Can you create a not-defined enum value in Java, like in .NET?

Question

I don't have that much experience in Java compared to .NET. In .NET, enums are treated as thin wrappers over integers, so you can easily create an enum value that is unnamed. For example:

// C# code
public enum Colors { Red, Green, Blue }

Console.Writeline(Colors.Red + " " + Colors.Green + " " + Colors.Blue); // Red Green Blue

var unknown = (Colors)(-1);
Console.WriteLine(unknown); // -1

Is it possible to do the same thing in Java?

edit: This seems to be the case from the fact that this code won't compile:

// Java code
enum Colors { R, G, B }

static int f(Colors c) {
    switch (c) {
        case R: return 1;
        case G: return 2;
        case B: return 3;
    } // Compiler complains about a missing return statement
}

@JoeC I agree, too. I'm a little pedantic, so in my .NET code I always have a default: case where I throw an exception every time I switch on an enum, even if I've taken care of all the defined values. I'm wondering whether I have to do the same thing for Java. — James Ko
– James Ko, Commented Sep 10, 2017 at 18:18
Absolutely NOT. In Java enums are type-safe and you cannot express this concept at all. — Jim Garrison
– Jim Garrison, Commented Sep 10, 2017 at 18:22
The code from the edit doesn't compile because the compiler defensively assumes that the Colors enum might be changed (extended) after creating the compiled class file that uses the enum. So a (while compiling) complete case list might turn incomplete when later running, if the Colors enum changed in between. — Ralf Kleberhoff
– Ralf Kleberhoff, Commented Sep 10, 2017 at 19:00
@RalfKleberhoff I see. Wouldn't the compiler be able to handle it if the enum and switch statement lived in the same jar, though? — James Ko
– James Ko, Commented Sep 11, 2017 at 0:19

Sergey Kalinichenko · Accepted Answer · 2017-09-10 18:28:09Z

4

Java implements enums differently from C#: rather than making them a thin wrapper over ints, it makes them thin wrappers around objects, a compiler-aided improvement on the type-safe enum pattern.

Compiler ensures that it is not possible to create an instance of an enum that is not included in the type.

There are advantages and disadvantages to each approach. C# stays closer to C and C++ enums, which behave like collections of named numeric constants. This makes it possible to create [Flag] enumerations - something that would not be possible with Java enums.

On the other hand, Java enums are fully working objects, complete with methods of their own, and an ability to implement interfaces. Methods can be added to C# enums as extensions, but it is not possible to implement an interface.

edited Sep 10, 2017 at 18:28

answered Sep 10, 2017 at 18:22

Sergey Kalinichenko

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3 Comments

James Ko Over a year ago

Thank you for your answer, it's very informative (I upvoted). Do you know why the code I posted in the edit won't compile, though?

Florian Over a year ago

You need to add the case default like in: 'default: throw new IllegalArgumentException();'

Sergey Kalinichenko Over a year ago

@JamesKo I think this is a shortcoming of the compiler. The only other value that you can pass to f() is null, but if you did, there would be a crash in the switch statement.

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