Why can't enum constructors be protected or public in Java?

Question

The whole question is in the title. For example:

enum enumTest {

        TYPE1(4.5, "string1"), TYPE2(2.79, "string2");
        double num;
        String st;

        enumTest(double num, String st) {
            this.num = num;
            this.st = st;
        }
    }

The constructor is fine with the default or private modifier, but gives me a compiler error if given the public or protected modifiers.

Answer 1

Because you cannot call the constructor yourself.

Here is what the tutorials on Enums has to say:

Note: The constructor for an enum type must be package-private or private access. It automatically creates the constants that are defined at the beginning of the enum body. You cannot invoke an enum constructor yourself.

Answer 2

The public modifier will work for sure as seen in the tutorial http://download.oracle.com/javase/tutorial/java/javaOO/enum.html Both private and protected will also work if used within a class. I did a private version example here: http://ideone.com/mb5Le

public class Main {
    private enum Test {
        TEST1, TEST2;
    }
    public Test getTest1() {
        return Test.TEST1;
    }
    public static void main(String[] args) {
        Main m = new Main();
        System.out.println(m.getTest1());
    }
}

Answer 3

Think of Enums as a class with a finite number of instances. There can never be any different instances beside the ones you initially declare.

Thus, you cannot have a public or protected constructor, because that would allow more instances to be created.

Note: this is probably not the official reason. But it makes the most sense for me to think of enums this way.

Answer 4

Enums contain a fixed set of values, which must all be known at compile-time. It doesn't make sense to create new literals at run-time, which would be possible if the constructor were visible.