Assertion

Check non null

It is recommended to use Objects.requireNonNull (with Objects being the one of Java 8):

Objects.requireNonNull(executionResult, "executionResult must not be null");

More advanced conditions

As for more advanced conditions, it is recommended to use Guava’s Preconditions.

In Wicket code

In Wicket code, you can use Args.notNull, Args.notEmpty, Args.isTrue, Args.isFalse. Be careful to use the Args class from Wicket.