Semantics

If assertions are disabled in a class, the assert statements contained in that class have no effect. If assertions are enabled, the first expression is evaluated. If it evaluates to false, an AssertionError is thrown. If the assertion contains a second expression (preceded by a colon), this expression is evaluated and passed to the constructor of the AssertionError; otherwise the parameterless constructor is used. (If the first expression evaluates to true, the second expression is not evaluated.)

If an exception is thrown while either expression is being evaluated, the assert statement completes abruptly, throwing this exception.