I am playing GWT. I am looking for basic argument checking. I do not require invariants or result ensures. 
What I am interested about it best practises on the topic.

For example, in c# I use one of this options:

1. `if (arg1 != null) throw new ArgumentNulException....; // Official for public API;`
2. `Args.NotNull(arg1); // Home grown.`
3. `Contracts.Requires(arg1 != null); // Internal contract validation.`

What is the best place for me to start?

Ok, what I found for now.

1. [Validate method arguments](http://www.javapractices.com/topic/TopicAction.do?Id=5)
2. [Programming With Assertions](http://java.sun.com/j2se/1.4.2/docs/guide/lang/assert.html)