设计模式学习（一）--简单工厂模式

工厂模式（Factory）调用者无需了解如何去实例化对象，仅需告知工厂类简单信息，调用工厂类方法生成具体的实现类。

举例 计算器开发过程中，通过工厂返回虚拟类Operation的具体运算实例，后续当需要修改某个运算的算法时，不会涉及其他运算类的修改，减少误修改因素。新增运算时，同时在工厂类方法中新增分支即可。

abstract Operation 运算虚拟类

public abstract class Operation {
    private double numberA = 0;
    private double numberB = 0;

    public double getNumberA() {
        return numberA;
    }

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

    public double getNumberB() {
        return numberB;
    }

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

    public abstract double getResult();
}

加法实现类

public class OperationAdd extends Operation {
    @Override
    public double getResult() {
        return this.getNumberA() + this.getNumberB();
    }
}

减法实现类

public class OperationSub extends Operation {
    @Override
    public double getResult() {
        return this.getNumberA() - this.getNumberB();
    }
}

乘法实现类

public class OperationMul extends Operation {
    @Override
    public double getResult() {
        return this.getNumberA() * this.getNumberB();
    }
}

除法实现类

public class OperationDiv extends Operation {
    @Override
    public double getResult() {
        return this.getNumberA() / this.getNumberB();
    }
}

工厂类

public class OperationFactory {
    public static Operation createOperation(String operate){
        switch (operate){
            case "+":
                return new OperationAdd();
            case "-":
                return new OperationSub();
            case "*":
                return new OperationMul();
            case "/":
                return new OperationDiv();
            default:
                return null;
        }
    }
}

调用

无需关心具体的实例化方法

public static void main(String[] args) throws IOException {
    Operation oper = OperationFactory.createOperation("+");
    oper.setNumberA(5);
    oper.setNumberB(2);
    System.out.println(oper.getResult());
}

但是简单工厂违背了封闭-开放原则，这一点会在后面使用工厂方法模式的介绍中克服。