The state pattern
The state pattern is structurally similar to the strategy pattern, but its intent and purpose are very different. The goal of the state pattern is to represent state-transition systems: systems where it is obvious that an object can be in a specific state, and that certain activities may drive it to a different state.
To make this work, we need a manager, or context class that provides an interface for switching states. Internally, this class contains a pointer to the current state; each state knows what other states it is allowed to be in and will transition to those states depending on actions invoked upon it.
So we have two types of classes, the context class and multiple state classes. The context class maintains the current state, and forwards actions to the state classes. The state classes are typically hidden from any other objects that are calling the context; it acts like a black box that happens to perform state management internally. Here's how it looks in UML...
