How to convert from String to Enum dynamically?

Question

I'm trying to build a dynamic wrapper that uses reflection to access properties of objects. My approach works with different kinds of objects – but I still have an issue with Enums.

Let's assume I already have the proper setter und getter and I want to invoke them in different situations. For example I'm trying to save a given value via the following code:

public void save() {
    try {
        // Enums come in as Strings... we need to convert them!
        if (this.value instanceof String && this.getter.getReturnType().isEnum()) {

            // FIXME: HOW?

        }
        this.setter.invoke(this.entity, this.value);
    }
    catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException ex) {
        throw new RuntimeException("Die Eigenschaft " + this.property + " von Entity " + this.entity.getClass().getSimpleName() + " konnte nicht geschrieben werden!", ex);
    }
}

How can I convert the String object to the proper Enum value?

I know about MyEnum.valueOf(String)... but what if I can't name the Enum in my source code? I've not managed to use something like

this.value = Enum.valueOf(this.getter.getReturnType(), this.value);

Well, I guess you can use .getDeclaredFields() from the class object... — fge
– fge, Commented Jul 22, 2015 at 8:59
I know the field... even getter and setter. I don't manage to convert to the proper Enum element at the moment. — Daniel Bleisteiner
– Daniel Bleisteiner, Commented Jul 22, 2015 at 9:01
what is the problem with saying this.value = YourEnum.valueOf(theString)? — dsharew
– dsharew, Commented Jul 22, 2015 at 9:03
You can't reference an Enum with this as they are implicitly static. — Edward J Beckett
– Edward J Beckett, Commented Jul 22, 2015 at 9:03
@DegenSharew I don't know YourEnum... it may be any of MyEnum1, MyEnum2, ... — Daniel Bleisteiner
– Daniel Bleisteiner, Commented Jul 22, 2015 at 9:06

wero · Accepted Answer · 2015-07-22 13:48:13Z

5

Given a Enum class and a string value you can convert to the Enum object corresponding to the string via the following static method defined in java.lang.Enum:

static <T extends Enum<T>> T valueOf(Class<T> enumType, String name)

e.g.

java.awt.Window.Type type = Enum.valueOf(java.awt.Window.Type.class, "NORMAL");

Note that Enum.valueOfthrows an IllegalArgumentException if there is no corresponding enum constant.

You can't invoke Enum.valueOf without a cast from your method, since getter.getReturnType() is a Class<?>.

So a helper function might handle the cast:

@SuppressWarnings("unchecked")
private static <E extends Enum<E>> E getEnumValue(Class<?> c, String value)
{
     return Enum.valueOf((Class<E>)c, value);
}

and you just use it in your code:

if (this.value instanceof String && this.getter.getReturnType().isEnum())
     this.value = getEnumValue(getter.getReturnType(), (String)this.value));

Note that any solution which loops over Enum.getEnumConstants() will create a temporary array for the constants, whereas Enum.valueOf uses an internal cache.

answered Jul 22, 2015 at 13:48

wero

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1 Comment

Daniel Bleisteiner Over a year ago

This helper function is a good point... the exakt syntax blocked my attempts so far. Thanks... I'll give this a try.

Daniel Bleisteiner · Accepted Answer · 2015-07-22 09:05:44Z

0

Just found one way... but it seems "dirty"... any better?

if (this.value instanceof String && this.getter.getReturnType().isEnum()) {
    for (Object v : this.getter.getReturnType().getEnumConstants()) {
        if (v.toString().equals(this.value)) {
            this.value = v;
            break;
        }
    }
}

answered Jul 22, 2015 at 9:05

Daniel Bleisteiner

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3 Comments

dsharew Over a year ago

how does this work then? getEnumConstants is only valid if the class is of type ENUM.

Daniel Bleisteiner Over a year ago

No, it is not... docs.oracle.com/javase/7/docs/api/java/lang/…

Daniel Bleisteiner Over a year ago

The method simply returns an empty list if it is not applicable. But I can call it on any class. It compiles...

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