An enum is a class with a fixed set of named instances. enum Size { SMALL, MEDIUM, LARGE } creates exactly three Size objects, and no code can create more, because enum constructors are always private (implicitly, if you write no modifier; writing public or protected is a compile error). Each constant is a public static final field, so you refer to them as Size.SMALL, and since there is only one instance of each, comparing enums with == is safe.
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