So this is the very simplified Lotus Domino Java agent code I am trying to run...
import lotus.domino.*;
enum SingletonTest {
INSTANCE;
public void he
Open the Designer preferences and select the Compiler settings.

Click on "Configure Project Specific Settings..."
Select your project and click OK.

Change the settings to 1.6 and default compliance settings.

Click OK. You should get a few prompts, select the default. Then open and save your agent again to recompile it.
Using your code I did this and ran it without error.