作业3-5;不懂错在哪。。看不懂

public class CharDemo {

    public static void main(String[] args) {

Scanner.sc=new Scanner(System.in);

System.out.println("请输入一个字母");

String cha=next();

cha=cha.toLowercase();

//定义一个字符变量并初始化

switch (cha)

{

   case "a":

       System.out.println("元音字母："+cha);break;

   case "e":

       System.out.println("元音字母："+cha);break;

   case "i":

       System.out.println("元音字母："+cha);break;

   case "o":

       System.out.println("元音字母："+cha);break;

   case "u":

       System.out.println("元音字母："+cha);break;

   default:

   System.out.println("不是元音字母");

}

//使用switch结构判断是否为元音字母，并输出对应的信息

}

}

CharDemo.java:3: error: cannot find symbol
Scanner.sc=new Scanner(System.in);
^
 symbol:   variable Scanner
 location: class CharDemo
CharDemo.java:3: error: cannot find symbol
Scanner.sc=new Scanner(System.in);
              ^
 symbol:   class Scanner
 location: class CharDemo
CharDemo.java:5: error: cannot find symbol
String cha=next();
          ^
 symbol:   method next()
 location: class CharDemo
CharDemo.java:6: error: cannot find symbol
cha=cha.toLowercase();
      ^
 symbol:   method toLowercase()
 location: variable cha of type String
4 errors