How to convert String to enum value when enum type reference is a Class<?>?

Question

I have code which is setting values on an object using its setter methods. One of the setters takes an Enum type as the method parameter. The code looks something like this:

    String value = "EnumValue1";
    Method setter = getBeanWriteMethod("setMyEnumValue");
    Class<?> type = setter.getParameterTypes()[0];
    Object convertedValue = null;
    if (type.isEnum()) {
       convertedValue = convertToEnum(value, type);
    } else {
       convertedValue = ClassUtils.convertType(value, type);
    }
    return convertedValue;

The question is what to put in the convertToEnum method. I know I could "brute force it" by iterating the enum constants (or the fields) of the type object, matching the value. Am I overlooking a simpler way to do it using Reflection? (I looked at several examples, but didn't find any where the enum was only know via Class).

Adrian Pronk · Accepted Answer · 2013-06-11 21:09:50Z

23

Off the top of my head:

  Enum<?> convertedValue = Enum.valueOf((Class<Enum>)type,  value);

This will convert a string to an enum constant of the Enum class of type

Edit: Now that I have a computer handy, I can see what actually works. Either of the below cases ran correctly without compiler warnings:

Enum<?> convertedValueA = Enum.valueOf(type, value);
Enum<?> convertedValueB = Enum.valueOf(type.asSubclass(Enum.class), value);

The second one calls asSubClass() which would do a runtime check to ensure that type is some enum class, but the valueOf() method has to make that check anyway in order to work correctly.

The following gave me a compile error:

Enum<?> convertedValueC = Enum.valueOf((Class<? extends Enum <?>>)type, value);

java: Foo.java:22: <T>valueOf(java.lang.Class<T>,java.lang.String) in java.lang.Enum cannot be applied to (java.lang.Class<capture#134 of ? extends java.lang.Enum<?>>,java.lang.String)

The intricacies of casting to wildcard types confuses me so I've probably tried the wrong cast. Plus the fact that it has no runtime effect means that it's easy to get it wrong and never find out.

edited Jun 11, 2013 at 21:09

answered Jun 11, 2013 at 18:52

Adrian Pronk

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5 Comments

Sam Goldberg Over a year ago

Thanks, I must have gotten the cast wrong when I first tried that. This is correct solution. Unfortunately, after trying this, I found that I need to do a case-insensitive match, which Enum.valueOf() won't do. So back to the brute force approach...

Tom Hawtin - tackline Over a year ago

That case is awful. type isn't Enum.class. In any case, asSubclass is better than a hack cast.

Sam Goldberg Over a year ago

@TomHawtin-tackline: using as asSubclass gives me compiler warnings.

Class<? extends Enum> enumClass = type.asSubclass(Enum.class); Enum<?> convertedValue = Enum.valueOf(enumClass, "Value1");

for both Raw Type, and also Unvhecked Invocation. So I'm not sure I see how that is better.

Sam Goldberg Over a year ago

The issue is that if the type variable is declared as Class<?>, then it has to be cast or it won't compile. If it is declared as Class (no wildcard), then it works without casting. I got messed up because the return type of Method.getParameterTypes() is Class<?>[], which set me up to have the messiness of needing the cast.

Paŭlo Ebermann Over a year ago

Enum.valueOf(type, value); gives me "Bound mismatch: The generic method valueOf(Class<T>, String) of type Enum<E> is not applicable for the arguments (Class<capture#2-of ?>, String). The inferred type capture#2-of ? is not a valid substitute for the bounded parameter <T extends Enum<T>>". The other one gives me a warning: "Type safety: Unchecked invocation valueOf(Class<capture#3-of ? extends Enum>, String) of the generic method valueOf(Class<T>, String) of type Enum" – because the "asSubClass" returns a Class<? extends Enum> instead of Class<? extends Enum<E>>.

Peter Lawrey · Accepted Answer · 2013-06-11 20:25:55Z

4

You can use

Class enumType = ....
String name = ....

Enum e = Enum.valueOf(enumType, name);

e.g.

import java.lang.annotation.*;

public class Main {
    public static void main(String[] args) {
        Class enumType = RetentionPolicy.class;
        String name = "SOURCE";

        Enum e = Enum.valueOf(enumType, name);
        System.out.println(e);
    }
}

prints

SOURCE

edited Jun 11, 2013 at 20:25

answered Jun 11, 2013 at 18:52

Peter Lawrey

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5 Comments

Tom Hawtin - tackline Over a year ago

@SamGoldberg That's a raw type. So I think it will work. Just badly.

Sam Goldberg Over a year ago

@TomHawtin-tackline: I can't get it to compile without the cast.

Peter Lawrey Over a year ago

@SamGoldberg See my example. No cast is required.

Sam Goldberg Over a year ago

Yes, I see. If I change Class<?> to Class, then it works and compiles fine.

Peter Lawrey Over a year ago

@SamGoldberg and the code is simpler, but does the same thing.

NimChimpsky · Accepted Answer · 2013-06-11 18:52:16Z

1

Enum.valueOf(yrEnum, myString)

Returns the enum where myString is the name of the enum instance you want. And yrEnum.values() returns an array of all the different possible Enums instances.

answered Jun 11, 2013 at 18:52

NimChimpsky

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1 Comment

Sam Goldberg Over a year ago

As with previous, the cast to Class<Enum> is required to make this work.

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