Exception Handling & Mechanism In Java

Exception Handling is the mechanism to handle runtime malfunctions. We need to handle such exceptions to prevent abrupt termination of program. The term exception means exceptional condition, it is a problem that may arise during the execution of program. A bunch of things can lead to exceptions, including programmer error, hardware failures, files that need to be opened cannot be found, resource exhaustion etc.

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Exception

A Java Exception is an object that describes the exception that occurs in a program. When an exceptional events occurs in java, an exception is said to be thrown. The code that's responsible for doing something about the exception is called an exception handler.

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Exception class Hierarchy

All exception types are subclasses of class Throwable, which is at the top of exception class hierarchy.

• Exception class is for exceptional conditions that program should catch. This class is extended to create user specific exception classes.
• RuntimeException is a subclass of Exception. Exceptions under this class are automatically defined for programs.

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Exception are categorized into 3 category.

• Checked Exception
The exception that can be predicted by the programmer.Example : File that need to be opened is not found. These type of exceptions must be checked at compile time.
• Unchecked Exception
Unchecked exceptions are the class that extends RuntimeException. Unchecked exception are ignored at compile time. Example : ArithmeticException, NullPointerException, Array Index out of Bound exception. Unchecked exceptions are checked at runtime.
• Error
Errors are typically ignored in code because you can rarely do anything about an error. Example : if stack overflow occurs, an error will arise. This type of error is not possible handle in code.

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Uncaught Exceptions

When we don't handle the excetions, they lead to unexpected program termination. Lets take an example for better understanding.

class UncaughtException
{
 public static void main(String args[])
 {
  int a = 0;
  int b = 7/a;     // Divide by zero, will lead to exception
 }
}

This will lead to an exception at runtime, hence the Java run-time system will construct an exception and then throw it. As we don't have any mechanism for handling exception in the above program, hence the default handler will handle the exception and will print the details of the exception on the terminal.

Exception Handling Mechanism

In java, exception handling is done using five keywords,

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

Exception handling is done by transferring the execution of a program to an appropriate exception handler when exception occurs.

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Using try and catch

Try is used to guard a block of code in which exception may occur. This block of code is called guarded region. A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in guarded code, the catch block that follows the try is checked, if the type of exception that occured is listed in the catch block then the exception is handed over to the catch block which then handles it.

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Example using Try and catch

class Excp
{
 public static void main(String args[])
 {
  int a,b,c;
  try
  {
   a=0;
   b=10;
   c=b/a;
   System.out.println("This line will not be executed");
  }
  catch(ArithmeticException e)
  {
   System.out.println("Divided by zero"); 
  }
  System.out.println("After exception is handled");
 }
}
output:
Divided by zero
After exception is handled

An exception will thrown by this program as we are trying to divide a number by zero inside try block. The program control is transfered outside try block. Thus the line "This line will not be executed" is never parsed by the compiler. The exception thrown is handle in catch block. Once the exception is handled the program controls continue with the next line in the program. Thus the line "After exception is handled" is printed.

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Multiple catch blocks:

A try block can be followed by multiple catch blocks. You can have any number of catch blocks after a single try block.If an exception occurs in the guarded code the exception is passed to the first catch block in the list. If the exception type of exception, matches with the first catch block it gets caught, if not the exception is passed down to the next catch block. This continue until the exception is caught or falls through all catches.

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Example for Multiple Catch blocks

class Excep
{
 public static void main(String[] args)
 {
  try
  {
   int arr[]={1,2};
   arr[2]=3/0;
  } 
  catch(ArithmeticException ae)
  {
   System.out.println("divide by zero");
  }
  catch(ArrayIndexOutOfBoundsException e)
  {
   System.out.println("array index out of bound exception");
  }
 }
}
Output:
divide by zero

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Example for Unreachable Catch block

While using multiple catch statements, it is important to remember that exception sub classes inside catchmust come before any of their super classes otherwise it will lead to compile time error.

class Excep
{
 public static void main(String[] args)
 {
  try
  {
   int arr[]={1,2};
   arr[2]=3/0;
  }
  catch(Exception e)    //This block handles all Exception
  {
   System.out.println("Generic exception");
  }
  catch(ArrayIndexOutOfBoundsException e)    //This block is unreachable
  {
   System.out.println("array index out of bound exception");
  }
 }
}

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Nested try statement

try statement can be nested inside another block of try. Nested try block is used when a part of a block may cause one error while entire block may cause another error. In case if inner try block does not have a catchhandler for a particular exception then the outer try is checked for match.

class Excep
{
 public static void main(String[] args)
 {
  try
  {
   int arr[]={5,0,1,2};
   try
   {
    int x=arr[3]/arr[1];
   }
   catch(ArithmeticException ae)
   {
    System.out.println("divide by zero");
   }
   arr[4]=3;
  }
  catch(ArrayIndexOutOfBoundsException e)
  {
   System.out.println("array index out of bound exception");
  }
 }
}

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Important points to Remember

1. If you do not explicitly use the try catch blocks in your program, java will provide a default exception handler, which will print the exception details on the terminal, whenever exception occurs.
2. Super class Throwable overrides toString() function, to display error message in form of string.
3. While using multiple catch block, always make sure that exception subclasses comes before any of their super classes. Else you will get compile time error.
4. In nested try catch, the inner try block, uses its own catch block as well as catch block of the outer try, if required.
5. Only the object of Throwable class or its subclasses can be thrown.

Try with Resource Statement

JDK 7 introduces a new version of try statement known as try-with-resources statement. This feature add another way to exception handling with resources management,it is also referred to as automatic resource management.

Syntax

try(resource-specification)
{
//use the resource
}catch()
{...}

This try statement contains a paranthesis in which one or more resources is declare. Any object that implements java.lang.AutoCloseable or java.io.Closeable, can be passed as a parameter to try statement. A resource is an object that is used in program and must be closed after the program is finished. The try-with-resources statement ensures that each resource is closed at the end of the statement, you do not have to explicitly close the resources.

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Example without using try with Resource Statement

import java.io.*;
class Test
{
public static void main(String[] args)
{
try{
String str;
//opening file in read mode using BufferedReader stream
BufferedReader br=new BufferedReader(new FileReader("d:\\myfile.txt"));   
while((str=br.readLine())!=null)
 {
System.out.println(str);
 }      
br.close(); //closing BufferedReader stream
}catch(IOException ie)
{  System.out.println("exception");  }
 }
}

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Example using try with Resource Statement

import java.io.*;
class Test
{
public static void main(String[] args)
{
try(BufferedReader br=new BufferedReader(new FileReader("d:\\myfile.txt")))
{
String str;
while((str=br.readLine())!=null)
{
System.out.println(str);
}
}catch(IOException ie)
{  System.out.println("exception");  }
}
}

NOTE: In the above example, we do not need to explicitly call close() method to close BufferedReader stream.