The state pattern, which closely resembles Strategy Pattern, is a behavioral software design pattern, also known as the objects for states pattern. This pattern is used in computer programming to represent the state of an object. This is a clean way for an object to partially change its type at runtime.

Take, for example, a drawing program. The program has a mouse cursor, which at any point in time can act as one of several tools. Instead of switching between multiple cursor objects, the cursor maintains an internal state representing the tool currently in use. When a tool-dependent method is called (say, as a result of a mouse click), the method call is passed on to the cursor's state.

Each tool corresponds to a state. The shared abstract state class is AbstractTool:

According to this definition, each tool must handle movement of the mouse cursor and also the start and end of any click or drag.

Using that base class, simple pen and selection tools could look like this:

For this example, the class for the context is called Cursor. The methods named in the abstract state class (AbstractTool in this case) are also implemented in the context. In the context class, these methods invoke the corresponding method of the current state, represented by current_tool.