Null Object Pattern to avoid Null checks?

问题:

Lately I have come across Null Object design pattern and my colleagues say it can be used to do away with the null pointer checks that are encountered throughout the code.

for e.g suppose a DAO class returns information on Customer (in a value object called CustomerVO). My main class is supposed to extract the firstName and emailId and send email to customer.

...
CustomerVO custVO = CustomerDAO.getCustomer(customerID);
if(custVO != null) { // imp, otherwise we may get null ptr exception in next line
     sendEmail(custVO.getFirstName(), custVO.getEmailID());
}
...

This is very simple example, but such null checks can quickly spread throughout your code based on the complexity of the value objects.

I have two issues with null check, - they tend tmake the code ugly and hard to read - lesser experienced developers put unnecessary null checks when in fact they are supposed to throw exceptions. for e.g. in above code, it would be better to throw exception from getCustomer() itself because if its not able to find customer info for given CustID, it indicates the CustID was invalid.

okay, coming back to null object pattern, can we use a 'null' CustomerVO object to hide the null check?

CustomerVO {
   String firstName = "";
   String emailID = ""; 
}

Don't it would make sense. what do you think?

And what are the things you follow to minimize null checks in your app.

回答1:

In this case a null object may be inappropriate since the default may infact hide what is in actuality an exception. If you find yourself having to check to see if your have safe null to perform some other activity the null pattern isn't buying you anything.

As you stated many new developers spend time trying to protect their code from exception situations that are worse then a halting program.

回答2:

While the null object pattern has it's uses, you're still going to need to make a check here, otherwise you're going to try to sendEmail() to an email address that's the empty string (or you push the check into sendEmail(), which could just as easily check for null).

Where the null object pattern would be really helpful here is if the CustomerVO class implemented the sendEmail() method. then you could simply chain the calls together, since the contract of getCustomer() would ensure that a null reference wouldn't be returned:

CustomerDAO.getCustomer(customerID).sendEmail();

In that case the sendEmail() method would check that its been asked to act on the special 'null object' and simply do nothing (or whatever's appropriate).

回答3:

Should your getCustomer method throw an exception if the customer is not found, rather than returning null?

The answer, of course, is that it depends: should it almost never be the case that a customer ID does not exist? In other words, is it an exceptional circumstance? If so, an exception is appropriate.

However, in a data-access layer it is often quite normal for something to not exist. In that case, it is better not to throw since it isn't an unexpected, exceptional circumstance.

The 'return a non-null object that has empty fields' is probably not any better. How do you know if the returned object is 'valid' or not without now adding some checking code that is probably worse than the null check?

So, if it might be a normal state that something being fetched does not exist, the null check pattern is probably best. If it is something that is unexpected then having the data access method throw a NotFound exception is probably better.

回答4:

I have an issue with this type of code, which is a common pattern. You don't need the null checks at all if you decompose what you're actually doing. The problem here, in my view, is that you're violating SRP.

The method CustomerVO custVO = CustomerDAO.getCustomer(customerID); does two things.

Firstly it returns a customer, if the customer exists and secondly returns null if there is no such customer. These are two distinct operations and should be coded as such.

A better approach would be:

bool customerExists = CustomerDAO.exists(customerID);

if (customerExists)
{
  CustomerVO custVO = CustomerDAO.getCustomer(customerID);
  sendEmail(custVO.getFirstName(), custVO.getEmailID());
}
else
{
   // Do whatever is appropriate if there is no such customer.
}

}

So, split the method into two, one which checks whether the requested object exists and the second which actually retrieves it. No need for any exceptions and the design and semantic is very clear (which, with a does-not-exist-returns-null pattern it isn't, in my view). Moreover, in this approach, the CustomerDAO.getCustomer(customerID) method throws an ArgumentException, if the requested customer does not exist. After all, you've asked for a Customer and there isn't one.

Futhermore, in my view, no method should return null. Null is explicitly, 'I don't know what the correct answer is, I have no value to return'. Null isn't a meaning, it's a lack of meaning. Ask yourself, 'why am I returning something that I know shouldn't really happen? Your method GetCustomer should return a Customer obviously. If you return null, you're simply pushing the responsibility of what to do back up the call-chain and breaking the contract. If there's meaningful default, use that, but throwing an exception here may make you think a bit harder about what the correct design is.

回答5:

The name is NULL object design pattern and not NULL check design pattern. Null object signifies absence of a object. This should be used when you have objects working in COLLABORATION. In your case you are checking existence of a object for which NULL check should be fine.

NULL design pattern is not meant to replace NULL exception handling. It’s one of the side benefits of NULL design pattern but the intention is to provide a default behavior.

NULL checks should not be replaced with NULL design pattern objects as it can lead to silent defects in application.

Do see the below article which goes in details of DO's and Donts of NULL design pattern.

http://www.codeproject.com/Articles/1042674/NULL-Object-Design-Pattern