设计模式——简单工厂模式

1 概述

将创造对象的工作交给一个单独的类来实现，这个单独的类就是工厂。

2 实现

假设要做一个计算器的需求，通常我们想到的是这样写：

package com.example.easyfactory;import java.util.Scanner;public class Demo1 {public static void main(String[] args) {try {Scanner sc =new Scanner(System.in);System.out.println("请输入数字A：");double numberA = Double.parseDouble(sc.nextLine());System.out.println("请输入运算符（+、-、*、/）：");String strOperate = sc.nextLine();System.out.println("请输入数字B：");double numberB = Double.parseDouble(sc.nextLine());double result = 0d;switch (strOperate) {case "+":result = numberA + numberB;break;case "-":result = numberA - numberB;break;case "*":result = numberA * numberB;break;case "/":result = numberA / numberB;break;}System.out.println("结果是：" + result);}catch (Exception e) {System.out.println("输入有错");}}
}

但这段代码的问题在于，其一显示和业务逻辑没有分离，其二是没有使用面向对象的编程思想，可扩展性和可重用性都很差。
可以考虑使用简单工厂模式，将各个计算逻辑进行分离，成为单独的类，在工厂类中根据传入的参数构造不同预算符对象。
运算符抽象类：

public abstract class Operation {public double getResult(double numberA, double numberB) {return 0d;}
}

接下来是四个具体的运算符类

public class Add extends Operation{@Overridepublic double getResult(double numberA, double numberB) {return numberA + numberB;}
}

public class Sub extends Operation{@Overridepublic double getResult(double numberA, double numberB) {return numberA - numberB;}
}

public class Mul extends Operation{@Overridepublic double getResult(double numberA, double numberB) {return numberA * numberB;}
}

public class Div extends Operation {@Overridepublic double getResult(double numberA, double numberB) {if (numberB == 0) {System.out.println("除数不能为0");throw new ArithmeticException();}return numberA / numberB;}
}

工厂类

public class OperationFactory {public static Operation createOperation(String operate) {Operation oper = null;switch (operate) {case "+":oper = new Add();break;case "-":oper = new Sub();break;case "*":oper = new Mul();break;case "/":oper = new Div();break;}return oper;}
}

测试类

public class Test {public static void main(String[] args) {Scanner sc =new Scanner(System.in);System.out.println("请输入数字A：");double numberA = Double.parseDouble(sc.nextLine());System.out.println("请输入运算符（+、-、*、/）：");String strOperate = sc.nextLine();System.out.println("请输入数字B：");double numberB = Double.parseDouble(sc.nextLine());double result = 0d;Operation oper = OperationFactory.createOperation(strOperate);result = oper.getResult(numberA, numberB);System.out.println("结果为：" + result);}
}