An exception (or exceptional event) is a problem that arises during the execution of a program. When an Exception occurs the normal flow of the program is disrupted and the program/Application terminates abnormally, which is not recommended, therefore these exceptions are to be handled.
An exception can occur for many different reasons, below given are some scenarios where exception occurs.
- A user has entered invalid data.
- A file that needs to be opened cannot be found.
- A network connection has been lost in the middle of communications or the JVM has run out of memory.
Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.
Based on these we have three categories of Exceptions you need to understand them to know how exception handling works in Java,
Checked exceptions: A checked exception is an exception that occurs at the compile time, these are also called as compile time exceptions. These exceptions cannot simply be ignored at the time of compilation, the Programmer should take care of (handle) these 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 an FileNotFoundException occurs, and compiler prompts the programmer to handle the exception.
import java.io.File;
import java.io.FileReader;
public class FilenotFound_Demo {
public static void main(String args[]){
File file=new File("E://file.txt");
FileReader fr = new FileReader(file);
}
}
If you try to compile the above program you will get exceptions as shown below.
C:\>javac FilenotFound_Demo.java
FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown
FileReader fr = new FileReader(file);
^
1 error
Note: Since the methods read() and close() of FileReader class throws IOException, you can observe that 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, these include programming bugs, such as logic errors or improper use of an API. runtime exceptions are ignored at the time of compilation.
For example, if you have declared an array of size 5 in your program, and trying to call the 6th element of the array then an ArrayIndexOutOfBoundsExceptionexception occurs.
public class Unchecked_Demo {
public static void main(String args[]){
int num[]={1,2,3,4};
System.out.println(num[5]);
}
}
If you compile and execute the above program you will get exception as shown below.
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
at Exceptions.Unchecked_Demo.main(Unchecked_Demo.java:8)
Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.
Catching Exceptions:
A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:
try
{
//Protected code
}catch(ExceptionName e1)
{
//Catch block
}
The code which is prone to exceptions is placed in the try block, when an exception occurs, that exception occurred is handled by catch block associated with it. Every try block should be immediately followed either by a class block or finally block.
A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter.
Example:
The following is an array is declared with 2 elements. Then the code tries to access the 3rd element of the array which throws an exception.
// File Name : ExcepTest.java
import java.io.*;
public class ExcepTest{
public static void main(String args[]){
try{
int a[] = new int[2];
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
System.out.println("Out of the block");
}
}
This would produce the following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3
Out of the block
An exception can occur for many different reasons, below given are some scenarios where exception occurs.
- A user has entered invalid data.
- A file that needs to be opened cannot be found.
- A network connection has been lost in the middle of communications or the JVM has run out of memory.
Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.
Based on these we have three categories of Exceptions you need to understand them to know how exception handling works in Java,
Checked exceptions: A checked exception is an exception that occurs at the compile time, these are also called as compile time exceptions. These exceptions cannot simply be ignored at the time of compilation, the Programmer should take care of (handle) these 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 an FileNotFoundException occurs, and compiler prompts the programmer to handle the exception.
import java.io.File;
import java.io.FileReader;
public class FilenotFound_Demo {
public static void main(String args[]){
File file=new File("E://file.txt");
FileReader fr = new FileReader(file);
}
}
If you try to compile the above program you will get exceptions as shown below.
C:\>javac FilenotFound_Demo.java
FilenotFound_Demo.java:8: error: unreported exception FileNotFoundException; must be caught or declared to be thrown
FileReader fr = new FileReader(file);
^
1 error
Note: Since the methods read() and close() of FileReader class throws IOException, you can observe that 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, these include programming bugs, such as logic errors or improper use of an API. runtime exceptions are ignored at the time of compilation.
For example, if you have declared an array of size 5 in your program, and trying to call the 6th element of the array then an ArrayIndexOutOfBoundsExceptionexception occurs.
public class Unchecked_Demo {
public static void main(String args[]){
int num[]={1,2,3,4};
System.out.println(num[5]);
}
}
If you compile and execute the above program you will get exception as shown below.
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
at Exceptions.Unchecked_Demo.main(Unchecked_Demo.java:8)
Errors: These are not exceptions at all, but problems that arise beyond the control of the user or the programmer. Errors are typically ignored in your code because you can rarely do anything about an error. For example, if a stack overflow occurs, an error will arise. They are also ignored at the time of compilation.
Catching Exceptions:
A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:
try
{
//Protected code
}catch(ExceptionName e1)
{
//Catch block
}
The code which is prone to exceptions is placed in the try block, when an exception occurs, that exception occurred is handled by catch block associated with it. Every try block should be immediately followed either by a class block or finally block.
A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter.
Example:
The following is an array is declared with 2 elements. Then the code tries to access the 3rd element of the array which throws an exception.
// File Name : ExcepTest.java
import java.io.*;
public class ExcepTest{
public static void main(String args[]){
try{
int a[] = new int[2];
System.out.println("Access element three :" + a[3]);
}catch(ArrayIndexOutOfBoundsException e){
System.out.println("Exception thrown :" + e);
}
System.out.println("Out of the block");
}
}
This would produce the following result:
Exception thrown :java.lang.ArrayIndexOutOfBoundsException: 3
Out of the block
