需要常量表达式？ （Java switch语句）

Question

I have no idea what is causing this error. My teacher and I went over it and couldn't find what was wrong.

import java.util.Scanner;
public class MailAssignment
{
public static void main(String [] args){

    Scanner userinput = new Scanner(System.in);
    char p;
    char f;
    double price = 0;
    System.out.println("First class or priority?");
    char type = userinput.next().charAt(0);
    System.out.println("How much does the package weigh? (in ounces)");
    double weight = userinput.nextDouble();

    switch (type){
     case p:
     if (weight > 16)
        price = weight * 3.95;

        else if (weight > 32) 
            price = (1.20 * (weight / 16));
       else
            price = 3.50 * weight;


        break;


     case f: 
     if (weight < 1 )
     price = 0.34;

     else if ( weight > 1)
     price = 0.34 + (weight * 21);

     else if (weight > 13)
     price = weight * 3.95;

        else if (weight > 32) 
            price = 1.20 * (weight / 16);
        else
            price = 3.50 * weight;

            break;

        }     

    System.out.println("Your price is: " +price);
    }
}

It throws a "Constant expression required" error when it's compiled and it points to the case p: line, however, it also throws it for f: if I switch them so I must be doing something completely off.

Answer 1

Yes, a case expression has to be a constant (or an enum constant name) - you can't use a variable. See the Java Language Specification section 14.11 (switch statements) for more details. (You hadn't even initialized the variables, so it's not clear what you expected to happen, to be honest.)

Did you mean:

case 'p':
    ...

case 'f':
    ...

? This will match your input ( type ) against the character literals 'p' and 'f' .

(As an aside, if this stumped your teacher, I have concerns about how suitable they are to be teaching Java. This is reasonably basic stuff.)

Answer 2

你不能在case语句中使用变量，它必须是文字字符（例如'p' ， 'f' ）。

Answer 3

In case statement
You should use a (character switch) or a (number switch) only
For example
You can use a (character switch)

switch (type) {
case 'p':
.......
case 'f':
......
}

NOT

switch (type) {
case p:
.......
case f:
......
}