通过一个计算器的例子来感受一下面向对象的三大特性

下面看一个计算器逻辑的实现：

public static void main(String[] args) {
        try{
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入第一个数字");
            double numA = scanner.nextDouble();
            System.out.println("请输入运算符（+-*/）");
            String operate = scanner.next();
            System.out.println("请输入第一个数字");
            double numB = scanner.nextDouble();
            String result = "";
            switch (operate){
                case "+":
                    result = String.valueOf(numA + numB);
                    break;
                case "-":
                    result = String.valueOf(numA - numB);
                    break;
                case "*":
                    result = String.valueOf(numA * numB);
                    break;
                case "/":
                    if(numB != 0){
                        result = String.valueOf(numA / numB);
                    }else {
                        System.out.println("您输入的参数有误，除数不能为0");
                    }
                    break;
            }
            System.out.println("结果为："+result);
        }catch(Exception e){
            System.out.println("您输入的参数有误");
        }
    }

乍一看没什么问题，该判断的逻辑也都判断了，也实现了一个简单计算器的基本功能，这样的编码方式是很直接的用计算机能够理解的逻辑来解决问题，这个逻辑本身没有错，但是这样的思维使得我们的程序只能满足当前的需求，程序不易维护、不易拓展、不易复用，这样的代码质量低、没有营养。

根据面向对象的三大特性来改造这段代码

1. 封装

   首先我们把计算机的业务逻辑封装起来，使得业务逻辑和界面逻辑分开
   Operation类

public class Operation {
    public Double getResult(double numberA,double numberB,String operate){
        Double result = "";
            switch (operate){
                case "+":
                    result = numA + numB;
                    break;
                case "-":
                    result = numA - numB;
                    break;
                case "*":
                    result = numA * numB;
                    break;
                case "/":
                    if(numB != 0){
                        result = numA / numB;
                    }else {
                        System.out.println("您输入的参数有误，除数不能为0");
                    }
                    break;
            }
            return result;
    }
}```
	**客户端类**
```java
public static void main(String[] args) {
        try{
            Operation operation = new Operation();
            Scanner scanner = new Scanner(System.in);
            System.out.println("请输入第一个数字");
            double numA = scanner.nextDouble();
            System.out.println("请输入运算符（+-*/）");
            String operate = scanner.next();
            System.out.println("请输入第一个数字");
            double numB = scanner.nextDouble();
            double result = operation.getResult(numA,numB);
            System.out.println("结果为："+result);
        }catch(Exception e){
            System.out.println("您输入的参数有误");
        }
    }

2. List item