一、前言

简单工厂模式不属于GoF23种设计模式之一，但是实际中用途广泛，并且可以作为学习“工厂方法模式”以及“抽象工厂模式”的基础。在简单工厂模式中，工厂类提供一个创建产品的工厂方法，用于创建各种产品。客户端只需传入对应的参数，利用该方法即可根据传入参数的不同返回不同的具体产品对象。

二、简单工厂模式

简单工厂模式（Simple Factory Pattern）：定义一个工厂类，它可以根据参数的不同返回不同类的实例，被创建的实例通常都具有共同的父类。

因为在简单工厂模式用于创建实例的方法是静态的方法，因此简单工厂模式又被称为静态工厂方法模式，它属于类创建型模式。

在介绍简单工厂之前先来看一个简单的计算器小栗子。

1、计算器例子

public static void main(String[] args) {Scanner scanner = new Scanner(System.in);System.out.println("请输入数字A：");String A = scanner.nextLine();System.out.println("请选择运算符号(+、-、*、/)：");String B = scanner.nextLine();System.out.println("请输入数字B：");String C = scanner.nextLine();String D = "";if(B == "+"){D = String.valueOf(Double.parseDouble(A) + Double.parseDouble(C));}if(B == "-"){D = String.valueOf(Double.parseDouble(A) - Double.parseDouble(C));}if(B == "*"){D = String.valueOf(Double.parseDouble(A) * Double.parseDouble(C));}if(B == "/"){D = String.valueOf(Double.parseDouble(A) / Double.parseDouble(C));}System.out.println("运算结果："+D);
}

这段代码存在的问题：

这样命名是非常不规范的。像A、B、C、D这些，别人扎眼一看肯定不知道这是在做什么。
判断分支，这样的写法，意味着每个条件都要做判断，等于计算机做了三次无用功。
如果除数时，客户输入了0怎么办，如果用户输入的是字符符号而不是数字怎么办。

2、优化后版本

    public static void main(String[] args) {try {Scanner scanner = new Scanner(System.in);System.out.println("请输入数字A：");String numberA = scanner.nextLine();System.out.println("请选择运算符号(+、-、*、/)：");String operate = scanner.nextLine();System.out.println("请输入数字B：");String numberB = scanner.nextLine();String result = "";switch (operate ){case "+":result = String.valueOf(Double.parseDouble(numberA) + Double.parseDouble(numberB));break;case "-":result = String.valueOf(Double.parseDouble(numberA) - Double.parseDouble(numberB));break;case "*":result = String.valueOf(Double.parseDouble(numberA) * Double.parseDouble(numberB));break;case"/":if (numberB != "0"){result = String.valueOf(Double.parseDouble(numberA) / Double.parseDouble(numberB));}else{result = "除数不能为0";}break;}System.out.println("运算结果："+result);}catch (Exception e){System.out.println(e);}}

可以看到这一版中优化了变量的命名，将if判断更换成了switch case语句，增加了除数判断是否为0。

3、结合面向对象进行优化（封装）

3.1、Operation运算类

public class Operation {public static String getResult(double numberA, double numberB, String operate){String result = "";switch (operate){case "+":result = String.valueOf(numberA+ numberB);break;case "-":result = String.valueOf(numberA -numberB);break;case "*":result = String.valueOf(numberA * numberB);break;case"/":if (numberB != 0){result = String.valueOf(numberA / numberB);}else{result = "除数不能为0";}break;}return result;}
}

3.2、客户端

public class Program {public static void main(String[] args) {try {Scanner scanner = new Scanner(System.in);System.out.println("请输入数字A：");String numberA = scanner.nextLine();System.out.println("请选择运算符号(+、-、*、/)：");String operate = scanner.nextLine();System.out.println("请输入数字B：");String numberB = scanner.nextLine();String result = "";Operation.getResult(Double.parseDouble(numberA), Double.parseDouble(numberB),operate);System.out.println("运算结果："+result);}catch (Exception e){System.out.println(e);}}
}

4、利用面向对象三大特性（继承和多态）

4.1、Operation类

public abstract class Operation {private double numberA;private double numberB;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() throws Exception;
}

4.2、加法类

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

4.3、减法类

public class OperationSub extends Operation{@Overridepublic double getResult() {return getNumberB() - getNumberB();}
}

4.4、乘法类

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

4.5、除法类

public class OperationDiv extends Operation{@Overridepublic double getResult() throws Exception {if (getNumberB() != 0){return getNumberA() / getNumberB();}throw new Exception("除数不能为0");}
}

4.6、简单工厂

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

4.7、客户端

public class Program {public static void main(String[] args) throws Exception {Operation operation;operation = OperationFactory.createOperate("+");operation.setNumberA(2);operation.setNumberB(4);double result = operation.getResult();System.out.println(result);}
}