CODESAMPLE

State - Kotlin

State Kotlin

The State pattern allows an object to alter its behavior when its internal state changes. It encapsulates each state as a separate class and provides a way to transition between them. This avoids complex conditional logic and makes the code more maintainable and extensible by adding new states without modifying the original context class.

Here, the TrafficLight is the context, and RedState, YellowState, and GreenState represent the possible states. Each state handles the display() and nextState() logic specific to that state. The state transition is managed within the TrafficLight class, delegating behavior to the current state object. This implementation leverages Kotlin’s class structure and object-oriented approach, utilizing interfaces for state definition and clear, concise function implementations.

// Define the State interface
interface State {
    fun display()
    fun nextState()
}

// Concrete states
class RedState : State {
    override fun display() {
        println("RED")
    }

    override fun nextState() {
        println("Red -> Green")
    }
}

class YellowState : State {
    override fun display() {
        println("YELLOW")
    }

    override fun nextState() {
        println("Yellow -> Red")
    }
}

class GreenState : State {
    override fun display() {
        println("GREEN")
    }

    override fun nextState() {
        println("Green -> Yellow")
    }
}

// Context class
class TrafficLight(private var state: State = RedState()) {
    fun setState(state: State) {
        this.state = state
    }

    fun display() {
        state.display()
    }

    fun nextState() {
        state.nextState()
        when (state) {
            is RedState -> setState(GreenState())
            is GreenState -> setState(YellowState())
            is YellowState -> setState(RedState())
        }
    }
}

fun main() {
    val trafficLight = TrafficLight()
    trafficLight.display()
    trafficLight.nextState()
    trafficLight.display()
    trafficLight.nextState()
    trafficLight.display()
    trafficLight.nextState()
    trafficLight.display()
}