Enum and generics in Java

Question

I need to provide some kind of facilities from enums and generics. Since enums cannot have type parameters (it's just forbidden by the grammar) and actually acts like public static final members I tend to write something like this:

public class GenericEnumLikeClass<T>{

    public static GenericEnumLikeClass<MyType> TYPE_INSTANCE;
    public static GenericEnumLikeClass<AnotherType> ANOTHER_INSTANCE;

    //other things
}

The thing is I've never done something similar before and strictly speaking not sure about the correctness from the Java Conventions standpoint. Will it be considered as something strange or it's a commmon technique used when we need to extend enum-like behavior with providing Type Paramters?

Ingo Kegel · Accepted Answer · 2015-10-26 07:13:27Z

3

This is the technique that was used before Java 1.5 introduced enums, so it does not look strange. Older APIs have a lot of these enum-style classes. I would recommend to make the constructor private and provide a values() method that returns an array of all available instances.

answered Oct 26, 2015 at 7:13

Ingo Kegel

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2 Comments

Tagir Valeev Over a year ago

And don't forget the final modifier.

St.Antario Over a year ago

Although, there was no generics before Java 1.5

Soven K Rout · Accepted Answer · 2015-10-26 07:24:38Z

1

Agreed with @Ingo, you can use as constructor and provide a value() method just like:

public enum GenericEnumLikeClass { MyType("TYPE_INSTANCE"), AnotherType("ANOTHER_INSTANCE");

answered Oct 26, 2015 at 7:24

Soven K Rout

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Comments

fluminis · Accepted Answer · 2015-10-26 08:40:16Z

1

Don't know exactly what you want to do with your 'generic' argument, but this could give you some ideas:

Edit: As you don't explain how you use this generic parameter I could not give real example. Just add some methods, to explain what I have in mind:

public enum MyGenericEnum {

    TYPE_INSTANCE(MyClass.class),
    ANOTHER_INSTANCE(AnotherClass.class);

    private Class<? extends SomeSuperClass> clazz;

    private MyGenericEnum(Class<? extends SomeSuperClass> clazz) {
        this.clazz = clazz;
    }

    public Class<? extends SomeSuperClass> getType() {
        return clazz;
    }

    @SuppressWarnings("unchecked")
    public <T extends SomeSuperClass> T getObject() {
        try {
            return (T) clazz.newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return null;
    }
}

edited Oct 26, 2015 at 8:40

answered Oct 26, 2015 at 7:42

fluminis

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3 Comments

St.Antario Over a year ago

Unfortunately, ClassCastException will be discovered only at runtime. But I need that aech enumerator be awared of some type at compile-time.

fluminis Over a year ago

Instead of just Class<?> you can use Class<? extends SomeClass>

St.Antario Over a year ago

Maybe I just didn't understand what you meant, but I think it doesn't provide compile-time type safety anyway.

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