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The State design pattern allows an object to dramatically change its behavior that depends on the state of this object. It replaces complex branching inside the object and separates the behavior of concrete states into separate classes. It's very similar to the Strategy design pattern where, on the contrary, it's the client who chooses the specific strategy (the behavior given by the object instance).


If we perform an algorithm that reacts strongly to the internal state of the object, we usually have to perform a complex branching of the state inside its methods using the switch construct or the if ... else if ... sequence. Imagine, for example, that we program the ghost AI in the Pacman game. They usually change their behavior to chase Pacman. After a while, they just move randomly in a maze, and if the ghost is blue (the player picked a power-up), they even run away from Pacman. The ghost will have, for example,


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The State design pattern allows an object to dramatically change its behavior, which depends on the object's state. Replaces complex branching.

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Article has been written for you by David Capka
The author is a programmer, who likes web technologies and being the lead/chief article writer at He shares his knowledge with the community and is always looking to improve. He believes that anyone can do what they set their mind to.
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