State

September 10, 2014

Author：Eric
Source：http://blog.wjin.org/posts/state.html
Declaration: this work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

State

Introduction

The state pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class. And the UML graph is similar to Strategy pattern.

Example

Last weekend, I watched a F1 match. And noticed that Hamilton ran into an issue when starting because the car’s start mode button did not work. A conspiracy?

Also I noticed that there is a burst mode of the engine that driveres can use to get much more power with a little more fuel.

So here I take car as an example as I always take the things I am familar with. Normally, a car has three modes: start mode, normal mode and burst mode. As a state pattern, it is easy to add other modes, so auto mode is added to my fancy car because of Google’s advanced unmaned technology. I wish there is a fly mode in the near future :(

Code

Cpp

As there are two classes referenced with each other, to pass build we need to split header file and cpp file.

// File State.h
class Ferrari;

// abstract car state
class CarState
{
public:
    virtual void drive() = 0;
    virtual ~CarState() {}
};

class StartState : public CarState
{
private:
    Ferrari* m_car;

public:
    StartState(Ferrari* car);
    void drive();
};

class NormalState : public CarState
{
private:
    Ferrari* m_car;

public:
    NormalState(Ferrari *car);
    void drive();
};

class BurstState : public CarState
{
private:
    Ferrari* m_car;

public:
    BurstState(Ferrari *car);
    void drive();
};

class AutoState : public CarState
{
private:
    Ferrari* m_car;

public:
    AutoState(Ferrari *car);
    void drive();
};

class Ferrari
{
private:
    // all states
    CarState* m_startState; // start state
    CarState* m_normalState; // normal state
    CarState* m_burstState; // burst state
    CarState* m_autoState; // auto state

    CarState* m_state; // current car state
    int m_speed; // current car speed

    void setState(CarState* m)
    {
        m_state = m;
    }

public:
    void setSpeed(int n)
    {
        m_speed = n;
    }

    Ferrari();
    ~Ferrari();

    void pushStartStateButton();
    void pushNormalStateButton();
    void pushBurstStateButton();
    void pushAutoStateButton();

    void drive();
};


// File State.cpp

#include <iostream>
#include "State.h"

using namespace std;

StartState::StartState(Ferrari* car)
{
    m_car = car;
}

void StartState::drive()
{
    m_car->setSpeed(10);
    cout << "Start State: help start smoothly" << endl;
}

NormalState::NormalState(Ferrari *car)
{
    m_car = car;
}

void NormalState::drive()
{
    m_car->setSpeed(50);
    cout << "Normal State: have fun" << endl;
}

BurstState::BurstState(Ferrari *car)
{
    m_car = car;
}

void BurstState::drive()
{
    m_car->setSpeed(100);
    cout << "Burst State: please concentrate" << endl;
}

AutoState::AutoState(Ferrari *car)
{
    m_car = car;
}

void AutoState::drive()
{
    m_car->setSpeed(30);
    cout << "Auto State: relax, just enjoy" << endl;
}

Ferrari::Ferrari()
    : m_startState(new StartState(this)),
      m_normalState(new NormalState(this)),
      m_burstState(new BurstState(this)),
      m_autoState(new AutoState(this))
{
    setState(m_normalState);
    setSpeed(0);
}

Ferrari::~Ferrari()
{
    delete m_startState;
    delete m_normalState;
    delete m_burstState;
    delete m_autoState;
}

void Ferrari::pushStartStateButton()
{
    setState(m_startState);
}

void Ferrari::pushNormalStateButton()
{
    setState(m_normalState);
}

void Ferrari::pushBurstStateButton()
{
    // security check
    if (typeid(*m_state) != typeid(NormalState)) {
        cout << "Warnning: cannot enter burst state" << endl;
        return;
    }

    setState(m_burstState);
}

void Ferrari::pushAutoStateButton()
{
    setState(m_autoState);
}

void Ferrari::drive()
{
    m_state->drive();
    cout << "Current Speed:" << m_speed << endl;
}

int main(int argc, char *argv[])
{
    Ferrari car;

    car.pushStartStateButton();
    car.drive();

    car.pushNormalStateButton();
    car.drive();

    car.pushBurstStateButton();
    car.drive();

    car.pushAutoStateButton();
    car.drive();

    car.pushBurstStateButton();
    car.drive();

    car.pushNormalStateButton();
    car.drive();

    return 0;
}

Java

// abstract car state
interface CarState {
	public void drive();
}

class StartState implements CarState {
	private final Ferrari car;

	public StartState(Ferrari car) {
		this.car = car;
	}

	public void drive() {
		car.setSpeed(10);
		System.out.println("Start State: help start smoothly");
	}
}

class NormalState implements CarState {
	private final Ferrari car;

	public NormalState(Ferrari car) {
		this.car = car;
	}

	public void drive() {
		car.setSpeed(50);
		System.out.println("Normal State: have fun");
	}
}

class BurstState implements CarState {
	private final Ferrari car;

	public BurstState(Ferrari car) {
		this.car = car;
	}

	public void drive() {
		car.setSpeed(100);
		System.out.println("Burst State: please concentrate");
	}
}

class AutoState implements CarState {
	private final Ferrari car;

	public AutoState(Ferrari car) {
		this.car = car;
	}

	public void drive() {
		car.setSpeed(30);
		System.out.println("Auto State: relax, just enjoy");
	}
}

class Ferrari {
	// all states
	private final CarState m_startState; // start state
	private final CarState m_normalState; // normal state
	private final CarState m_burstState; // burst state
	private final CarState m_autoState; // auto state

	private CarState m_state; // current car state
	private int m_speed; // current car speed

	private void setState(CarState state) {
		m_state = state;
	}

	void setSpeed(int speed) {
		m_speed = speed;
	}

	public Ferrari() {
		m_startState = new StartState(this);
		m_normalState = new NormalState(this);
		m_burstState = new BurstState(this);
		m_autoState = new AutoState(this);

		setState(m_normalState);
		setSpeed(0);
	}

	public void pushStartStateButton() {
		setState(m_startState);
	}

	public void pushNormalStateButton() {
		setState(m_normalState);
	}

	public void pushBurstStateButton() {
		// security check
		if (!(m_state instanceof NormalState)) {
			System.out.println("Warnning: cannot enter burst state");
			return;
		}

		setState(m_burstState);
	}

	public void pushAutoStateButton() {
		setState(m_autoState);
	}

	public void drive() {
		m_state.drive();
		System.out.println("Current Speed:" + m_speed);
	}
}

public class State {
	public static void main(String[] args) {
		Ferrari car = new Ferrari();

		car.pushStartStateButton();
		car.drive();

		car.pushNormalStateButton();
		car.drive();

		car.pushBurstStateButton();
		car.drive();

		car.pushAutoStateButton();
		car.drive();

		car.pushBurstStateButton();
		car.drive();

		car.pushNormalStateButton();
		car.drive();

	}
}

State

State

Introduction

Example

Code

Cpp

Java

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