Use the exception classes in the API whenever possible. Create custom exception classes if the predefined classes are not sufficient.

How to create your own exception class

• To define a checked exception, inherit the Exception class or any of its subclasses.
• To define an unchecked exception, inherit the RuntimeException class or any of its subclasses.
• By convention, each exception class should contain a default constructor that doesn't accept any arguments and another constructor that accepts a string argument.

Syntax to throw your own exception object

throw new ExceptionClassName(messageString);

When to define your own exceptions

• When a method requires an exception that isn't provided by any of Java's exception types
• When using a built-in Java exception would inappropriately expose details of a method's operation

Code for a user-defined exception class

public class MyDivisionByZeroException extends Exception
{
    public MyDivisionByZeroException()
    {
        super("Cannot divide by zero");
    }

    public MyDivisionByZeroException(String strMessage)
    {
        super(strMessage);
    }
}

Code for a user-defined exception class

public class ProductDAOException extends Exception
{
    public ProductDAOException()
    {
    }

    public ProductDAOException(String message)
    {
        super(message);
    }
}

A method that throws the ProductDAOException

public static Product getProduct(String productCode)
    throws ProductDAOException
{
    try
    {
        Product p = readProduct(productCode);
                              // may throw IOException
        return p;
    }
    catch (IOException e)
    {
        throw new ProductDAOException("An IO error
            occurred while retrieving the product.");
    }
}

Code that catches the ProductDAOException

try
{
    Product p = getProduct("1234");
}
catch (ProductDAOException e)
{
    System.out.println(e.getmessage());
}