State - Harsh4999/Design-Patterns GitHub Wiki

State

USE

• State design pattern allows our objects to behave differently based on its internal state.
• This pattern allows to define the state specific behaviors in seperate classes.
• State transition can be triggered by themselves in which case each state knows about at least one other state's existence.
• We can add new states witthout changing main class

UML

• Context: Class whose state is now an object. Client code works with this class delegates operation to current state.Context holds a state object which defines the actual behavior of the class
• State: Interface which defines operations
• Concrete State: Implementation of an unique behavior of object tied to this state. They represent specific values which our state can take.

Implementation

• Identify values which will be a state for an object. Each state value becomes a specific class.
• We have to decide how our state transition will happen from one state to other.
• Client will be unaware of states.
• Ask a client to initialize the state after that states should transit by themselves.
• Using flyweight pattern we can share states which do not have any instance variables.
• Eg: Java Server Face pattern.
• Implementing this may lead to lot of classes. It may be complicated if states are high in number