简单工厂模式

• 出现的背景：当一个模块中使用到了一类对象，如果在使用到的位置直接具体创建出各种对象（显示new），则不易于该模块的扩展，因为当该类对象出现一种新对象时，需要修改该模块的代码，而简单工厂模式可以解决这个问题。
• 作用：专门负责某类对象（实例）的创建。更具体的来说，就是在使用这类对象的模块中通过工厂中的方法，根据用户需求创建出相应的对象实例，并令父类引用指向这个对象实例（多态），从而实现了模块与这类对象的分离，当扩展了一种新对象时，只需要修改工厂方法，而这个模块无需变动。

• 实例如下：

  • 首先定义运算器接口

    public interface Operation {
    void setNumberA(double numberA);
    void setNumberB(double numberB);
    double getResult();
    }

  • 定义该运算器接口的加、减、乘、除四种实现类 ```java class OperationAdd implements Operation { public double numberA; public double numberB;

    @Override public double getResult() { return numberA + numberB; }

    @Override public void setNumberA(double numberA) { this.numberA = numberA; }

    @Override public void setNumberB(double numberB) { this.numberB = numberB; } }

class OperationSub implements Operation { public double numberA; public double numberB;

@Override
public double getResult() {
    return numberA - numberB;
}
@Override
public void setNumberA(double numberA) {
    this.numberA = numberA;
}
@Override
public void setNumberB(double numberB) {
    this.numberB = numberB;
}

}

class OperationMul implements Operation { public double numberA; public double numberB;

@Override
public double getResult() {
    return numberA * numberB;
}
@Override
public void setNumberA(double numberA) {
    this.numberA = numberA;
}
@Override
public void setNumberB(double numberB) {
    this.numberB = numberB;
}

}

class OperationDiv implements Operation { public double numberA; public double numberB;

@Override
public double getResult() {
    return numberA / numberB;
}
@Override
public void setNumberA(double numberA) {
    this.numberA = numberA;
}
@Override
public void setNumberB(double numberB) {
    this.numberB = numberB;
}

}

   - 定义简单工厂，负责Operation实现类的创建
```java
public class OperationSimpleFactory {
    public static Operation createOperation(String operator){
        switch (operator){
            case "+": return new OperationAdd();
            case "-": return new OperationSub(); 
            case "*": return new OperationMul();
            case "/": return new OperationDiv();
            default: return null;
        }
    }
}

• 客户端使用简单工厂创建运算器对象

  public class CalculatorClient {
  public static void main(String[] args) {
     try {
         Scanner scanner = new Scanner(System.in);
         System.out.println("请输入第一个运算数");
         double numberA = scanner.nextDouble();
         System.out.println("请输入运算符");
         String operator = scanner.next();
         System.out.println("请输入第二个运算数");
         double numberB = scanner.nextDouble();
         Operation operation = OperationSimpleFactory.createOperation(operator);
         operation.setNumberA(numberA);
         operation.setNumberB(numberB);
         double result = operation.getResult();
         System.out.println(numberA + " " + operator + " " + numberB + " = " + result);
     } catch (Exception e){
         e.printStackTrace();
     }
  }
  }