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Creational patterns organize object creation; behavioral patterns organize interactions and responsibility for behavior. A creational pattern controls how concrete software objects are selected, instantiated, or shared, so clients need not hard-code the entire construction policy. Examples include the factory method pattern and the singleton pattern.

A behavioral pattern describes how objects collaborate, receive requests, or distribute an algorithm's behavior. For example the observer pattern lets a subject notify dependants after its state changes, and the strategy pattern supplies interchangeable implementations of an operation. This distinction concerns the pattern's principal responsibility; a system can use both kinds together.

The Strategy pattern is a behavioral pattern that encapsulates alternative algorithms behind a shared object interface. A context holds a strategy and delegates the relevant operation to it, letting the chosen algorithm vary independently of its caller.

Use it when the same task has interchangeable policies, such as alternative pricing or route-selection algorithms. Supplying a different strategy changes the policy without expanding a chain of type tests in the context, and each policy can be checked independently. Object composition makes the selected strategy replaceable; clients rely on its contract rather than on one concrete implementation.

This is the Observer pattern, a behavioral pattern. Service stores subscribers through the Company object interface. SomeService.updateValue first changes the underlying value and then sends a notification to every registered company; each company's meth() queries the new value and computes its own response. This is a pull-style notification: the message announces the change, while the observer reads the data from its retained subject reference.