Difference between revisions of "Java/C3/Exception-Handling/English"

Exception Handling

Learning Objectives

• What is an Exception
• Checked and Unchecked Exceptions
• Handling the Exceptions using
  • try-catch block
  • finally block

• What is an exception
• Checked and unchecked exceptions
• Handling the exceptions using
  • try-catch block and
  • finally block

Software Requirements

• Ubuntu 16.04 OS
• JDK 1 .8
• Eclipse 4.3.1

• Ubuntu Linux 16.04 OS
• JDK 1 .8 and
• Eclipse 4.3.1

Prerequisites

• Basic knowledge of Java and Eclipse IDE
• For relevant tutorials, please visit http://www.spoken-tutorial.org

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

Exceptions

• An exception is an unexpected event, which occurs during the execution
• It interrupts the normal flow and results in an abnormal termination.
• Exceptions are classified as
• unchecked exceptions and
• checked exceptions.

It interrupts the normal flow of the program and results in an abnormal termination.

Based on their occurrence, exceptions are classified as

• unchecked exceptions and
• checked exceptions.

Inside this project we will create the necessary classes to demonstrate exception handling.

public static void main(String[] args) {

int[] marks={30,40,35,34,45};

int a=0;

System.out.println("Mark List");

for(int i=0;i<5;i++){

System.out.println(marks[i]);

}}

This program prints the marks of 5 students that is stored in an array marks.

Highlight the output

We can see that the values in the array are getting printed.

System.out.println("\t"+marks[50]);

Highlight the index 50

We know that there are only 5 elements in our array.

But in this statement we are trying to access the element at index 50 which is not existing.

Highlight the error Message

ArrayIndexOutOfBoundsException and

at Marks.main(Marks.java:7)

Highlight the print statement

We can see that the program terminates with an error message:

“ArrayIndexOutOfBoundsException “at line number 7.

The error message indicates the details of the exception like

• type of exception
• where it occurred
• and other details.

Note that the print statement is not executed as the program terminates after the error.

This is an example of Unchecked exception.

Unchecked Exception

• Unchecked Exceptions are called as Runtime Exceptions
• They handle the programming bugs and logical errors such as
  • Dividing a number by zero
  • Accessing an array element which is not existing

• Unchecked exceptions are called as Runtime exception as it is checked only during the execution.
• They handle the programming bugs and logical errors such as
  • Dividing a number by zero and
  • Accessing an array element which is not existing

Handling Exceptions

Highlight the portion of the code which can raise the exception

Highlight Exception e

Highlight the portion of code inside catch block

The corresponding catch block can receive the exception details in object e.

Inside the catch block we can write the code for

• displaying error messages or
• recovering from the error

try{

System.out.println("\t"+marks[50]);

}

catch(ArrayIndexOutOfBoundsException e)

{

System.out.println("Array Overflow Exception occurred");

}

Highlight "Array Overflow Exception occurred"

Highlight the Exception object e

So type the following code.

Here we are printing a custom message “Array Overflow Exception occurred”

Inside the round brackets we have created an instance of ArrayIndexOutOfBoundsException.

So this block can catch exceptions of type ArrayIndexOutOfBoundsException.

Highlight the output ie Printing the array

We can see that the error message gets printed.

But this time, note that printing the marks array is also executed.

In this way we can handle exceptions.

We can use them when different types of exceptions are raised by a block.

System.out.println(marks[2]/a);

Highlight the above line of code

This line of code divides an array element by zero as the value of a is zero.

So an ArithmeticException will be raised first.

catch(ArithmeticException e)

{

System.out.println("Arithmetic Exception occurred");

}

So type the following code after the existing catch block.

Highlight the error message in the output

This time the error message "Arithmetic Exception occurred" gets printed as it is caught first.

The remaining portion of the code outside the try catch block executes.

Checked Exceptions

• They are checked at compile time.
• They must be handled before running the program

Example:

• Accessing a File which is not existing
• Accessing a Network System when the network is down

Checked exceptions are checked at compile time.

So they must be handled before running the program.

For example:

• Accessing a file which is not existing or
• Accessing a network system when the network is down

FileReader fr=null;

Highlight FileReader fr=null;

Highlight FileReader

So type the following code.

Here the FileReader object fr is initialized as null.

FileReader object can be used to access and read a particular file.

Click on the error and import the class

Go to the top of the code file and highlight the import statement

To rectify the error, click on it and double click import FileReader java dot io.

The FileReader class is imported from the java dot io package.

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

fr = new FileReader("/home/spoken/Marks");

Highlight "/home/spoken/Marks

Point the error

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

Now an error comes up. It indicates that this line of code can create a FileNotFoundException.

We can see that Eclipse automatically inserts the try catch block to rectify this error.

So we can understand that this is a checked exception.

finally{

System.out.println("Inside finally block");

}

finally block usually follows a try-catch block .

The code inside this block is executed whether exception has occurred or not.

It contains a print statement.

So type, fr dot close

Click on the error and double click Surround with try/catch

So click on the error and double click Surround with try/catch.

Highlight NullPointerException

Highlight the print statement

Inside finally block

This is because we don't have a file named Marks in our home folder.

We can also see a NullPointerException as fr still refers to a null value.

But we can see that the print statement inside finally block gets executed.

D:\\Marks.txt

For example it can be specified as D:\\Marks.txt

Inside finally block

And “Inside finally block” gets printed.

The cleanup operation i.e closing the FileReader Object fr is also executed successfully.

Let us summarize.

Summary

• What is an Exception
• Checked and Unchecked Exceptions
• Handling the Exceptions using
  • try-catch block
  • finally block

• What is an exception
• Checked and unchecked exceptions
• Handling the exceptions using the
  • try-catch block and
  • finally block

Assignment

Learn about another Runtime Exception called NullPointerException

Refer to the Java program named Demo.java provided in the assignment section.

An exception will be raised when you run this code.

Identify the code which is responsible for the exception.

Rectify it using a try-catch block

Learn about another Runtime Exception called NullPointerException.

Refer to the Java program named Demo.java provided in the Assignment link of this tutorial.

An exception will be raised when you run this code.

Identify the code which is responsible for the exception.

Rectify it using a try-catch block.

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