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URL: https://dev.to/eidher/state-pattern-3kki

⇱ State Pattern - DEV Community

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

👁 Alt Text

Participants

  • Context: defines the interface of interest to clients Maintains an instance of a ConcreteState subclass that defines the current state.
  • State: defines an interface for encapsulating the behavior associated with a particular state of the Context.
  • Concrete State: each subclass implements a behavior associated with a state of Context

Code 

public class Main {

 public static void main(String[] args) {
 Context c = new Context(new ConcreteStateA());
 c.request();
 c.request();
 c.request();
 c.request();
 }
}

public interface State {
 void handle(Context context);
}

public class ConcreteStateA implements State {

 @Override
 public void handle(Context context) {
 context.setState(new ConcreteStateB());
 }
}

public class ConcreteStateB implements State {

 @Override
 public void handle(Context context) {
 context.setState(new ConcreteStateA());
 }
}

public class Context {

 private State state;

 public Context(State state) {
 setState(state);
 }

 public State getState() {
 return state;
 }

 public void setState(State state) {
 this.state = state;
 System.out.println("State: " + state.getClass().getSimpleName());
 }

 public void request() {
 state.handle(this);
 }
}

Output 

State: ConcreteStateA
State: ConcreteStateB
State: ConcreteStateA
State: ConcreteStateB
State: ConcreteStateA