设计模式之简单工厂模式

　　在编写一个计算器程序时，可以将业务逻辑和显示分离，业务逻辑封装为一个类（封装）；如果要新添加一种运算，可以先创建一个Operation的基类，然后各种运算从Operation类继承，并实现GetResult()虚函数，这时添加新的运算只需要派生一个新的类，即不需要之前的运算参与编译。如何让计算器知道我是希望使用哪种运算呢？应该考虑单独的类来做这个创造实例的过程，这就是工厂。创建一个OperationFactory类，传入参数，函数createOperate就可以实例化出合适的对象。 

　　Java代码如下：

public class OperationFactory {
    public static abstract class Operation
    {        
        private double _numberA = 0;
        private double _numberB = 0;
        
        public double get_numberA() {
            return _numberA;
        }

        public void set_numberA(double _numberA) {
            this._numberA = _numberA;
        }

        public double get_numberB() {
            return _numberB;
        }

        public void set_numberB(double _numberB) {
            this._numberB = _numberB;
        }
        
        abstract double GetResult();
        // TODO Auto-generated constructor stub
    }
    
    public static class OperationAdd extends Operation
    {
        double GetResult() 
        {
            double result = get_numberA() + get_numberB();
            return result;
        }        
    }
    
    public static class OperationSub extends Operation
    {
        double GetResult() {
            double result = get_numberA() - get_numberB();
            return result;    
        }
    }
    
    
    public static Operation createOperate(String operate){
        Operation oper = null;
        if (operate.equals("+")) {
            oper = new OperationAdd();
        }
        else if (operate.equals("-")) {
            oper = new OperationSub();
        }

        return oper;
    }

    public static void main(String[] args) {
        Operation oper;
        oper = OperationFactory.createOperate("+");
        oper.set_numberA(1);
        oper.set_numberB(2);
        double result = oper.GetResult();
    }
}