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Most languages allow assertions to be enabled or disabled globally, and sometimes independently. Assertions are often enabled during development and disabled during final testing and on release to the customer. Not checking assertions avoids the cost of evaluating the assertions while (assuming the assertions are free of [[Side-effect (computer science)|side effects]]) still producing the same result under normal conditions. Under abnormal conditions, disabling assertion checking can mean that a program that would have aborted will continue to run. This is sometimes preferable.
Some languages, including [[C (programming language)|C]] and [[C++]], can completely remove assertions at compile time using the [[preprocessor]]. Java requires an option to be passed to the run-time engine in order to enable assertions. Absent the option, assertions are bypassed, but they always remain in the code unless optimised away by a JIT compiler at run-time or excluded by an <code>if (false)</code> condition at compile time, thus they need not have a run-time space or time cost in Java either.
Programmers can build checks into their code that are always active by bypassing or manipulating the language's normal assertion-checking mechanisms.
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