GetInstance

getInstance is a common name for a method that returns an instance of a class. In object-oriented programming, it is often used as a static accessor to provide controlled access to an object, hiding constructor details from callers.

Most frequently, getInstance is associated with the singleton pattern, where the method returns the single shared instance of a class. This can involve lazy initialization and requires attention to thread safety. Techniques include synchronized methods, double-checked locking, or the initialization-on-demand holder idiom to ensure that only one instance is created in a multithreaded environment.

Beyond singletons, getInstance can serve as a static factory method that returns an instance of a subclass or an implementation of an interface. In such cases the method may select among options based on configuration, input, or locale, and can cache or pool instances. A multiton variation may return distinct instances keyed by an identifier rather than a single shared object.

In practice, various libraries expose getInstance methods with framework-specific meanings. For example in Java APIs, Calendar.getInstance() yields a calendar object configured for the default time zone and locale, while other getInstance variants provide preconfigured or cached objects. The exact semantics depend on the class and library and are not universal.

Design considerations for getInstance include balancing simplicity with testability, avoiding hidden state, and ensuring thread-safe initialization when used as a singleton. Alternatives to getInstance include explicit constructors, dependency injection, or more explicit factory methods that make dependencies and lifecycles clearer.

See also: Singleton pattern, Factory method pattern, Static factory method, Multiton.