State Method

It allow an object to alter its behavior when its internal state changes. This pattern is particularly useful for implementing state machines, where the object behaves differently based on its current state. Instead of using a large number of conditional statements to manage state changes, this pattern encapsulates state-specific behavior in separate classes.

Structure

• Context: The class that has a current state and can change its behavior based on that state.
• State Interface: An interface that defines the methods that concrete state classes must implement.
• Concrete States: Classes that implement the State interface and define the behavior for a specific state.

Example

// State Interface
interface TrafficLightState {
    void change(TrafficLight context);  // Method to change state
    String getColor();                   // Method to get the current color
}

// Concrete State for Red Light
class RedLightState implements TrafficLightState {
    @Override
    public void change(TrafficLight context) {
        System.out.println("Changing light from Red to Green.");
        context.setState(new GreenLightState()); // Change to Green state
    }

    @Override
    public String getColor() {
        return "Red"; // Return current color
    }
}

// Concrete State for Green Light
class GreenLightState implements TrafficLightState {
    @Override
    public void change(TrafficLight context) {
        System.out.println("Changing light from Green to Yellow.");
        context.setState(new YellowLightState()); // Change to Yellow state
    }

    @Override
    public String getColor() {
        return "Green"; // Return current color
    }
}

// Concrete State for Yellow Light
class YellowLightState implements TrafficLightState {
    @Override
    public void change(TrafficLight context) {
        System.out.println("Changing light from Yellow to Red.");
        context.setState(new RedLightState()); // Change to Red state
    }

    @Override
    public String getColor() {
        return "Yellow"; // Return current color
    }
}

// Context Class
class TrafficLight {
    private TrafficLightState currentState; // Current state of the traffic light

    public TrafficLight() {
        currentState = new RedLightState(); // Initial state
    }

    public void setState(TrafficLightState state) {
        this.currentState = state; // Change the current state
    }

    public void change() {
        currentState.change(this); // Delegate to the current state's change method
    }

    public String getColor() {
        return currentState.getColor(); // Delegate to the current state's getColor method
    }
}

// Client code
public class Main {
    public static void main(String[] args) {
        TrafficLight trafficLight = new TrafficLight();

        // Current color
        System.out.println("Current Light: " + trafficLight.getColor());
        
        // Change the light
        trafficLight.change();
        System.out.println("Current Light: " + trafficLight.getColor());

        // Change the light
        trafficLight.change();
        System.out.println("Current Light: " + trafficLight.getColor());

        // Change the light
        trafficLight.change();
        System.out.println("Current Light: " + trafficLight.getColor());
    }
}