Question

Why is it that when I define an enum, I pass it a list of field names, and then somehow those field names (e.g. Days.MONDAY) end up referring to field values? I can pass along a field (e.g. Days.MONDAY) and then use a switch to get the field value. Even more strange, when I declare the enum fields, I don’t even have to put the names in quotes, even though they are actually values.

Answer 1

Think of the Java enum as a nice syntax for defining a class. Here is a shell transcript that might help:

$ cat > Direction.java
public enum Direction {NORTH, EAST, SOUTH, WEST}
$ javac Direction.java 
$ javap Direction
Compiled from "Direction.java"
public final class Direction extends java.lang.Enum{
    public static final Direction NORTH;
    public static final Direction EAST;
    public static final Direction SOUTH;
    public static final Direction WEST;
    public static Direction[] values();
    public static Direction valueOf(java.lang.String);
    static {};
}

So yes, you can say EAST is a field of the class Direction and its value is an instance of class Direction. We refer to this value as Direction.EAST the same way you refer to a value through a static field of any other class.

Perhaps the confusion is a result of the fact that you don’t see a declaration like

public static final Direction.EAST = SOMETHING_OR_OTHER_HERE;

This is understandable. Enums are designed to initialize these field values, but you don’t have to explicitly initialize them yourself. It is as if you said

public static final Direction.EAST = new Direction();

which is, as a matter of fact, a part of what is called the “Typesafe Enum” pattern which was commonly used before Java got the current enum syntax.

Regarding your comment about quotes, there is simply no reason to have to quote anything, because there are no strings involved.

Regarding your comment about switches, yes, it is somewhat interesting how the notion of field name and field value play out here. But that is only because the value looks like the field. In other words, the value of field EAST in Direction is, well, Direction.EAST. If you think about it, though, it is not too different from literals. What is the value of Float.NaN you may ask? Well, it is…. Float.NaN. Kind of an opaque value.