Java 为什么可以'；字符串变量不能在此程序中打印吗？

此代码用于将月份更改为相应的字母代码

这是我迄今为止编写的代码：

public static void main (String[ ] args) 

  {
  Scanner input = new Scanner(System.in); 
  System.out.print("Enter month number. [1..12] --> ");
  int month = input.nextInt();
  String MonthString;
  switch (month)
  {
   case 1 : MonthString = "ZS"; break;
   case 2 : MonthString = "CN"; break;
   case 3 : MonthString = "YH"; break;
   case 4 : MonthString = "MT"; break;
   case 5 : MonthString = "CL"; break;
   case 6 : MonthString = "SS"; break;
   case 7 : MonthString = "WM"; break;
   case 8 : MonthString = "WY"; break;
   case 9 : MonthString = "SH"; break;
   case 10 : MonthString = "YJ"; break;
   case 11 : MonthString = "XG"; break;
   case 12 : MonthString = "HZ"; break;
   default : System.out.print("This is not a valid month number.");
  }
  System.out.println(MonthString);/*This is where it won't compile*/
}

Enter month number. [1..12] --> 1
ZS

---

您需要先初始化

MonthString

：

String MonthString = null;

---

您需要在

String monthString=“”）上添加一些内容第一个

注意变量引导中的小写和大写字母：

初始化变量时，应按如下方式初始化：

String firstExample=“Hello world”和非：
stringfirstexample=“helloworld”，类以大写字母命名

public static void main (String[ ] args)

      {
      Scanner input = new Scanner(System.in); 
      System.out.print("Enter month number. [1..12] --> ");
      int month = input.nextInt();
      String monthString = "";
      switch (month)
      {
       case 1 : monthString = "ZS"; break;
       case 2 : monthString = "CN"; break;
       case 3 : monthString = "YH"; break;
       case 4 : monthString = "MT"; break;
       case 5 : monthString = "CL"; break;
       case 6 : monthString = "SS"; break;
       case 7 : monthString = "WM"; break;
       case 8 : monthString = "WY"; break;
       case 9 : monthString = "SH"; break;
       case 10 : monthString = "YJ"; break;
       case 11 : monthString = "XG"; break;
       case 12 : monthString = "HZ"; break;
       default : System.out.print("This is not a valid month number.");
      }
      System.out.println(monthString);
    }
}

输出：

public static void main (String[ ] args) 

  {
  Scanner input = new Scanner(System.in); 
  System.out.print("Enter month number. [1..12] --> ");
  int month = input.nextInt();
  String MonthString;
  switch (month)
  {
   case 1 : MonthString = "ZS"; break;
   case 2 : MonthString = "CN"; break;
   case 3 : MonthString = "YH"; break;
   case 4 : MonthString = "MT"; break;
   case 5 : MonthString = "CL"; break;
   case 6 : MonthString = "SS"; break;
   case 7 : MonthString = "WM"; break;
   case 8 : MonthString = "WY"; break;
   case 9 : MonthString = "SH"; break;
   case 10 : MonthString = "YJ"; break;
   case 11 : MonthString = "XG"; break;
   case 12 : MonthString = "HZ"; break;
   default : System.out.print("This is not a valid month number.");
  }
  System.out.println(MonthString);/*This is where it won't compile*/
}

Enter month number. [1..12] --> 1
ZS

您需要在switch语句的默认值中指定该值，而不是将其打印出来，因为它将在switch/case结束后打印。
变量名应以小写开头，这样它们就不会与以大写开头的类名混淆。默认的case不会将
MonthString
指定给值。如果号码错了怎么办？如果未初始化MonthString，则最后一条语句是错误的。要么在开关之前分配它，要么在默认情况下分配它，要么抛出异常。它将编译，但不会像intended@RandomCoder_01为什么？他有2个system.out.println（MonthString）。因此，输出将是“这不是一个有效的数字”，后跟“Null”。这没什么大不了的，但真正的问题是他是如何使用他的默认声明的。