Assertions are a new feature in the Merlin release. However, neither "java -help" nor "java -X" list the command line options necessary to turn assertion checking on, from JSR-41:

java [ -enableassertions | -ea ] [:<package name>"..." | :<class name> ]
java [ -disableassertions | -da ] [:<package name>"..." | :<class name> ]
java [ -enablesystemassertions | -esa ]
java [ -disablesystemassertions | -dsa ]

These options should be documented somewhere in the java command-line usage messages.