Switch语句eclipse错误：case表达式必须是常量表达式

Question

我写了一个程序，它会根据我输入的颜色输出一朵花。在 switch 语句中，我一直看到一个错误，指出“case 表达式必须是常量表达式”。我看不出我在哪里做错了。我还遇到了打印出花朵复数时态的问题（如果用户输入 2 或更高）。

代码如下：

Scanner input = new Scanner(System.in);

    int quantity;
    String color;
    String flowerType = "";
    char extra;

    System.out.print("Please enter a color: ");
    color = input.next();
    System.out.print("Please enter the quantity: ");
    quantity = input.nextInt();

    String red = "red";
    String blue = "blue";
    String yellow = "yellow";
    String purple = "purple";
    String white = "white";

    switch(color){
    case red:
        flowerType = "rose";
        break;
    case blue:
        flowerType = "iris";
        break;
    case yellow:
        flowerType = "daffodil";
        break;
    case purple:
        flowerType = "sage";
        break;
    case white:
        flowerType = "dogwood";
        break;
        default:
            System.out.println("Invalid color.");
    }
    switch(quantity){
    case 1:
        break;
    default:
        extra = 's';
        break;
    }
    System.out.println("You have " + quantity + flowerType + extra + ".");
}

}

Answer 1

将变量red、purple等标记为final。

final String red = "red";
final String blue = "blue";
final String yellow = "yellow";

Answer 2

对颜色名称使用字符串文字而不是使用变量。

switch(color) {
case "red":
     ...
case "blue":
     ...
... //other cases also with ""
}

Answer 3

switch 中的任何语句都必须是一个编译时常量，这意味着它必须是一个常量，它是一个字面量或由字面量赋值。文字是直接写入代码的所有值。常量是赋值后不能改变的值，这意味着任何最终值都是常量。强烈建议将静态修饰符用于数学常量（并减少实例使用的空间）。

final String red = "red";
case red:
     ... //insert code here
     break;

和

case "red":
     ... //insert code here
     break;

两者都可以。

另一方面，返回字符串的函数将不起作用：

static final String s = new String("abc");
final String s = new String("abcd");

不起作用，因为该方法无法在编译时执行。

Answer 4

char 的标准值显示为矩形框，如果您想将空字符作为标准值，请使用字符串。还可以将 if 用于条件而不是开关。

package pointless;

import java.util.Scanner;

public class SO {
    static final String red = "red";
    static final String blue = "blue";
    static final String yellow = "yellow";
    static final String purple = "purple";
    static final String white = "white";
    public static void main(String[] args) {
        final Scanner input = new Scanner(System.in);

        int quantity;
        String color;
        String flowerType = "";
        String extra = ""; //using a string is better cause it can be empty, a char can't be empty

        System.out.print("Please enter a color: ");
        color = input.next();
        System.out.print("Please enter the quantity: ");
        quantity = input.nextInt();

        switch(color){
            case red:
                flowerType = "rose";
                break;
            case blue:
                flowerType = "iris";
                break;
            case yellow:
                flowerType = "daffodil";
                break;
            case purple:
                flowerType = "sage";
                break;
            case white:
                flowerType = "dogwood";
                break;
            default:
                System.out.println("Invalid color.");
        }
        if(quantity>1) { // use a if for checking conditions like quantity>1
            extra="s";
        }
        System.out.println("You have " + quantity+ " " + flowerType + extra + ".");
    }
}

Answer 5

代码应该是这样的： 扫描仪输入 = new Scanner(System.in);

    int quantity;
    String color;
    String flowerType = "";
    char extra;

    System.out.print("Please enter a color: ");
    color = input.next();
    System.out.print("Please enter the quantity: ");
    quantity = input.nextInt();

    final String  red = "red";
    final String blue = "blue";
    final String yellow = "yellow";
    final String purple = "purple";
    final String white = "white";

    switch(color){
    case red:
        flowerType = "rose";
        break; 
    case blue:
        flowerType = "iris";
        break; 
    case yellow:
        flowerType = "daffodil";
        break; 
    case purple:
        flowerType = "sage";
        break; 
    case white:
        flowerType = "dogwood";
        break; 
        default: 
            System.out.println("Invalid color.");
    } 
    switch(quantity){
    case 1: 
        System.out.println("You have " + quantity + flowerType+".");
        break; 
    default: 
        extra = 's';
        System.out.println("You have " + quantity + flowerType + extra + ".");
        break; 
    } 

}