Java 7 - null as case expression

Question

Why is this failing to compile with an error of "case expressions must be constant expressions"? Isn't null a constant (known at compile time)? Explicitly casting the null value to String as in case ((String)null) does not help too (I get the same error).

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;
}

Related: [Why isn't null a compile time constant?](http://stackoverflow.com/q/17758796/1065197) — Luiggi Mendoza, May 18 '15 at 22:29
http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.28 — Sotirios Delimanolis, May 18 '15 at 22:30
http://stackoverflow.com/questions/18263489/why-doesnt-string-switch-statement-support-a-null-case/18264731#18264731 — ZhongYu, May 18 '15 at 22:43

score 9 · Answer 1 · edited Jun 20 '20 at 09:12

Yes, the case expressions must be constant expressions, but null is specifically prohibited by the JLS, Section 14.11, which describes the switch statement:

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.

(italics emphasis mine)

As a workaround, you can test for null outside of the switch statement.

I see, it's in the JLS. Thanks. – peter.petrov May 18 '15 at 22:30 — peter.petrov, May 18 '15 at 22:30

score -1 · Answer 2 · answered May 18 '15 at 22:32

-1

The switch statement will throw a NullPointerException if you pass it a null value. Use a separate test to check for null values.

answered May 18 '15 at 22:32

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It will indeed throw NPE but I have to compile it first (e.g. by removing that case null statement). So I find this relevant too. Thanks. – peter.petrov May 18 '15 at 22:35

Java 7 - null as case expression

2 Answers2