If you want a plain and simple basic support for expressing your contracts, have a look on valid4j (found on Maven Central as org.valid4j:valid4j). It lets you express your contracts using regular hamcrest-matchers in plain code (no annotations, nor comments).
For preconditions and postconditions (basically assertions -> throwing AssertionError):
import static org.valid4j.Assertive.*;
require(inputList, hasSize(greaterThan(0)));
...
ensure(result, lessThan(4.0));
If you are not happy with the default global policy (throwing AssertionError), valid4j provides a customization mechanism that let's you provide your own implementation of org.valid4j.AssertiveProvider.
Links:
- http://www.valid4j.org/
- https://github.com/helsing/valid4j
