When Should We Create Our Own Java Exception Classes

Why should I create my own Exceptions?

You should write your own exception classes if you answer yes to any of the following questions; otherwise, you can probably use someone else’s.

• Do you need an exception type that isn’t represented by those in the Java platform?
• Would it help users if they could differentiate your exceptions from those thrown by classes written by other vendors?
• Does your code throw more than one related exception?
• If you use someone else’s exceptions, will users have access to those exceptions?
• A similar question is, should your package be independent and self-contained?

Taken from "2. Creating custom Exception" in this article.

When should we create our own Java exception classes?

From Best Practices for Exception Handling:

Try not to create new custom exceptions if they do not have useful information for client code.

What is wrong with the following code?

public class DuplicateUsernameException extends Exception {}

It is not giving any useful information to the client code, other than an indicative exception name. Do not forget that Java Exception classes are like other classes, wherein you can add methods that you think the client code will invoke to get more information.

We could add useful methods to DuplicateUsernameException, such as:

public class DuplicateUsernameException
    extends Exception {
    public DuplicateUsernameException 
        (String username){....}
    public String requestedUsername(){...}
    public String[] availableNames(){...}
}

The new version provides two useful methods: requestedUsername(), which returns the requested name, and availableNames(), which returns an array of available usernames similar to the one requested. The client could use these methods to inform that the requested username is not available and that other usernames are available. But if you are not going to add extra information, then just throw a standard exception:

throw new IllegalArgumentException("Username already taken");

When should I create a new Exception class

Exceptions are a special case. In their case, the inheritance is not to add new functionality, but to add new classes of errors. This lets your code catch particular kinds of errors while ignoring others.

Say you are writing a large project. You have a Data component, and you have a Display component. They can both fail in various ways, and you want to throw exceptions for these failures. The Display component doesn't care about exceptions arising from the Data component, though, and vice versa. If all the classes just threw Exception, there'd be no way to figure out where the exception came from. However, if you subclass Exception with DataException and GraphicsException, even though they don't add new functionality, you can now throw and catch those particular types of exceptions, i.e. a graphics component can catch GraphicsException and not have to deal with data exceptions.

When should you create your own exception type?

Create your own exception types when:

1. you might want to differentiate them when handling. If they're different types, you have the option to write different catch clauses. They should still have a common base so you can have common handling when that is appropriate
2. you want to add some specific structured data you can actually use in the handler

Having separate list and vector exceptions doesn't seem worthwhile, unless there's something distinctively list-like or vectorish about them. Are you really going to have different catch handling depending on which type of container had an error?

Conversely it could make sense to have separate exception types for things that might be recoverable at runtime, versus things that are rolled-back but could be retried, versus things that are definitely fatal or indicate a bug.

When exception occur, what is responsibility of exception class?

1. What is exception class do when exception occur
   Nothing.

2. if i want to write my own MyException extend Exception, what method i write in exception class to handle my exception
   You don't need to implement anything, but you may want to have constructor to set message and store some relevant information.

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