简单工厂模式（四则运算）

1. 目的
       工厂模式就是专门负责将大量有共同接口的类实例化,而且不必事先知道每次是要实例化哪一个类的模式。它定义一个用于创建对象的接口，由子类决定实例化哪一个类。

2.举例（两个数的加减乘除运算）
/**
* 定义一个得到运算结果的接口
* @author Michael
*
*/
public interface Operation {
    public double getResult(double numberA,double numberB);
}

/**
* 加法运算
* @author Michael
*
*/
public class OperationAdd implements Operation {
@Override
public double getResult(double numberA, double numberB) {
        double result = 0;
        result = numberA + numberB;
        return result;      
}
}

/**
* 减法运算
* @author Michael
*
*/
public class OperationSub implements Operation {
@Override
public double getResult(double numberA, double numberB) {
        double result = 0;
        result = numberA - numberB;
        return result;      
}
}

/**
* 乘法运算
* @author Michael
*
*/
public class OperationMul implements Operation {
@Override
public double getResult(double numberA, double numberB) {
        double result = 0;
        result = numberA * numberB;
        return result;      
}
}

/**
* 除法运算
* @author Michael
*
*/
public class OperationDiv implements Operation {
@Override
public double getResult(double numberA, double numberB) {
        double result = 0;
        if (numberB==0) {
throw new IllegalArgumentException("除数不能为零");
}else {
result = numberA / numberB;
}     
        return result;      
}
}

/**
* 测试类
* @author Michael
*
*/
public class Test {
public static void main(String[] args) {
// TODO Auto-generated method stub

Operation operation = null;
operation = OperationFactory.createOperate("-");
double numberA = 3;
double numberB = 1;
System.out.println(operation.getResult(numberA, numberB));
}
}

参考大话设计模式，附件贴出代码。