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Handling Exceptions in Java

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 Java, an exception is an event that disrupts the normal flow of the program.
It is an object which is thrown at runtime.
Exception Handling is a mechanism to handle runtime errors such a ClassNotFoundException, IOException, SQLException, RemoteException, etc.

Advantage of Exception Handling

The core advantage of exception handling is to maintain the normal flow of the application.
An 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;  
          statement 3;  
          statement 4;  
          statement 5;//exception occurs  
          statement 6;  
          statement 7;  
          statement 8;  
          statement 9;  
          statement 10;

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

Types of Java Exceptions

There are mainly two types of exceptions: checked and unchecked. Here, an error is considered as the unchecked exception. According to Oracle, there are three types of exceptions:

Checked Exception
Unchecked Exception
Error

1) Checked Exception

The classes which directly inherit Throwable class except RuntimeException and Error are known as checked exceptions.
E.g. IOException, SQLException etc. Checked exceptions are checked at compile-time.

2) Unchecked Exception

The classes which inherit RuntimeException are known as unchecked exceptions.
E.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at compile-time, but they are checked at runtime.

3) Error

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

Java Exception Keywords

try : The try keyword is used to specify a block where we should place exception code. The try block must be followed by either catch or finally. It means, we can't use try block alone.
catch : The catch block is used to handle the exception. It must be preceded by try block which means we can't use catch block alone. It can be followed by finally block later.
finally : The finally block is used to execute the important code of the program. It is executed whether an exception is handled or not.
throw : The throw keyword is used to throw an exception.
throws : The throws keyword is used to declare exceptions. It doesn't throw an exception. It specifies that there may occur an exception in the method. It is always used with method signature.

Common Scenarios of Java Exceptions

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

1) A scenario where ArithmeticException occurs

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

int a=50/0;//ArithmeticException

2) A scenario where NullPointerException occurs

If we have a null value in any variable, performing any operation on the variable throws a NullPointerException.

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

3) A scenario where NumberFormatException occurs

The wrong formatting of any value may occurNumberFormatException.

String s="abc";  
          Inti=Integer.parseInt(s);//NumberFormatException

Java Exception Handling Example

Let's see an example of Java Exception Handling where we using a try-catch statement to handle the exception.In this example, 100/0 raises an ArithmeticException which is handled by a try-catch block.

public class JavaExceptionExample
          {  
          public static void main(String args[])
          {  
          try
          {
          //code that may raise exception  
          int data=100/0;  
          }
          catch(ArithmeticException e)
          {
          System.out.println(e);
          }  
          //rest code of the program   
          System.out.println("rest of the code...");  
          }  
          }

OUTPUT

Exception in thread main java.lang.ArithmeticException:/ by zero
          rest of the code...

Java Multi-catch block

A try block can be followed by one or more catch blocks. Each catch block must contain a different exception handler. So, if you have to perform different tasks at the occurrence of different exceptions, use java multi-catch block.

Points to remember

At a time only one exception occurs and at a time only one catch block is executed.
All catch blocks must be ordered from most specific to most general, i.e. catch for ArithmeticException must come before catch for Exception.

Example

Let's see a simple example of java multi-catch block.

public class MultipleCatchBlock1 {  
            
              public static void main(String[] args) {  
                    
                     try{    
                          int a[]=new int[5];    
                          a[5]=30/0;    
                         }    
                         catch(ArithmeticException e)  
                            {  
                             System.out.println("Arithmetic Exception occurs");  
                            }    
                         catch(ArrayIndexOutOfBoundsException e)  
                            {  
                             System.out.println("ArrayIndexOutOfBounds Exception occurs");  
                            }    
                         catch(Exception e)  
                            {  
                             System.out.println("Parent Exception occurs");  
                            }             
                         System.out.println("rest of the code");    
              }  
          }

OUTPUT

Arithmetic Exception occurs
          rest of the code