Java 7 - null 作为 case 表达式

Question

这个问题已经有答案了:

How to use null in switch (15 个回答)

已关闭 8 年前。

为什么编译失败并出现“case 表达式必须是常量表达式”错误？不是 null一个常量(在编译时已知)？显式转换 null值转换为字符串，如case ((String)null)也没有帮助(我得到同样的错误)。

public static String getTypeOfDayWithSwitchStatement(String dayOfWeekArg) {
    String typeOfDay;
    switch (dayOfWeekArg) {

        case null:
            typeOfDay = "NULL";
            break;

        case "Monday":
            typeOfDay = "Start of work week";
            break;
        case "Tuesday":
        case "Wednesday":
        case "Thursday":
            typeOfDay = "Midweek";
            break;
        case "Friday":
            typeOfDay = "End of work week";
            break;
        case "Saturday":
        case "Sunday":
            typeOfDay = "Weekend";
            break;
        default:
            throw new IllegalArgumentException("Invalid day of the week: " + dayOfWeekArg);
    }
    return typeOfDay;
}

Answer 1

是的，case 表达式必须是常量表达式，但 JLS, Section 14.11 明确禁止 null ，它描述了 switch 语句:

Given a switch statement, all of the following must be true or a compile-time error occurs:

• Every case constant associated with the switch statement must be assignment compatible with the type of the switch statement's Expression (§5.2).

• If the type of the switch statement's Expression is an enum type, then every case constant associated with the switch statement must be an enum constant of that type.

• No two of the case constants associated with the switch statement have the same value.

• No case constant associated with the switch statement is null.

• At most one default label is associated with the switch statement.

(斜体强调我的)

作为解决方法，您可以在 switch 语句之外测试 null。