java.lang.String无法转换为char

Question

我是新手，这是我第一次尝试输入char。我写了以下程序来输入成绩和显示结果：     import java.util.Scanner;     公共等级

{
    public static void main(String[] args)
    {
        Scanner obj = new Scanner(System.in);
        do
        {


     System.out.println("Enter grade(A/B/C/D/E/F): ");
            char grade = obj.next(".").charAt(0);
            switch(grade)
           {
               case "A": //The error is displayed here in "A"
               System.out.println("Excellent");
               break;
               case "B": case "C":
               System.out.println("Well done");
               break;
               case "D": case "E":
               System.out.println("You passed");
               break;
               case "F":
               System.out.println("Better try");
               break;
               default:
               System.out.println("Invalid");
               break;
           }
           System.out.println("Do you want to continue? (y/n): ");
           char cont = obj.nextChar();
        }
        while (cont == y);
    }
}

当我编译它时，它突出显示“A”并说java.lang.String不能转换为char。我该怎么办？