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An exception is nothing but an object of an exception class. We can create our own exception in the following ways:

• By creating a class which extends the superclass of all the exception classes - Exception class, or
• By creating a class which extends any of the subclasses of the Exception class, such as ArrayStoreException, IOException, ParseException, NullPointerException etc.

After creating our own exception class, we will be able to throw and catch our exception just as we throw and catch predefined checked/unchecked exceptions. Let's see how.

Creating a user-defined Exception by extending Exception class



• In the upcoming code, we are creating our own exception class by extending the Exception class, which is the mother of all exception classes.
• Next, we will throw our exception out of a try block and catch it within a catch block, by using the throw keyword.



//Java - Example of user-defined exception

class myException1 extends Exception
{
public String toString()
{
	return "User-Defined Exception";
}

public static void main(String... ar)
{
	myException1 ob= new myException1();
	try
	{
		throw ob;
	}
	catch(myException1 e)
	{
		System.out.println("Exception handled - "+ e);
	}
}
}





Output




Exception handled - User-Defined Exception








Creating a user-defined Exception by extending IOException class



• In the upcoming code, we are creating our own exception class by extending a checked exception class - IOException.
• Next, we will throw our exception out of a try block and catch it within a catch block, by using the throw keyword.



//Java - Example of user-defined exception

import java.io.*;

class myException1 extends IOException
{
public String toString()
{
	return "User-Defined Exception by extending IOException";
}

public static void main(String... ar)
{
	myException1 ob= new myException1();
	try
	{
		throw new myException1();
	}
	catch(myException1 e)
	{
		System.out.println("Exception handled - "+ e);
	}
}
}





Output




Exception handled - User-Defined Exception by extending IOException






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Throwing a user-defined exception by throws keyword



• In the upcoming code, we are creating our own exception class by extending the Exception class, which is the mother of all the exception classes.
• Next, we will use throws keyword, to throw the exception out of a method from where our defined exception was generated.



//Java - Example of user-defined exception

class myException2 extends Exception
{
int acc_bal=0; 		//current account balance

public String toString()
{
	return "Cannot debit. Low Account Balance.";
}


public static void main(String... ar)
{
	myException2 ob= new myException2();
	try
	{
		ob.withdraw(100);
	}
	catch(myException2 e)
	{
		System.out.println("Exception handled - "+ e);
	}
}


public void withdraw(int debitMoney) throws myException2
{
	if(acc_bal>1000)  //if account balance is greater than 1000$ then you can dedit money
	{
		acc_bal = acc_bal - debitMoney;
	}
	else 	// if account balance is less than 1000$ then debiting throws our user defined exception.
		throw new myException2();
	}
}





Output




Exception handled - Cannot debit. Low Account Balance.


In the last code, we have made our own exception class, myException2 by extending the Exception class. This class throws our defined exception, i.e. myException2, when acc_balance is less than 1000$ and if we still want to debit money by calling the withdraw() method.





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