code.

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| 02:00
|This '''class''' contains only one '''variable''' named '''marks.'''

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| 02:04
|This '''constructor''' initializes the value of '''marks.'''

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| 02:09
| Let us now add a '''method''' to validate the marks.

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| 02:13
|The normal range of marks is from 0 to 100.

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| 02:18
|If '''marks less than 0 or greater than 100''' is processed, '''InvalidMarkException''' will be thrown.

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| 02:25
|For this, we need to use the '''throw keyword''' explicitly to throw a '''custom exception'''.

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| 02:33
|If the mark is a valid one, the message '''“Entry OK” '''will be displayed.

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| 02:39
| We can see that there is an error '''InvalidMarkException.'''

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| 02:43
|Let us check and resolve it.

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| 02:46
| So click on the error and double click '''“Add throws declaration'''”.

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| 02:51
|We can see that the error disappears once the '''“throws InvalidMarkException”''' is added to the '''method signature.'''

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| 03:00
| Here we can see that the '''throws keyword''' is used along with '''methods'''.

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| 03:06
| It indicates that the '''method''' will raise the '''specified exception.'''

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| 03:11
| We have to provide the '''exception handling''' code when such a '''method''' is called.

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| 03:16
| Next, let us perform a '''file access operation''' which will raise a '''FileNotFoundException'''.

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| 03:23
|So type the following code to create an instance of a '''FileReader class'''.

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|03:29
| '''Eclipse''' will show some errors as we have not imported the corresponding '''Java packages.'''

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| 03:36
|To rectify the same, click on the error and then double click '''import 'FileReader' (java.io).'''

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| 03:44
|We will learn about '''package''' and its usage in detail in a later tutorial.

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| 03:50
|To allow '''fr '''to access a file called''' Marks''' which is located in the home folder, type the following code.

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| 03:59
|The path shown here is to be replaced with that of your system's home folder.

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| 04:05
| An error shows that this line of code can raise a '''FileNotFoundException.'''

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| 04:10
|We can resolve it by adding this exception in the '''throws clause'''.

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|04:16
| We can see that '''FileNotFoundException''' is also added to the '''throws clause'''.

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| 04:22
|We can handle multiple '''exceptions''' using '''throws''' as shown here.

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| 04:28
| We will now create the '''main method''' inside the '''StudentMarks class''' and verify the results.

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| 04:34
| Here we create an object '''m1''' initialized with 40 as the value for '''marks.'''

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| 04:41
|In04: the next line we invoke the method '''validate''' using the object '''m1.'''

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| 04:47
| We can see that there is an error when the '''validate method''' is invoked.

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| 04:52
|It says that this '''method''' will raise the '''InvalidMarkException''' and '''FileNotFoundException'''

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| 04:59
| To resolve the error, we can add '''throws clause''' to '''main method''' as we did earlier

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| 05:05
|But it is recommended to use '''try''' and '''catch block.'''

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| 05:10
|So, double click '''Surround with try/catch'''.

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| 05:14
|Now the necessary '''try-catch blocks''' are added and the '''exception''' has been handled.

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| 05:20
| Now let us run this program.

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|05:23
| It displays “Entry OK” and “rest of the code”.

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| 05:27
|This happens because the value of '''marks''' 40 is a valid entry.

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| 05:32
| Let us now change the value to -10 which is an invalid entry.

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| 05:37
| We will run the program again.

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| 05:40
| Now we can see that the '''InvalidMarkException''' is thrown as -10 is an invalid entry.

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| 05:47
|Since we have handled the '''exception''', we can see the message “rest of the code”

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| 05:53
|Instead if we use '''“throws” clause''', this message “rest of the code” will not be printed.

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| 06:00
|Also the program will be terminated.

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| 06:03
|So it is better to use a '''try catch block''' when a '''method''' is called inside the '''main method'''.

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| 06:10
|With this we come to the end of this tutorial.

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| 06:13
| Let us summarize.

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| 06:15
|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'''.

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| 06:26
| As an assignment :Create a '''custom exception class''' called '''InvalidAgeException'''

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| 06:33
| Create another '''class Age''' and create a '''constructor''' to initialize the value of age

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| 06:39
| Also create a '''method validate''' to throw an '''exception''' if the age is less than 18

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| 06:45
| Create objects inside the '''main method''' and invoke the '''validate() method'''

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| 06:51
|Provide '''exception handling''' using '''try-catch blocks''' wherever required.

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| 06: 56
|Verify the '''custom exception class'''.

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| 07:00
| The video at the following link summarizes the''' Spoken Tutorial Project.''' Please download and watch it.

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| 07:06
| 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.

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| 07:18
| '''Spoken Tutorial Project''' is funded by the NMEICT, MHRD, Government of India. More information on this mission is available at the link shown.

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| 07:29
| This script has been contributed by: '''Department of Information Technology, Amal Jyothi College of Engineering'''

|-
| 07:36
| This is Priya from IIT Bombay, signing off. Thanks for joining.

|}

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 |Verify the '''custom exception class'''.

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

Jyotisolanki at 06:55, 14 September 2017

Jyotisolanki: Created page with "{| border=1 | Time | Narration |- | 00:01 | Welcome to the spoken tutorial on''' custom exceptions.''' |- | 00:05 | In this tutorial we..."