Using json.org JSON, can you use a string to help recreate an enum?

Question

I am attempting to recreate from a JSON string, a collection that holds enum constants (the source of the JSON string). I'm having difficulty figuring out how to recreate the enum from a string. The enums hold a variety of methods which I need access to.

The answers I've seen this and this one, result in a class which enables you to iterate through the constants rather than recreating the original enum. I want to pass the enums around other existing classes without having to rewrite code to cope with a new class.

Is this even possible? Can you use a JSON string to help recreate an enum in such a way you can use it as an enum?

EDIT: Bizarre, I have a big red banner telling me Stack Overflow requires external Javascript and now I cannot comment on posts...

Thanks Brian for your reply below, but it appears in essence to be the same as the two I linked to here, it does not result in you actually having the enum object, just the constants via a different method.

Erm, it does result in you having your enum object ... there's no such thing as "just the constants" in Java. I've edited the answer to demonstrate that fully. — Brian Roach, Feb 18 '13 at 17:23
In essence I was using the basic JSON Java library from json.org, which is errr... basic. Brian in a roundabout way has pointed me to Gson as a better alternative for what I want to do (recreate classes easily from JSON strings). — Matt Stevens, Feb 18 '13 at 20:09

Brian Roach · Accepted Answer · 2013-02-18T20:12:26.830

2

An enum in Java is a class, and the compiler adds a static valueOf(String name) method. So if you have:

enum MyEnum  { 
      ONE(1.0), TWO(2.0), THREE(3.0); 

      private double myDouble; 

      MyEnum(double d) {  
          myDouble = d;
      }

      public double getDouble() {
          return myDouble;
      }
}

You can say:

MyEnum e = MyEnum.valueOf("ONE");
assert(e.equals(MyEnum.ONE));
System.out.println(e.getDouble());

The more popular JSON parsers for Java handle enums just fine. Here's an example with Gson:

public class App
{

    public static void main(String[] args) 
    {
        // Array containing your enum constants
        String json = "[\"ONE\",\"TWO\",\"THREE\"]";

        Type t = new TypeToken<Collection<MyEnum>>(){}.getType();
        Collection<MyEnum> c = new Gson().fromJson(json, t);

        for (MyEnum me : c)
        {
            System.out.println(me);
            switch(me)
            {
                case ONE:
                    System.out.println("This is equal to MyEnum.ONE");
                    break;
                case TWO:
                    System.out.println("This is equal to MyEnum.TWO");
                    break;
                case THREE:
                    System.out.println("This is equal to MyEnum.THREE");
                    break;
            }
            System.out.println(me.getDouble());

        }
    }    
}

Output:

ONE
This is equal to MyEnum.ONE
1.0
TWO
This is equal to MyEnum.TWO
2.0
THREE
This is equal to MyEnum.THREE
3.0

edited Feb 18 '13 at 20:12

answered Feb 18 '13 at 06:58

Brian Roach

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Fixed the big red banner thing (I had to edit my post earlier as I could not comment on your post). Thanks for your reply, but it doesn't provide me with the enum, just the constants, admittedly in a collection which is quite a good idea. My enums also hold methods and need to be used in other classes as enums. This way would require a lot of re-writing. – Matt Stevens Feb 18 '13 at 08:40
PS. I'm very aware I could be looking for a non-existant answer. I'm picking I may need to do a major re-write and turn the enums into classes :( – Matt Stevens Feb 18 '13 at 08:51
1

Huh? I'm not even sure what you're talking about - I think you many not understand how enums work in Java? I demonstrated in the first example in my answer that you do indeed have your enum *object*. There's no such thing as "just the constants". – Brian Roach Feb 18 '13 at 17:08
I didn't bother looking at Gson as "Any of the JSON parsers for Java handle enums just fine." It would seem the one from JSON.org doesn't. I was quite surprised when I tracked down the TypeToken class and found the first stated goal of Gson is to enable doing exactly what I want (& by implication some other parsers don't). In a nutshell, I'll take your advice as a roundabout way of saying "go look at Gson". A very genuine thanks for the pointer, I should be ok from here. – Matt Stevens Feb 18 '13 at 19:47
@MattStevens - I haven't used that in ages but I'm surprised it doesn't, sorry; I'll edit. I know Jackson can as well (though it's a bit more clunky IIRC). – Brian Roach Feb 18 '13 at 20:11
maybe it does, at this point though I like what I'm reading in the Gson user guide. After a day of bashing my head against a brick wall, I'm just very pleased to be moving forward again :) cheers! – Matt Stevens Feb 18 '13 at 20:22

Using json.org JSON, can you use a string to help recreate an enum?

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