Thursday, 26 September 2013

Exception Handling in Java

An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:
  • 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.
To understand how exception handling works in Java, you need to understand the three categories of exceptions:
  • Checked exceptions: A checked exception is an exception that is typically a user error or a problem that cannot be foreseen by the programmer. For example, if a file is to be opened, but the file cannot be found, an exception occurs. These exceptions cannot simply be ignored at the time of compilation.
  • Runtime exceptions: A runtime exception is an exception that occurs that probably could have been avoided by the programmer. As opposed to checked exceptions, runtime exceptions are ignored at the time of compilation.
  • 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.

Exceptions Methods:

Following is the list of important medthods available in the Throwable class.
SNMethods with Description
1public String getMessage()
Returns a detailed message about the exception that has occurred. This message is initialized in the Throwable constructor.
2public Throwable getCause()
Returns the cause of the exception as represented by a Throwable object.
3public String toString()
Returns the name of the class concatenated with the result of getMessage()
4public void printStackTrace()
Prints the result of toString() along with the stack trace to System.err, the error output stream.
5public 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.
 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.

The throws/throw Keywords:

If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature.
You can throw an exception, either a newly instantiated one or an exception that you just caught, by using the throw keyword. Try to understand the different in throws and throw keywords.
The following method declares that it throws a RemoteException:

import java.io.*;
public class className
{
   public void deposit(double amount) throws RemoteException
   {
      // Method implementation
      throw new RemoteException();
   }
   //Remainder of class definition
}
A method can declare that it throws more than one exception, in which case the exceptions are declared in a list separated by commas. For example, the following method declares that it throws a RemoteException and an InsufficientFundsException:

import java.io.*;
public class className
{
   public void withdraw(double amount) throws RemoteException,
                              InsufficientFundsException
   {
       // Method implementation
   }
   //Remainder of class definition
}
 
 

Declaring you own Exception:

You can create your own exceptions in Java. Keep the following points in mind when writing your own exception classes:
  • All exceptions must be a child of Throwable.
  • If you want to write a checked exception that is automatically enforced by the Handle or Declare Rule, you need to extend the Exception class.
  • If you want to write a runtime exception, you need to extend the RuntimeException class.
We can define our own Exception class as below:
class MyException extends Exception{
}
You just need to extend the Exception class to create your own Exception class. These are considered to be checked exceptions. The following InsufficientFundsException class is a user-defined exception that extends the Exception class, making it a checked exception. An exception class is like any other class, containing useful fields and methods.

Example:

// File Name InsufficientFundsException.java
import java.io.*;

public class InsufficientFundsException extends Exception
{
   private double amount;
   public InsufficientFundsException(double amount)
   {
      this.amount = amount;
   } 
   public double getAmount()
   {
      return amount;
   } 
 }

 

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