通俗易懂设计模式之状态模式

直接上代码：

Before

3-speed ceiling fan state machine

// Not good: unwieldy "case" statementclass CeilingFanPullChain{    private int m_current_state;    public CeilingFanPullChain()    {        m_current_state = 0;    }    public void pull()    {        if (m_current_state == 0)        {            m_current_state = 1;            System.out.println("   low speed");        }        else if (m_current_state == 1)        {            m_current_state = 2;            System.out.println("   medium speed");        }        else if (m_current_state == 2)        {            m_current_state = 3;            System.out.println("   high speed");        }        else        {            m_current_state = 0;            System.out.println("   turning off");        }    }}public class StateDemo{    public static void main(String[] args)    {        CeilingFanPullChain chain = new CeilingFanPullChain();        while (true)        {            System.out.print("Press ");            get_line();            chain.pull();        }    }    static String get_line()    {        BufferedReader in = new BufferedReader(new InputStreamReader(System.in))          ;        String line = null;        try        {            line = in.readLine();        }        catch (IOException ex)        {            ex.printStackTrace();        }        return line;    }}

Output

Press    low speedPress    medium speedPress    high speedPress    turning offPress    low speedPress    medium speedPress    high speedPress    turning off

After

The CeilingFanPullChain class is now a wrapper’ that delegates to its m_current_state reference. Each clause from the “before” case statement is now captured in a State derived class.

For this simple domain, the State pattern is probably over-kill.

class CeilingFanPullChain{    private State m_current_state;    public CeilingFanPullChain()    {        m_current_state = new Off();    }    public void set_state(State s)    {        m_current_state = s;    }    public void pull()    {        m_current_state.pull(this);    }}interface State{  void pull(CeilingFanPullChain wrapper);}class Off implements State{    public void pull(CeilingFanPullChain wrapper)    {        wrapper.set_state(new Low());        System.out.println("   low speed");    }}class Low implements State{    public void pull(CeilingFanPullChain wrapper)    {        wrapper.set_state(new Medium());        System.out.println("   medium speed");    }}class Medium implements State{    public void pull(CeilingFanPullChain wrapper)    {        wrapper.set_state(new High());        System.out.println("   high speed");    }}class High implements State{    public void pull(CeilingFanPullChain wrapper)    {        wrapper.set_state(new Off());        System.out.println("   turning off");    }}public class StateDemo{    public static void main(String[] args)    {        CeilingFanPullChain chain = new CeilingFanPullChain();        while (true)        {            System.out.print("Press ");            get_line();            chain.pull();        }    }    static String get_line()    {        BufferedReader in = new BufferedReader(new InputStreamReader(System.in))          ;        String line = null;        try        {            line = in.readLine();        }        catch (IOException ex)        {            ex.printStackTrace();        }        return line;    }}

State Pattern 主要解决的问题是对象的行为依赖于它的状态（属性），并且可以根据它的状态改变而改变它的相关行为。

使用场景：1、行为随状态（成员变量）改变而改变的场景。 2、条件、分支语句的代替者。

具体方法：将各种具体的状态类抽象出来。

优点：自行体会。

缺点：1、状态模式的使用必然会增加系统类和对象的个数。 2、状态模式的结构与实现都较为复杂，如果使用不当将导致程序结构和代码的混乱。 3、状态模式对”开闭原则”的支持并不太好，对于可以切换状态的状态模式，增加新的状态类需要修改那些负责状态转换的源代码，否则无法切换到新增状态，而且修改某个状态类的行为也需修改对应类的源代码。