简单的工厂模式

                                  ## 简单工厂模式 ##

什么是简单工厂模式

**简单的工厂模式**：到底要实例谁，将来会不会增加实例化的对象，都应该考虑用一个单独的类来做这个创造实例的过程，这就是工厂。

步骤一

public class OperationObject {
    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;
    }
    public double getResult(){
        double result = 0;
        return result;
    }
}

步骤二

//加法
public class OperationAdd extends OperationObject {
    public double getResult(){
        double result = 0;
        result = get_numberA()+get_numberB();
        return result;
    }
}
//加法
public class OperationDiv extends OperationObject{
    public double getResult() {
        double result = 0;
        if (get_numberB() == 0){
            try {
                throw new Exception("除数不能为零");
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        result = get_numberA() / get_numberB();
        return result;
    }
}
//乘法
public class OperationMul extends OperationObject {
    public double getResult(){
        double result = 0;
        result = get_numberA() * get_numberB();
        return result;
    }
}
//除法
public class OperationSub extends OperationObject {
    public double getResult(){
        double result = 0;
        result = get_numberA() - get_numberB();
        return result;
    }
}

步骤三

//简单的工厂模式：到底要实例谁，将来会不会增加实例化的对象，都应该考虑用一个单独的类来做这个创造实例的过程，这就是工厂。
//一句话就是一个工厂里面装着一堆需要实例的对象
public class OperationFactory {
    public static OperationObject createOperation(String operation){
        //负责生产加减乘除
        OperationObject operationObject = null;
        switch (operation){
            case "+":
                operationObject = new OperationAdd();
                break;
            case "-":
                operationObject = new OperationSub();
                break;
            case "*":
                operationObject = new OperationMul();
                break;
            case "/":
                operationObject = new OperationDiv();
                break;
        }
        return operationObject;
    }
}

步骤四

public class factoryDesign {
    public static void main(String[] args) {
        //实例所需要的计算类型
        OperationObject operationObject = OperationFactory.createOperation("-");
        operationObject.set_numberA(2);
        operationObject.set_numberB(4);
        double result = operationObject.getResult();
        System.out.println(result);
    }
}

简单工厂的问题就在于swich case（或者if else)。每当新增一种产品时，你都需要去维护工厂中的判断语句，造成该工厂类不断增长，各种判断挤在一起，给扩展和维护带来麻烦。产品和工厂没有实现完全解耦，绑定在一起。
以上内容为个人学习大话设计模式后笔记，不喜勿喷。