应用switch时出错，烦请解决。

public class CharDemo {

    public static void main(String[] args) {

               //定义一个字符变量并初始化

               byte ch=65;

               //使用switch结构判断是否为元音字母，并输出对应的信息

               switch(ch){

                   case a:

                       System.out.println("元音字母a");

                       break;

                   case b:

                   System.out.println("元音字母b");

                   break;

                   case i:

                   System.out.println("元音字母i");

                   break;

                   case o:

                   System.out.println("元音字母o");

                   break;

                   case u:

                   System.out.println("元音字母u");

                   break;

                   default:

                     System.out.println("不是元音字母");

               }

        }

}

/7/5381116/95/1787/CharDemo.java:7: error: cannot find symbol

                   case a:

                        ^

  symbol:   variable a

  location: class CharDemo

/7/5381116/95/1787/CharDemo.java:10: error: cannot find symbol

                   case b:

                        ^

  symbol:   variable b

  location: class CharDemo

/7/5381116/95/1787/CharDemo.java:13: error: cannot find symbol

                   case i:

                        ^

  symbol:   variable i

  location: class CharDemo

/7/5381116/95/1787/CharDemo.java:16: error: cannot find symbol

                   case o:

                        ^

  symbol:   variable o

  location: class CharDemo

/7/5381116/95/1787/CharDemo.java:19: error: cannot find symbol

                   case u:

                        ^

  symbol:   variable u

  location: class CharDemo

5 errors