ContainsKey

ContainsKey (often written containsKey) is a method used on map-like containers in several programming languages to test whether a given key has an associated value in the container. The method returns a boolean: true if the key exists, false otherwise. It does not retrieve the value.

In Java, the Map interface provides boolean containsKey(Object key) to check for a mapping. Example: Map<String, Integer> map = new HashMap<>(); map.put("x", 1); boolean hasX = map.containsKey("x");

In Kotlin, Map<K, V> exposes containsKey(key: K): Boolean. Example: val map = mapOf("x" to 1); val hasX = map.containsKey("x")

In C#, many Dictionary<TKey, TValue> implementations include ContainsKey on their IDictionary<TKey, TValue> interface. Example: var dict = new Dictionary<string, int>(); dict["x"] = 1; bool hasX = dict.ContainsKey("x");

Semantics and performance considerations: containsKey generally indicates membership without materializing the value. For hash-based maps, the operation is typically O(1) on average; for tree-based maps, it is O(log n). The exact performance depends on the container’s internal data structure and current load factor.

Edge cases and notes: Some languages distinguish between keys that exist with a null value versus keys that do not exist. Null keys may be allowed or disallowed depending on the language and implementation. Equality semantics for keys (such as how objects are compared) determine whether two keys are considered the same.

See also: containsValue, get, hasKey, and membership tests in other container types.