Java Glossary : idempotent

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idempotent: In mathematics it means a number that when multiplied by itself gives the same number, e.g. 0 * 0 = 0 and 1 * 1 = 1.
The computing science world uses the term for any operation than you can use repeatedly safely, and still have the effect of using it only once. This would include operations having no lasting observable effect on the state of the world e.g. a database query with no history of it recorded, and no change to the database, also an operation that if run more than once makes no further changes to the state, e.g. C header files that are safe to include multiple times. It would include an initialisation routine that allocated a singleton only the first time it is run, and thereafter just returns a reference to that singleton. The idempotent operation is permitted to have side effects or change state. It is just that it must be safe to repeat the operation without any further consequential effect.

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