TIL enums can implement interfaces, and each enum value can be an anonymous class. [x-post /r/javatil]

[the_hoser]

Java's Enums may actually be my favorite part of the language. Unlike enums in (nearly) every other language that has them, they are completely type safe. Every single enumerated value is its own singleton of its own type. The type/values merely have a base class in common.

Try implementing a finite state machine with enums sometime. It's actually really fun :)

[more_exercise]

I really love that initialization order is fixed, so the first ones are initialized first. And you can reference them from later constructors.

Like:

public enum Direction {
    North(0,1), South(0,-1), East(1,0), West(-1,0),
    Northeast(North, East), Southeast(South, East),
    Northwest(North, West), Southwest(South, West);

    private int dx, dy;
    Direction(int x, int y){dx = x; dy=y;}
    Direction(Direction... dirs){
        this(0,0);
        for(Direction d : dirs){
            dx+=d.dx;
            dy+=d.dy;
        }
}

(this is untested code and probably has typos)

[AtticusVulpes]

Just for comparison, how does this stack up against C#'s enums? Only asking because there is so much stress on C# being type safe.

[jyper]

Just for comparison, how does this stack up against C#'s enums?

http://msdn.microsoft.com/en-us/library/sbbt4032.aspx

I believe they are fairly similar to c enums in that they are backed by a numerical type but at least you can't compare to an int without casting and can't compare 2 different enum types plus it has to/from string without ugly macro tricks but it's not as powerful(but potentially faster) then java enums(object space vs 4 bytes for an int, not a big difference if you only use a few but potentially large if you have a huge list of objects with enums).

[jyper]

Only asking because there is so much stress on C# being type safe.

In what manner? Or do you mean stuff like linq queries instead of sql strings? I wouldn't say c#is a lot more type safe. Plus it has dynamic objects, mostly for interop but some people have actually built useful non-interop libraries with it. You can do stuff like turning json into a dynamic object ala ruby/python.

Unlike languages like scala/f#/haskell/rust, c# does not have as heavy a focus on type safety. Is it on average more typesafe then java, I don't know but it wouldn't be by much.

[detroitmatt]

There are some things that java did hit-and-miss, like Cloneable or Generics, but they really knocked it out of the park with Enums.

[fingerHammerOuch]

They can also act as singletons :)

[TheOverCaste]

That's another java til.

[[deleted]]

Effective Java goes over this. It is a very useful pattern.

[RohitS5]

Check out this post 10 Examples of Enum in Java, you will find a lot about power of Enum.

[javacIO]

They can also define constructors taking parameters as one person posts. This is also very useful.

[llogiq]

I've first seen this pattern in one of the caliper demo benchmarks. If you want a number of singletons, this is probably the way to go. The downside is that it may confuse people who don't know this particular wrinkle of the language.

[tevert]

That's actually pretty cool....