What is exception

Dictionary Meaning : Exception is an abnormal condition.

In java, exception is an event that disrupts the normal flow of the program. It is an object which is thrown at runtime.

Checked exceptions

A checked exception is an exception that is checked (notified) by the compiler at compilation-time, these are also called as compile time exceptions.

For example, if you use FileReader class in your program to read data from a file, if the file specified in its constructor doesn't exist, then a FileNotFoundException occurs, and the compiler prompts the programmer to handle the exception.


package com.ashu;
import java.io.File;
import java.io.FileReader;
public class FilenotFound_java64bit {
public static void main(String args[]) {
File file = new File("E://file.txt");
FileReader fr = new FileReader(file);
}
}


Output: error: unreported exception FileNotFoundException; must be caught or declared to be thrown FileReader fr = new FileReader(file);

Imortant

the methods read() and close() of FileReader class throws IOException, you can observe that the compiler notifies to handle IOException, along with FileNotFoundException.

Unchecked exceptions

An unchecked exception is an exception that occurs at the time of execution. These are also called as Runtime Exceptions.


package com.ashu;
public class Unchecked_java64bit {
public static void main(String args[]) {
int num[] = {1, 2, 3, 4,5};
System.out.println(num[6]);
}
}


Output: Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 6 at exceptions.Unchecked_java64bit.main(Unchecked_java64bit.java:6)

Exception Handling in Java

The exception handling in java is one of the powerful mechanism to handle the runtime errors so that normal flow of the application can be maintained.

In this page, we will learn about java exception, its type and the difference between checked and unchecked exceptions.


Exceptions Methods

Following is the list of important methods available in the Throwable class.

1- public String getMessage()
Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.
2- public Throwable getCause()
Returns the cause of the exception as represented by a Throwable object.
3- public String toString()
Returns the name of the class concatenated with the result of getMessage().
4- public void printStackTrace()
Prints the result of toString() along with the stack trace to System.err, the error output stream.
5- public StackTraceElement [] getStackTrace()
Returns an array containing each element on the stack trace. The element at index 0 represents the top of the call stack, and the last element in the array represents the method at the bottom of the call stack.
6- public Throwable fillInStackTrace()
Fills the stack trace of this Throwable object with the current stack trace, adding to any previous information in the stack trace.

What is exception handling

Exception Handling is a mechanism to handle runtime errors such as ClassNotFound, IO, SQL, Remote etc.

Advantage of Exception Handling

The core advantage of exception handling is to maintain the normal flow of the application. Exception normally disrupts the normal flow of the application that is why we use exception handling. Let's take a scenario:

statement 1;
statement 2; //exception occurs
statement 3;
statement 4;


Suppose there is 10 statements in your program and there occurs an exception at statement 5, rest of the code will not be executed i.e. statement 6 to 10 will not run. If we perform exception handling, rest of the statement will be executed. That is why we use exception handling in java.


Hierarchy of Java Exception classes




Types of Exception

There are mainly two types of exceptions: checked and unchecked where error is considered as unchecked exception. The sun microsystem says there are three types of exceptions:

  1. Checked Exception
  2. Unchecked Exception
  3. Error

Difference between checked and unchecked exceptions

  1. 1) Checked Exception

    The classes that extend Throwable class except RuntimeException and Error are known as checked exceptions e.g.IOException, SQLException etc. Checked exceptions are checked at compile-time.

  2. 2) Unchecked Exception

    The classes that extend RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at compile-time rather they are checked at runtime.

  3. 3) Error

    Error is irrecoverable e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc.


Common scenarios where exceptions may occur

There are given some scenarios where unchecked exceptions can occur. They are as follows:

1) Scenario where ArithmeticException occurs

If we divide any number by zero, there occurs an ArithmeticException.

int a=50/0;//ArithmeticException


2) Scenario where NullPointerException occurs

If we have null value in any variable, performing any operation by the variable occurs an NullPointerException.

int s=null;
System.out.println(s.length());//NullPointerException


3) Scenario where NumberFormatException occurs

If you are inserting any value in the wrong index, it would result ArrayIndexOutOfBoundsException as shown below:

int a[]=new int[5];
a[10]=50; //ArrayIndexOutOfBoundsException



Java Exception Handling Keywords

There are 5 keywords used in java exception handling

  1. try
  2. catch
  3. finally
  4. throw
  5. throws

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