Five months ago I reported that the Javadocs are not being built for the Google Checkout SDK. This issue is still not fixed, yet should be simple to fix. At least one other person in the forum was looking for the Javadocs, so I decided to build them and host them until the Google Checkout SDK team gets it together. It is somewhat frustrating. :-|
So I have temporarily put the Javadocs at http://www.gentomi.com/google_checkout_sdk_250_javadocs .
(And I just put the 2.5.1 docs at http://www.gentomi.com/google_checkout_sdk_251_javadocs )
For those that care, below is exactly what I did to produce these Javadocs. I got some warnings when I built these, but I think the warnings are simply because I did not have HttpServletRequest and HttpServletResponse available on my path, so it did not link to those classes.
nebula:/tmp/checkout$ wget http://google-checkout-java-sdk.googlecode.com/files/checkout-sdk-2.5.0.jar
nebula:/tmp/checkout$ mkdir checkout-sdk-2.5.0