工厂模式

意图：定义一个用于创建对象的接口，让子类决定实例化哪一个类。使一个类的实例化延迟到其子类。

### 实现（简单工厂）：

public class OperationFactory {
    public static Operation createOperate(String operate) {
        Operation oper = null;

        switch (operate) {
            case "+": oper = new OperationAdd(); break;
            case "-": oper = new OperationSub(); break;
            case "*": oper = new OperationMul(); break;
            case "/": oper = new OperationDiv(); break;
        }

        return oper;
    }

    // 客户端
    public static void main(String[] args) {
        Operation oper = OperationFactory.createOperate("+");
        oper.numberA = 1;
        oper.numberB = 2;
        double result = oper.getResult();
    }

}

### 实现（工厂方法）：

// 工厂接口
public interface IFactory {
    Operation createOperation();
}

// 加法工厂
public class AddFactory implements IFactory {
    public Operation createOperation() {
        return new OperationAdd();
    }
}

// 减法工厂、乘法工厂、除法工厂
...

// 客户端
IFactory operFactory = new AddFactory();
Operation oper = operFactory.CreateOperation();
oper.numberA = 1;
oper.numberB = 2;
double result = oper.getResult();