assert

Assertions tests conditions. If the conditions fails (result=false) an exceptions (AssertionError) is thrown.
Example 1:

public void doSomething(String str) {
   assert str != null;
   if (str.length > 0)
      ...
}

Example 2 (with message text for the stacktrace):

public void doSomething(String str) {
   assert str != null : "Parameter must not be null";
   if (str.length > 0)
      ...
}

VM-Parameter for asserts:
-ea (enable assertions)
-ea:packagename
-da (disable assertions)
-da:packagename