为什么我不能在开关中使用相同的变量？

Question

例如，如果我写这样的代码：

double result = 0; 
    switch (operation) { 
     case "divide": 
      double result = (double) first/second; 
      string = Double.toString(result); 
      break; 
     case "multiply": 
      double result = (double) first * second; 
      string = Double.toString(result); 
      break; 
     case "substract": 
      double result = (double) first - second; 
      string = Double.toString(result); 
      break; 
     case "add": 
      double result = (double) first + second; 
      string = Double.toString(result); 
      break; 
    } 

它返回错误：可变“结果”在范围已定义。

我知道如何解决这个问题。我可以在每种情况下创建一个不同的变量，但是这样会通过必须为每个变量编写代码来延长代码长度。

如何使这项工作在给定的代码中书写？

Answer 1

这是由result已被定义在switch语句顶部的事实引起的，并且您正在声明中再次声明它。

尝试这样：

switch (operation) { 
    case "divide": 
     double result = (double) first/second; 
     string = Double.toString(result); 
     break; 
    case "multiply": 
     double result = (double) first * second; 
     string = Double.toString(result); 
     break; 
    case "substract": 
     double result = (double) first - second; 
     string = Double.toString(result); 
     break; 
    case "add": 
     double result = (double) first + second; 
     string = Double.toString(result); 
     break; 
} 

换句话说，你有过什么，只是从你最初的声明。

---

或者，你可以声明double，并用它在你的每一个案件：

double result = 0; 
switch (operation) { 
    case "divide": 
     result = (double) first/second; 
     string = Double.toString(result); 
     break; 

    ... 

---

由于亚辛Hajaj在评论中指出，这将是，如果要好得多您在switch语句的末尾使用了string = Double.toString(result);行一次，而不是多次重复。

这将是这个样子：

double result = 0; 
switch (operation) { 
    case "divide": 
     result = (double) first/second; 
     break; 
    case "multiply": 
     result = (double) first * second; 
     break; 
    ... 
} 
string = Double.toString(result); 

Answer 2

实际上，你可以通过使用括号。

switch (operation) { 
     case "divide": { 
      double result = (double) first/second; 
      string = Double.toString(result1); 
      break; 
     } 
     case "multiply": { 
      double result = (double) first * second; 
      string = Double.toString(result2); 
      break; 
     } 
     default: 
      break; 
    } 

Answer 3

在java中，{}内定义的任何局部变量都可用，直到最后一行，因此您可以在块内定义之后重用任何地方。所以在你的情况下没有用来重新定义结果变量。只需在第一行中定义并随处使用即可。

double result = 0; 
    switch (operation) { 
     case "divide": 
      result = (double) first/second; 
      string = Double.toString(result1); 
      break; 
     case "multiply": 
      result = (double) first * second; 
      string = Double.toString(result2); 
      break; 
     case "substract": 
      result = (double) first - second; 
      string = Double.toString(result3); 
      break; 
     case "add": 
      result = (double) first + second; 
      string = Double.toString(result4); 
      break; 
    } 

Answer 4

becaue您已经定义双重结果变量，以便做到像下面希望它会工作

double result = 0; <----- already define here 
switch (operation) { 
    case "divide": 
     result = (double) first/second; <------ there is no need to 
                define it again 
     string = Double.toString(result1); 
     break; 
    case "multiply": 
     result = (double) first * second; 
     string = Double.toString(result2); 
     break; 
    case "substract": 
     result = (double) first - second; 
     string = Double.toString(result3); 
     break; 
    case "add": 
     result = (double) first + second; 
     string = Double.toString(result4); 
     break; 
} 

Answer 5

如果要使用公共变量，那么你可以这样做。

double result = 0; 
    switch (operation) { 
     case "divide": 
      result = (double) first/second; 
      string = Double.toString(result); 
      break; 
     case "multiply": 
      result = (double) first * second; 
      string = Double.toString(result); 
      break; 
     case "substract": 
      result = (double) first - second; 
      string = Double.toString(result); 
      break; 
     case "add": 
      result = (double) first + second; 
      string = Double.toString(result); 
      break; 
    } 

Answer 6

你不能在你的代码的一个范围内声明相同的变量名，使用全局变量的结果，你也万一范围内使用同一个变量。

double result = 0; 
    switch (operation) { 
     case "divide": 
      result = (double) first/second; 
      string = Double.toString(result); 
      break; 
     case "multiply": 
      result = (double) first * second; 
      string = Double.toString(result); 
      break; 
     case "substract": 
      result = (double) first - second; 
      string = Double.toString(result); 
      break; 
     case "add": 
      result = (double) first + second; 
      string = Double.toString(result); 
      break; 
    }