Chapter 7.1.5 : Declaring your 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;

   }

}

To demonstrate using our user-defined exception, the following CheckingAccount class contains a withdraw() method that throws an InsufficientFundsException.

// File Name CheckingAccount.java

import java.io.*;

public class CheckingAccount

{

   private double balance;

   private int number;

   public CheckingAccount(int number)

   {

      this.number = number;

   }

   public void deposit(double amount)

   {

      balance += amount;

   }

   public void withdraw(double amount) throws

                              InsufficientFundsException

   {

      if(amount <= balance)

      {

         balance -= amount;

      }

      else

      {

         double needs = amount - balance;

         throw new InsufficientFundsException(needs);

      }

   }

   public double getBalance()

   {

      return balance;

   }

   public int getNumber()

   {

      return number;

   }

}

The following BankDemo program demonstrates invoking the deposit() and withdraw() methods of CheckingAccount.

// File Name BankDemo.java

public class BankDemo

{

   public static void main(String [] args)

   {

      CheckingAccount c = new CheckingAccount(101);

      System.out.println("Depositing $500...");

      c.deposit(500.00);

      try

      {

         System.out.println("\nWithdrawing $100...");

         c.withdraw(100.00);

         System.out.println("\nWithdrawing $600...");

         c.withdraw(600.00);

      }catch(InsufficientFundsException e)

      {

         System.out.println("Sorry, but you are short $"

                                  + e.getAmount());

         e.printStackTrace();

      }

    }

}

Compile all the above three files and run BankDemo, this would produce the following result:

Depositing $500...

Withdrawing $100...

Withdrawing $600...

Sorry, but you are short $200.0

InsufficientFundsException

        at CheckingAccount.withdraw(CheckingAccount.java:25)

        at BankDemo.main(BankDemo.java:13)

Common Exceptions:

In Java, it is possible to define two catergories of Exceptions and Errors.

·         JVM Exceptions: - These are exceptions/errors that are exclusively or logically thrown by the JVM. Examples : NullPointerException, ArrayIndexOutOfBoundsException, ClassCastException,

·         Programmatic exceptions: - These exceptions are thrown explicitly by the application or the API programmers Examples: IllegalArgumentException, IllegalStateException.