getAndAdd

GetAndAdd is an atomic operation used in concurrent programming. It combines two actions into one indivisible step: it reads the current value, adds a specified delta, and stores the result, all without interruption. The method returns the value that existed before the addition, making it useful for implementing lock-free counters and indices.

Semantics and behavior: getAndAdd guarantees that no other thread observes an intermediate state between reading the old value and writing the new one. Because the operation is atomic, multiple threads can perform getAndAdd concurrently, and the outcomes appear in some serial order. The return value is the previous content of the variable before the addition. The delta may be positive or negative, allowing both increments and decrements. If the underlying type is an integer, overflow behavior follows the language’s standard arithmetic rules (often wrap-around).

Common variants and naming: many libraries expose a method with a similar purpose under names such as fetch_add, getAndAdd, or addAndGet. A closely related operation is addAndGet, which applies the delta and then returns the new value. These variants are useful depending on whether the caller needs the old or the new value after the update.

Typical use cases: implementing thread-safe counters, indexes for work-stealing queues, or other shared numeric state without explicit locking. GetAndAdd is especially valuable when multiple producers and consumers update a shared counter or position in a predictable, low-contention way.

Implementation notes: the operation is typically supported directly by atomic hardware primitives or built atop compare-and-swap loops. Its availability and exact memory ordering guarantees can vary by language and platform, but the core idea remains the same: read-old, add, write-new atomically, returning the old value.