Difference between revisions of "Java/C3/Custom-Exceptions/English"

Learning Objectives

• Custom exceptions
• Usage of throw and throws keywords

• Custom exceptions and
• Usage of throw and throws keywords

Software Requirements

• Ubuntu Linux 16.04
• JDK 1 .7
• Eclipse 4.3.1

• Ubuntu Linux 16.04 OS
• JDK 1 .7 and
• Eclipse 4.3.1

Prerequisites

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

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

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.

• 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.

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

Type

extends Exception

To do so, type extends Exception.

Click OK button.

Highlight the inserted code

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

<<PAUSE>>

int marks;

public StudentMarks(int marks) {

this.marks = marks;

}

Highlight “int marks;”

Highlight “public StudentMarks(int marks) { this.marks = marks;}”

This class contains only one variable named marks.

This constructor initializes the value of marks.

public void validate()

{

if (marks<0 || marks>100)

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

else

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

}

Highlight “if (marks<0 || marks>100)”

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

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

Highlight "Entry OK"

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.

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

Let us check and resolve it.

Highlight the “throws InvalidMarkException”

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

public void validate() throws InvalidMarkException

InvalidMarkException

<<PAUSE>>

FileReader fr=null;

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

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

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.

Highlight "/home/spoken/Marks

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

d

Click on the error and double click Add throws declaration

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

Highlight throws InvalidMarkException, FileNotFoundException

We can handle multiple exceptions using throws as shown here.

<<PAUSE>>

public static void main(String[] args) {

StudentMarks m1=new StudentMarks(40);

m1.validate();

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

}

Highlight m1.validate();

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

It says that this method will raise the

InvalidMarkException and FileNotFoundException

Double click Surround with try/catch

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.

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

InvalidMarkException: -10 is not a valid entry

Highlight the message “rest of the code”

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.

<<PAUSE>>

Summary

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

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

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 throw the above exception if age < 18

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

Assignment

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

• Provide exception handling using try-catch blocks

• Verify the custom exception class

Provide exception handling using try-catch blocks wherever required.

Verify the custom exception class.

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