state

State Design Pattern

Intent: Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.

Checklist

• Define a "context" class to present a single interface to the outside world.

• Define a State abstract base class.

• Represent the different "states" of the state machine as derived classes of the State base class.

• Define state-specific behavior in the appropriate State derived classes.

• Maintain a pointer to the current "state" in the "context" class.

To change the state of the state machine, change the current "state" pointer.

Example

WallFan has a pulling chain. Initially, it's off. If user pull chain, then current state Off change to next state.

Then, if user pull again, it's state change to next state. All states Off, Low, Medium, High changes its state to next one using context object WallFanPullingChain.

Test

WallFanPullingChain pullingChain = new WallFanPullingChain();

// initial state is Off
Assertions.assertInstanceOf(Off.class, pullingChain.getCurrentState());

// pull chain will change state to Low
pullingChain.pull();
Assertions.assertInstanceOf(Low.class, pullingChain.getCurrentState());

// pull again will change state to Medium        
pullingChain.pull();
Assertions.assertInstanceOf(Medium.class, pullingChain.getCurrentState());

// pull again will change state to High            
pullingChain.pull();
Assertions.assertInstanceOf(High.class, pullingChain.getCurrentState());
   
// pull again will change state to Off                       
pullingChain.pull();
Assertions.assertInstanceOf(Off.class, pullingChain.getCurrentState());