What is the difference between strategy design pattern and State design pattern?

State Pattern defines the “what” and “when” part of an Object. Example: What can an object when it’s in a certain state. Strategy pattern defines the “How” part of an Object. Example: How a Sorting object sorts data.

State design pattern is used to define and manage state of an object, while Strategy pattern is used to define a set of interchangeable algorithm and lets client to choose one of them. So Strategy pattern is a client driven pattern while Object can manage there state itself.

The strategy pattern allows you to change the implementation of something used at runtime . The decorator pattern allows you augment (or add to) existing functionality with additional functionality at run time.