Java: instantiating an enum using reflection

Question

Suppose you have a text file like:

my_setting = ON
some_method = METHOD_A
verbosity = DEBUG
...

That you wish to to update a corresponding object accordingly:

Setting my_setting = ON;
Method some_method = METHOD_A;
Verbosity verbosity = DEBUG;
...

Where all are different kind of enums.

I would like to have a generic way to instantiate the enum values. That is, at runtime using reflection, and without knowing the enum types of the object in advance.

I would have imagined something like this:

for (ConfigLine line : lines)
{
   String[] tokens = line.string.split("=", 2);
   String name = tokens[0].trim();
   String value = tokens[1].trim();

   try
   {
      Field field = this.getClass().getDeclaredField(name);   
      if(field.getType().isEnum())
      {
         // doesn't work (cannot convert String to enum)
         field.set(this, value);
         // invalid code (some strange generics issue)
         field.set(this, Enum.valueOf(field.getType().getClass(), value));
      }
      else
      { /*...*/ }
   }
   catch //...
}

The question is: what should there be instead? Is it even possible to instantiate an unknown enum given its String representation?

Bozho · Accepted Answer · 2010-09-17T14:04:17.240

115

field.set(this, Enum.valueOf((Class<Enum>) field.getType(), value));

getClass() after getType() should not be called - it returns the class of a Class instance
You can cast Class<Enum>, to avoid generic problems, because you already know that the Class is an enum

edited Sep 17 '10 at 14:04

answered Sep 17 '10 at 14:01

Bozho

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this results in a generics compiler error: The method valueOf(Class, String) in the type Enum is not applicable for the arguments (Class, Object) – onejigtwojig Apr 25 '11 at 17:05
11

Would `field.getType().asSubclass(Enum.class)` be better? – Tom Anderson May 20 '12 at 23:03
1

This will have problems if you use declared fields - isEnumConstant may not work. Better to use valueOf method invocation as suggested by @rebzie – amrk7 Oct 05 '17 at 04:47

score 16 · Answer 2 · answered Feb 04 '16 at 23:39

16

Alternative solution with no casting

try {
    Method valueOf = field.getType().getMethod("valueOf", String.class);
    Object value = valueOf.invoke(null, param);
    field.set(test, value);
} catch ( ReflectiveOperationException e) {
    // handle error here
}

answered Feb 04 '16 at 23:39

Rebzie

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I think that this is the best solution for this question – allemattio Nov 28 '18 at 13:23
Just spent two hours trying to make selected answer work and couldn't because i didn't have the enum type to cast to - if only i scrolled down earlier... thank you – Quinn Oct 12 '19 at 03:43

Matthew Flaschen · Answer 3 · 2010-09-17T14:08:27.743

5

You have an extra getClass call, and you have to cast (more specific cast per Bozho):

field.set(test, Enum.valueOf((Class<Enum>) field.getType(), value));

edited Sep 17 '10 at 14:08

answered Sep 17 '10 at 14:01

Matthew Flaschen

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1

the generics problem still occurs – Bozho Sep 17 '10 at 14:03

score 2 · Answer 4 · answered Nov 06 '17 at 22:53

The accepted answer results in warnings because it uses the raw type Enum instead of Enum<T extends Enum<T>>.

To get around this you need to use a generic method like this:

@SuppressWarnings("unchecked")
private <T extends Enum<T>> T createEnumInstance(String name, Type type) {
  return Enum.valueOf((Class<T>) type, name);
}

Call it like this:

Enum<?> enum = createEnumInstance(name, field.getType());
field.set(this, enum);

score 0 · Answer 5 · answered Sep 18 '10 at 11:53

You may code your Enum similar tho this:

public enum Setting {

    ON("ON"),OFF("OFF");

    private final String setting;

    private static final Map<String, Setting> stringToEnum = new ConcurrentHashMap<String, Setting>();
    static {
        for (Setting set: values()){
            stringToEnum.put(set.setting, set);
        }
    }

    private Setting(String setting) {
        this.setting = setting;
    }

    public String toString(){
        return this.setting;
    }

    public static RequestStatus fromString(String setting){
        return stringToEnum.get(setting);
    }   
}

Then you may easily create Enum from String without reflection:

Setting my_settings = Setting.fromString("ON");

This solution is not originated from myself. I read it from somewhere else, but I can't recall the source.

Not really helpful if you're using reflection, because you don't know if the enum in question is using this setup. — Kolky, Oct 06 '10 at 08:54

Java: instantiating an enum using reflection

5 Answers5

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