TrafficLight

// State interface
interface TrafficLightState {
    void display();
}

// Concrete State: RedLightState
class RedLightState implements TrafficLightState {
    @Override
    public void display() {
        System.out.println("Red Light - Stop");
    }
}

// Concrete State: YellowLightState
class YellowLightState implements TrafficLightState {
    @Override
    public void display() {
        System.out.println("Yellow Light - Prepare to Stop or Go");
    }
}

// Concrete State: GreenLightState
class GreenLightState implements TrafficLightState {
    @Override
    public void display() {
        System.out.println("Green Light - Go");
    }
}

// Context: TrafficLight
class TrafficLight {
    private TrafficLightState currentState;

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

    public void setState(TrafficLightState state) {
        this.currentState = state;
    }

    public void changeState() {
        if (currentState instanceof RedLightState) {
            setState(new GreenLightState());
        } else if (currentState instanceof GreenLightState) {
            setState(new YellowLightState());
        } else if (currentState instanceof YellowLightState) {
            setState(new RedLightState());
        }
        currentState.display();
    }
}

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

        // Initial state: Red
        trafficLight.changeState();

        // Transition to next state: Green
        trafficLight.changeState();

        // Transition to next state: Yellow
        trafficLight.changeState();

        // Transition to next state: Red
        trafficLight.changeState();
    }
}

VendingMachine

// State interface
interface VendingMachineState {
    void insertCoin();
    void ejectCoin();
    void dispense();
}

// Concrete State: NoCoinState
class NoCoinState implements VendingMachineState {
    @Override
    public void insertCoin() {
        System.out.println("Coin inserted");
    }

    @Override
    public void ejectCoin() {
        System.out.println("No coin to eject");
    }

    @Override
    public void dispense() {
        System.out.println("Insert a coin first");
    }
}

// Concrete State: HasCoinState
class HasCoinState implements VendingMachineState {
    @Override
    public void insertCoin() {
        System.out.println("Coin already inserted");
    }

    @Override
    public void ejectCoin() {
        System.out.println("Coin ejected");
    }

    @Override
    public void dispense() {
        System.out.println("Product dispensed");
    }
}

// Concrete State: SoldState
class SoldState implements VendingMachineState {
    @Override
    public void insertCoin() {
        System.out.println("Please wait, product is being dispensed");
    }

    @Override
    public void ejectCoin() {
        System.out.println("Sorry, product is already dispensed");
    }

    @Override
    public void dispense() {
        System.out.println("Product dispensed. Enjoy!");
    }
}

// Context: VendingMachine
class VendingMachine {
    private VendingMachineState currentState;

    public VendingMachine() {
        // Initial state: NoCoinState
        currentState = new NoCoinState();
    }

    public void setState(VendingMachineState state) {
        this.currentState = state;
    }

    public void insertCoin() {
        currentState.insertCoin();
        if (currentState instanceof NoCoinState) {
            setState(new HasCoinState());
        } else if (currentState instanceof HasCoinState) {
            setState(new SoldState());
        }
    }

    public void ejectCoin() {
        currentState.ejectCoin();
        if (currentState instanceof HasCoinState) {
            setState(new NoCoinState());
        } else if (currentState instanceof SoldState) {
            setState(new NoCoinState());
        }
    }

    public void dispense() {
        currentState.dispense();
        if (currentState instanceof SoldState) {
            setState(new NoCoinState());
        }
    }
}

// Client Code
public class StatePatternExample {
    public static void main(String[] args) {
        VendingMachine vendingMachine = new VendingMachine();

        vendingMachine.insertCoin();
        vendingMachine.ejectCoin();

        vendingMachine.insertCoin();
        vendingMachine.insertCoin();
        vendingMachine.dispense();

        vendingMachine.insertCoin();
        vendingMachine.ejectCoin();
        vendingMachine.dispense();
    }
}