computeIfAbsent

computeIfAbsent is a method in the Java Map interface that enables lazy initialization of values. It provides a way to compute and insert a value for a key only if the key is not already associated with a value. This pattern is commonly used for caching and lazy loading in a thread-safe manner.

The method signature is: V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction). Behavior: if the key is already mapped to a non-null value, that value is returned. If not, the mappingFunction is applied to the key to produce a value. If the function returns a non-null value, it is associated with the key and returned; if it returns null, no mapping occurs and null is returned. The mappingFunction is not invoked if a value already exists and is non-null (or, in some maps, non-null values only).

In concurrent maps, such as ConcurrentHashMap, computeIfAbsent performs the computation and insertion atomically, ensuring that only one thread initializes the value for a given key. If another thread inserts a value for the same key during the operation, the existing value is returned and the new value is discarded. The method also adheres to the map’s null-value policy; for maps that disallow null values, a null result from the mappingFunction will not create a mapping.

Common use cases include implementing caches, memoization, or resource initialization where heavy computation should occur only once and subsequent accesses should reuse the result. Related methods include computeIfPresent, which updates values only if a key is already present.