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.

Answer 1

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