5/18/11

Java: Declaring Your own Exceptions tutorial

You can create your own exceptions in Java. Keep the following points in mind when writing your own exception classes:
  1. All exceptions must be a child of Throwable.
  2. 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.
  3. 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 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)

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