Why doesn't java.lang.Enum::valueOf check for the null name first?

Question

I have checked the source code for java.lang.Enum and the method T valueOf(Class<T> enumType, String name) beginning on line 232 (the implementation in both java-8 and java-11 seems equal; here is the source for Java 8).

public static <T extends Enum<T>> T valueOf(Class<T> enumType, String name) {
    T result = enumType.enumConstantDirectory().get(name);
    if (result != null)
        return result;
    if (name == null)
        throw new NullPointerException("Name is null");
    throw new IllegalArgumentException(
        "No enum constant " + enumType.getCanonicalName() + "." + name);
}

What is the reason the null check for name happens after finding the enumeration by name in the Map get using enumConstantDirectory()? As far as I know, one cannot define a null enum; therefore the following call makes no sense:

MyEnum myEnum = Enum.valueOf(MyEnum.class, null);     // The NPE should be thrown

Although the HashMap implementation allows null as a key, I would expect the implementation would check for null before iterating the Map. What is the reason for this implementation and is there a case where searching for the null key before comparing name == null makes sense?

I think this is a good question. Without knowing it, my assumption is that this part was implemented afterwards to make the API 'better'. From my oppinion there is no rease why this check is doing afterwards. — Mafick
– Mafick, Commented Jun 24, 2020 at 10:31
It's an interesting question, though I don't think fits SO because we are not the authors, and we can only guess, not give an authoritative answer. My guess: They wanted to give precedence to the exception that is thrown if the class is not a real Enum class. That is, if both arguments are incorrect, you want to throw an exception for the first argument first, and only if it's legal, throw an exception for the second argument. — RealSkeptic
– RealSkeptic, Commented Jun 24, 2020 at 10:58
Maybe to make faster the happy case (generally that will be used most), especially when the previous operation don't make any problem with null value — Eklavya
– Eklavya, Commented Jun 24, 2020 at 11:59

Boann · Accepted Answer · 2020-06-24 14:18:01Z

11

99.999+% of calls to that method will be valid enum names. Passing a null name is always a blunder and is not the case that you want to optimize for. So moving the null check three lines down means that the check will be skipped in the common case that the enum name is valid. It doesn't change the result; it's just a tiny optimization.

answered Jun 24, 2020 at 14:18

Boann

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