State Pattern

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

GIF

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

eidherjulian61 / design-patterns

Main Design Patterns

eidher ・ Sep 27 '20 ・ 1 min read

#designpatterns #creational #structural #behavioral