How to respond based on arguments in java constructor

Question

I have a MVC type architecture for new user registration to my app. The user submits the registration form to the controller servlet. This servlet extracts the user parameters and passes them to a class constructor, which represents (and returns) a template(or dummy) user with all the fields initialized based on the arguments.

I remember people insisting on argument validation in methods or constructors before using them. Now I try to check the arguments for their validity(for example they are not null). So when I find any argument having invalid value, How do I notify the servlet that this particular argument is invalid, because I cannot return a value from the constructor.

Please help me.

How about an Exception? For instance IllegalArgumentException? — Burkhard, Feb 08 '13 at 06:59
Ya, but how would the servlet know that this particular parameter is invalid? — me_digvijay, Feb 08 '13 at 07:00

score 1 · Answer 1 · answered Feb 08 '13 at 06:58

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You should use another service or some class to validate your data, and throw exception from method or pass message back to servlet

answered Feb 08 '13 at 06:58

jmj

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How would I tell the servlet about that particular argument that has caused the problem? – me_digvijay Feb 08 '13 at 07:05
By returning message from or by throwing exception – jmj Feb 08 '13 at 07:08
Ok, but in future my user class will always need to use the services of the validator class. – me_digvijay Feb 08 '13 at 07:12
Thanx a lot your answer helped me a lot. I need implement validation in both places before calling the constructor and in the constructor also. – me_digvijay Feb 08 '13 at 07:26

score 1 · Accepted Answer · answered Feb 08 '13 at 06:59

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Validate before calling the constructor. If anything is wrong get back to user with error. If everything is fine then only call constructor and create objects.

answered Feb 08 '13 at 06:59

Ajinkya

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But, someday some other person may use my user class and suppose he doesn't implement validation before calling the constructor(relying on me to have implemented the validation in the constructor.) – me_digvijay Feb 08 '13 at 07:04
@DigvijayYadav: As others mentioned Throwing exception will be a good way of handling this scenario. Also while creating your class thinking about what class gonna do is more important than what data others gonna provide to it. – Ajinkya Feb 08 '13 at 07:12
Ok, now I got what you mean. So I need to implement validation before calling the constructor and also throw an exception from the constructor for users who don't implement validation before calling the constructor. Thanx. – me_digvijay Feb 08 '13 at 07:24
@DigvijayYadav: Yes. Because validating every possible case in constructor will make it very complex. – Ajinkya Feb 08 '13 at 08:46

score 0 · Answer 3 · answered Feb 08 '13 at 06:58

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Do validation in servlet before calling the constructor. If validation fails forward to an error.jsp where you display which input parameter was null or invalid.

answered Feb 08 '13 at 06:58

Subin Sebastian

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How to respond based on arguments in java constructor

3 Answers3