Question

When I’m reading Effective Java, the author told me that a single-element enum type is the best way to implement a singleton, because we don’t have to consider sophisticated serialization or reflection attacks. This means we cannot create an instance of enum using reflection, right?

I have done some tests, with an enum class here:

public enum Weekday {}

Then I tried to create an instance of Weekday:

Class<Weekday> weekdayClass = Weekday.class;
Constructor<Weekday> cw = weekdayClass.getConstructor(null);
cw.setAccessible(true);
cw.newInstance(null);

As you know, it doesn’t work. When I change the key word enum to class, it works. I want to know why. Thank you.

Answer 1

This is built into the language. From the Java Language Specification (§8.9):

It is a compile-time error to attempt to explicitly instantiate an enum type (§15.9.1). The final clone method in Enum ensures that enum constants can never be cloned, and the special treatment by the serialization mechanism ensures that duplicate instances are never created as a result of deserialization. Reflective instantiation of enum types is prohibited. Together, these four things ensure that no instances of an enum type exist beyond those defined by the enum constants.

The whole purpose of this is to allow the safe use of == to compare Enum instances.

EDIT: See the answer by @GotoFinal for how to break this “guarantee” using reflection.