设计模式之状态模式

发布时间：2020-12-13 20:55:49 所属栏目：百科来源：网络整理

导读：情景：有一个自动投币的糖果机。它有以下几种状态（每个圆圈代表一个状态）：如果用常量值来定义几个变量 SOLD_OUT = 0 NO_QUARTER = 1 HAS_QUARTER = 2 SOLD = 3 span style="color: #0000ff;"int state = SOLD_OUT; 这样状态变化就可以写成： (state == (

情景：有一个自动投币的糖果机。它有以下几种状态（每个圆圈代表一个状态）：

如果用常量值来定义几个变量

SOLD_OUT = 0 NO_QUARTER = 1 HAS_QUARTER = 2 SOLD = 3<span style="color: #0000ff;">int state = SOLD_OUT;

这样状态变化就可以写成：

(state == (state ==

每一个动作都要先判断状态，改后再决定反馈以及下一个状态是什么。

虽然可以实现要求，但是当出现新的状态的时候，很不利于扩展。

我们把状态变成一个类，把不同的状态封装在各自的类中，然后在动作发生时委托给当前状态。

首先，定义状态接口

然后定义各种状态

NoQuarterState

GumballMachine gumballMachine;

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; NoQuarterState(GumballMachine gumballMachine) {
    </span><span style="color: #0000ff;"&gt;this</span>.gumballMachine =<span style="color: #000000;"&gt; gumballMachine; 
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; insertQuarter() {
    System.out.println(</span>"You insert a quarter"<span style="color: #000000;"&gt;);
    gumballMachine.setState(gumballMachine.getHasQuarterState());
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; ejectQuarter() {
    System.out.println(</span>"You haven't inserted a quarter"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; turnCrank() {
    System.out.println(</span>"You turned,but there's no quarter"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; dispense() {
    System.out.println(</span>"You need to pay first"<span style="color: #000000;"&gt;);
}

}

  HasQuarterState GumballMachine gumballMachine;

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; HasQuarterState(GumballMachine gumballMachine) {
    </span><span style="color: #0000ff;"&gt;this</span>.gumballMachine =<span style="color: #000000;"&gt; gumballMachine;
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; insertQuarter() {
    System.out.println(</span>"You can't insert another quater"<span style="color: #000000;"&gt;);        
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; ejectQuarter() {
    System.out.println(</span>"Quarter returned"<span style="color: #000000;"&gt;);
    gumballMachine.setState(gumballMachine.getNoQuarterState());
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; turnCrank() {
    System.out.println(</span>"You turned..."<span style="color: #000000;"&gt;);
    gumballMachine.setState(gumballMachine.getSoldState());
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; dispense() {
    System.out.println(</span>"No gumball dispensed"<span style="color: #000000;"&gt;);
}

}

  SoldOutState GumballMachine gumballMachine;

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; SoldOutState(GumballMachine gumballMachine) {
    </span><span style="color: #0000ff;"&gt;this</span>.gumballMachine =<span style="color: #000000;"&gt; gumballMachine;
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; insertQuarter() {
    System.out.println(</span>"You can't insert a quarter,the machine is sold out"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; ejectQuarter() {
    System.out.println(</span>"You can't eject,you haven't inserted a quarter yet"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; turnCrank() {
    System.out.println(</span>"You turned,but there are no gumballs"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; dispense() {
    System.out.println(</span>"No gumball dispensed"<span style="color: #000000;"&gt;);
}

}

  SoldState GumballMachine gumballMachine;

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; SoldState(GumballMachine gumballMachine) {
    </span><span style="color: #0000ff;"&gt;this</span>.gumballMachine =<span style="color: #000000;"&gt; gumballMachine;
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; insertQuarter() {
    System.out.println(</span>"Please wait,we're already giving you a gumball"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; ejectQuarter() {
    System.out.println(</span>"Sorry,you already turned the crank"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; turnCrank() {
    System.out.println(</span>"Turning twice doesn't get you another gumball!"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; dispense() {
    gumballMachine.releaseBall();
    </span><span style="color: #0000ff;"&gt;if</span> (gumballMachine.getCount() > 0<span style="color: #000000;"&gt;) {
        gumballMachine.setState(gumballMachine.getNoQuarterState());
    } </span><span style="color: #0000ff;"&gt;else</span><span style="color: #000000;"&gt; {
        System.out.println(</span>"Oops,out of gumballs!"<span style="color: #000000;"&gt;);
        gumballMachine.setState(gumballMachine.getSoldOutState());
    }
}

}

最后写一下糖果机

State soldOutState;
State noQuarterState;
State hasQuarterState;
State soldState;

State state </span>=<span style="color: #000000;"&gt; soldOutState;
</span><span style="color: #0000ff;"&gt;int</span> count = 0<span style="color: #000000;"&gt;;

</span><span style="color: #0000ff;"&gt;public</span> GumballMachine(<span style="color: #0000ff;"&gt;int</span><span style="color: #000000;"&gt; numberGumballs) {
    soldOutState </span>= <span style="color: #0000ff;"&gt;new</span> SoldOutState(<span style="color: #0000ff;"&gt;this</span><span style="color: #000000;"&gt;);
    noQuarterState </span>= <span style="color: #0000ff;"&gt;new</span> NoQuarterState(<span style="color: #0000ff;"&gt;this</span><span style="color: #000000;"&gt;);
    hasQuarterState </span>= <span style="color: #0000ff;"&gt;new</span> HasQuarterState(<span style="color: #0000ff;"&gt;this</span><span style="color: #000000;"&gt;);
    soldState </span>= <span style="color: #0000ff;"&gt;new</span> SoldState(<span style="color: #0000ff;"&gt;this</span><span style="color: #000000;"&gt;);

    count </span>=<span style="color: #000000;"&gt; numberGumballs;
    </span><span style="color: #0000ff;"&gt;if</span> (numberGumballs > 0<span style="color: #000000;"&gt;) {
        state </span>=<span style="color: #000000;"&gt; noQuarterState;
    }
}

</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; insertQuarter() {
    state.insertQuarter();
}

</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; ejectQuarter() {
    state.ejectQuarter();
}

</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; turnCrank() {
    state.turnCrank();
    state.dispense();
}

</span><span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; setState(State state) {
    </span><span style="color: #0000ff;"&gt;this</span>.state =<span style="color: #000000;"&gt; state;
}

</span><span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; releaseBall() {
    System.out.println(</span>"A gumball comes rolling out the slot..."<span style="color: #000000;"&gt;);
    </span><span style="color: #0000ff;"&gt;if</span> (count != 0<span style="color: #000000;"&gt;) {
        count </span>-= 1<span style="color: #000000;"&gt;;
    }
}

</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;int</span><span style="color: #000000;"&gt; getCount() {
    </span><span style="color: #0000ff;"&gt;return</span><span style="color: #000000;"&gt; count; 
}

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; State getSoldOutState() {
    </span><span style="color: #0000ff;"&gt;return</span><span style="color: #000000;"&gt; soldOutState;
}

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; State getNoQuarterState() {
    </span><span style="color: #0000ff;"&gt;return</span><span style="color: #000000;"&gt; noQuarterState;
}

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; State getHasQuarterState() {
    </span><span style="color: #0000ff;"&gt;return</span><span style="color: #000000;"&gt; hasQuarterState;
}

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; State getSoldState() {
    </span><span style="color: #0000ff;"&gt;return</span><span style="color: #000000;"&gt; soldState;
}

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; String toString() {    
    </span><span style="color: #0000ff;"&gt;return</span> "nMighty Gumball,Incn" + "Inventory: " +<span style="color: #000000;"&gt; 
            count </span>+ " gumballsn"<span style="color: #000000;"&gt;;
}

}

测试类：

= GumballMachine(5

    System.out.println(gumballMachine);

    gumballMachine.insertQuarter();
    gumballMachine.turnCrank();

    System.out.println(gumballMachine);

    gumballMachine.insertQuarter();
    gumballMachine.ejectQuarter();
    gumballMachine.turnCrank();

    System.out.println(gumballMachine);

    gumballMachine.insertQuarter();
    gumballMachine.turnCrank();
    gumballMachine.insertQuarter();
    gumballMachine.turnCrank();
    gumballMachine.ejectQuarter();

    System.out.println(gumballMachine);

    gumballMachine.insertQuarter();
    gumballMachine.insertQuarter();
    gumballMachine.turnCrank();
    gumballMachine.insertQuarter();
    gumballMachine.turnCrank();
    gumballMachine.insertQuarter();
    gumballMachine.turnCrank();

    System.out.println(gumballMachine);

}

}

将每一个状态局部化到它自己的类中。将容易产生问题的if语句删除，方便日后的维护。

这样，每一个状态“对修改关闭”，糖果机“对扩展开放”

状态模式：允许对象在内部状态改变时改变他的行为，对象看起来好像修改了它的类。

类图：

现在我们改变需求，希望有10%的概率中奖，也就是投一个币，获得两个糖果。

我们只需要添加一个新的状态类。

WinnerState

GumballMachine gumballMachine;

</span><span style="color: #0000ff;"&gt;public</span><span style="color: #000000;"&gt; WinnerState(GumballMachine gumballMachine) {
    </span><span style="color: #0000ff;"&gt;this</span>.gumballMachine =<span style="color: #000000;"&gt; gumballMachine;
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; insertQuarter() {
    System.out.println(</span>"Please wait,you already turned the crank"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; turnCrank() {
    System.out.println(</span>"Turning twice doesn't get you another gumball!"<span style="color: #000000;"&gt;);
}

@Override
</span><span style="color: #0000ff;"&gt;public</span> <span style="color: #0000ff;"&gt;void</span><span style="color: #000000;"&gt; dispense() {
    System.out.println(</span>"YOU'RE A WINNER! YOU GET TWO GUMBALLS FOR YOUR QUARTERS"<span style="color: #000000;"&gt;);
    gumballMachine.releaseBall();
    </span><span style="color: #0000ff;"&gt;if</span> (gumballMachine.getCount() == 0<span style="color: #000000;"&gt;) {
        gumballMachine.setState(gumballMachine.getSoldOutState());
    } </span><span style="color: #0000ff;"&gt;else</span><span style="color: #000000;"&gt; {
        gumballMachine.releaseBall();
        </span><span style="color: #0000ff;"&gt;if</span> (gumballMachine.getCount() > 0<span style="color: #000000;"&gt;) {
            gumballMachine.setState(gumballMachine.getNoQuarterState());
        } </span><span style="color: #0000ff;"&gt;else</span><span style="color: #000000;"&gt; {
            System.out.println(</span>"Oops,out of gumballs!"<span style="color: #000000;"&gt;);
            gumballMachine.setState(gumballMachine.getSoldOutState());
        }
    }
}

}

在HasQuarterState中有几率跳到WinnerState状态：

HasQuarterState

GumballMachine gumballMachine;
Random randomWinner </span>= <span style="color: #0000ff;"&gt;new</span><span style="color: #000000;"&gt; Random(System.currentTimeMillis());

</span><span style="color: #008000;"&gt;//</span><span style="color: #008000;"&gt; ....</span>

<span style="color: #000000;">
@Override
<span style="color: #0000ff;">public <span style="color: #0000ff;">void<span style="color: #000000;"> turnCrank() {
System.out.println("You turned..."<span style="color: #000000;">);
<span style="color: #0000ff;">int winner = randomWinner.nextInt(10<span style="color: #000000;">);
<span style="color: #0000ff;">if (winner == 0 && (gumballMachine.getCount() > 1<span style="color: #000000;">)) {
gumballMachine.setState(gumballMachine.getWinnerState());
} <span style="color: #0000ff;">else<span style="color: #000000;"> {
gumballMachine.setState(gumballMachine.getSoldState());
}
}

</span><span style="color: #008000;"&gt;//</span><span style="color: #008000;"&gt; ....</span>

<span style="color: #000000;">
}

记得在糖果机类中添加一个WinnerState状态类，这个新的需求就实现了。

状态模式允许Context随着状态的改变而改变行为。
状态装换可以由State类或者Context类控制。
使用状态模式通常会导致设计中的类数目大量增加。
状态类可以被多个Context实例共享。

（编辑：李大同）

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